Adolescence a transition stage between Childhood and adulthood

Adolescence is that period of the human development that occurs during the time span between childhood and adulthood. More specifically, it spans from 10 to 19 years. Apart from physical and sexual changes, adolescence also marks the beginning of more independent attitude in maturing girls and boys as they move towards creating their unique identity.The human body undergoes both physiological and mental changes during adolescence. Thus changes occur in parts of body including brain, sexual organs during this period. The growth spurt is observed in puberty, characterized by rapid growth in extremities such as hands, head and feet fallowed by legs,arms, torso and shoulders.This gives an adolescent body an out of proportion look.In adolescents the rate of growth nearly becomes equal to growth of a child.

Sex hormones

The male and female sexual development is governed by two different hormones viz. testosterone and estrogen respectively. Prior to explaining the two, it is important to study a frequently used term in adolescence development called androgens.
Androgens

Androgens are produced by the adrenal glands and ovaries in females. In males, androgen is produced by testis. Androgens provide for the activity of accessory sex organ .Androgens also help in the development of secondary sexual characteristic. Secondary sexual characteristics show that a child has grown into an adult. Some of the visible secondary sexual characteristics are appearance of Adam’s apple and facial hair in males and enlargement of breasts in females. The natural male androgen is testosterone and natural female androgens are estrogen/oestrogen and progesterone.
Testosterone

Testosterone is a natural androgen found in human males (androgens can also be synthetically made).  Chemically, testosterone is C 10 sterol compound. This hormone is synthesized by the interstitial cells of the testis .Testis are two flat and oval bodies, found one on each side of the scrotum sac beneath the penis. Interstitial cells from the endocrine part of the testis.

Physical changes in boys during adolescence

Testosterone acts to cause growth of accessory male sex organs like testis and penis, the seminal vesicles, vas deferens, prostate, epididymis etc. testosterone also facilitates sperm production. The hormone exerts myotrophic effect and helps in muscular development of certain muscles. Testosterone also provides for normal development of secondary sexual characteristics. Secondary sexual characteristics exhibited by human males by action of testosterone are listed below.Growth of facial, body, underarm, abdominal, chest and pubic hair.  

Androgenic alopecia can cause loss of scalp hair.Boys usually complete puberty changes between 16 to 17 years.

1.Ejaculation of semen through penis starts. Ejaculation  due to spontaneous orgasm during sleep (Wet dreams) nocturnal emissions becomes common.

2.Deepening of voice as a result of enlargement of larynx or Adam’s apple.

3.Development of facial hair like mustache and beard, body hair like hair on chest, hands, legs and Pubic hair in the groin.

4.Heavier bone structure and skull, increase of mass of thigh muscles located in front of the femur

5.Thinning of waist.Increased muscle mass and strength , enlarged body and    stature along with squaring of face.

6.Growth of penis and enlargement of testis.

7.Rapid growth  and increase in height (growth spurt)  with lower body fat percentage.

8.Widening of shoulders and chest.

9.Increase in sweat and oil secretion from the body causing body odor and acne formation.

Physical changes in girls during adolescence

Progesterone and oestrogen/estrogen constitute female sex hormones.

Oestrogens or estrogen are sterol derivatives. They are secreted by the Graafian follicles, corpus luteum, placenta and interstitial cells found in ovaries. Oestradiol and oestriol constitute oestrogens.  Oestrogen is responsible for all the puberty changes that occur in females. The puberty changes include growth of vagina, uterus and stratification, of vaginal epithelial cells. Oestrogen proliferates the ducts of the breast and causes Breasts enlargement. The hormone also causes increases in secretions of fallopian tubes. Oestrogen is responsible for proliferative changes in menstruation. Oestrogen also develops secondary sexual characteristics in female. Girls usually complete puberty changes between 15 to 17 years.Progesterone in females is secreted by the corpus luteum, placenta and adrenal cortex. The hormone is a sterol derivative having a side chain at the 17 C position. Progesterone causes an in vaginal cellular proliferation and acts synergistically with oestrogens to cause development of secondary sexual characteristics in females. Progesterone is responsible for premenstrual changes that occur in uterine mucosa. The hormone causes an increase in ovulation and causes enlargement of breasts.

1.Secondary sexual characteristics appear in females because of the female  androgen estrogen.

2.Erection of nipples and enlargement of breasts occurs between 8-13 years.

3.Growth of the pubic hair occurs between 8- 14 years and underarm hair fallows after two years . Pubic hair starts growing first along the lips of vagina in the shape of inverted triangle and gradually grow in to thighs.

4.Increase in thigh muscle behind the femur (opposite to males).

5.Increase in width of hips resulting in a lower waist to hip ratio than males.

6.Growth of uterus, the inner lips of vulva, clitoris and labia minora  show color change.

7.Increase in length of upper arms.Rounding of face.

8.Menstrual cycle  starts  between 10 -16 years.

Stages of Adolescence

Adolescence occur in three stages namely Early adolescence (12-14 years), Middle adolescence (15-17 years) and Late adolescene (18-21 years).

During early adolescence the youngsters tend to remain moody, more aware about their body changes and compare their body physique with idealized personalities. They develop shyness and crave for privacy. They express their  inner emotions non verbally in spite of poor communication skills. Fantasies, day dreaming are common.


Middle adolescents invest lot of time in self grooming, body shaping and experimenting with new hair styles. Imitating the mannerisms and gestures of their favorite role models (Hero worship) is frequently observed. They imagine themselves as well grown adults with self concepts and esteem and often dislike parent's interference with their freedom and privacy.Risky behavior like trying alcohol, Cigarettes, drugs and intercourse  develop during this stage. Speaking skills develop consistently and begins better use of verbal communication. There are more chances to contract HIV and STDs because of emotional ignorance at this stage.

Late adolescents eventually turn into responsible young adults. They recognize and respect the suggestions of their elderly parents. They stay focused in achieving career goals and  start a family. Maximum mental maturity is observed in this stage. They avoid risky behavior and pay little interest in grooming. Becomes acclimatized to ways of life, pleasure and pain and develops compromising nature. Becomes more concerned about serious relationships.

Main health issues during adolescence

Adolescence has its own set of health issues. The major heath issues are early pregnancy has complications and can cause death of both mother and fetus. Birth control pills, procedures and techniques provide solution to this menace.Lack of knowledge about sexual diseases like HIV leads to their spread. Caution during sexual activities like use of condoms provides some relief.  Other HIV control measures such as not using used syringes, infected blood etc must be practices. Spread of education in masses regarding STDs can eradicate these diseases.

Depression

Depression is one of the leading causes of suicidal death in adolescent individuals. Hunger, violence, poverty and other causes are related to depression. Counseling is one of the measures for treating depression. Drugs are also available.
Androgen related disorders

Low androgen levels cause decrease in interest for sex, bone diseases, fatigue and decrease in sense of well being. Low androgens levels in females mostly occur during menopause. Low androgen levels are treated with estrogen containing medications.High levels of female androgens cause hirsutism, acne and thinning of hairs. High levels of female androgens can also cause PCOS or poly systic ovary syndrome. PCOS results in infertility, irregular or totally absent menstrual periods and blood sugar disorders.Hyperandrogenism can also cause a genetic disorder called. Congenital adrenal hyperplasia. Other adrenal abnormalities can also occur. Ovarian tumors are also formed in some cases.

Acne a big problem in adolescence

Adolescents mostly experience  acne on their face, neck, back, chest and shoulders. Acne spoils self esteem and appearance and that is what most teenagers are bothered about. The reason for these  pimples is elevated levels of androgen hormones in body stimulates  Oil glands (Sebacious glands) in skin.  These glands become hyper active and  secrete oily substance called Sebum which usually comes out from hair follicle, but excess amounts of sebum clog the follicle passage and soon attracts bacteria around to thrive on it. This causes infection, pus accumulation causing a swelling. Some times these pimples burst and leave permanent ugly scars on skin.


General treatment of acne include Ointments, creams and  lotions with formulations of salicylic acid, benzoyl peroxide.  Mild form of acne can be treated with benzoyl peroxide,tazarotene, adapalene, dapsone, azelaic acid and antibiotics. Doctors may prescribe oral antibiotics to inhibit the bacterial growth on skin. It is advised to take two gentle face washes per day with mild soap.  Too much cleansing of face should be avoided as it may irritate skin and trigger the activity of oil glands, making the situation much more worse. Squeezing the pimples may push bacteria into deeper layers of skin so, it is better to stop it.

Changes in the Brain during adolescence


The human brain reaches 90% of its size during the first 6 years of life. During the years 10 to 25, the brain undergoes those changes that are important for behavior. The creases of the brain start taking a complex shape during the late teen age. Changes in folds of brain cortex during adolescence result in the cognitive and emotional improvements. During adolescence, the white matter content of the brain increases. Synaptic pruning also occurs which is the elimination of unnecessary neuronal connections. Increase in myelination makes the brain more efficient. Both synaptic pruning and myelination that occur in the prefrontal cortex improves the efficiency of the brain in processing information. The brain also evaluates “risks and rewards "in a better way as the neural connections in brain (specially the prefrontal cortex) get strengthened. “Planning ahead” functions are created by developments that occur in the dorsolateral prefrontal cortex. Decision making gets improved as the ventromedial prefrontal cortex develops further.Various neurotransmitters are involved in adolescent brain functioning. Glutamate is secreted during synaptic pruning. Dopamine effects decision making, risk taking and boredom. Serotin effects “rewards and punishment”, mood and behavior.  

Cognitive development during adolescence

Cognitive development includes reduction in egocentric thoughts and start of occurring of more abstract forms of thought. Thus during adolescence an individual starts to think in wider terms. These functions are associated with prefrontal cortex. The cognitive developments play a major role in forming character and personality.Other cognitive developments are improvement in attention, memory (both working and long term), faster processing of knowledge and information, organization and metacognition or thinking about self.Considerable improvement in selective attention-ability to focus on one stimulus and divided attention - ability to focus on more than one stimuli simultaneously are observed.

Psychological development during adolescence


During adolescence the individuals start to form their own beliefs and values. They set aim for the rest of their life. During this period the individual gets highly self conscious. The children start to lose parental control and began experimenting with new ideas. It is important that the newly turned adolescents use their newly acquired skills to gain self confidence. The adolescent takes more control of the vocational and educational aspects of his/her life. The adolescent strive for self interdependence and self esteem. During late stages of adolescence an individual feels psychologically integrated and develops a consistent view of the outside world. The individual concludes the adolescent stage by setting goal and aspirations and starts to work for them in his/her later periods of life.  

They tend to develop self-esteem and self-concepts to gain recognition in society and among peers. They spend much time in personal grooming, making them selves look presentable.They tend to stay updated in fashion clothing, hair styling, stylish mannerisms and accent to mark their identity.Girls make efforts to look charming and presentable, spend much of their time before dressing table in applying cosmetics, hair styling etc. It is during this age most of the girls choose modeling as their career. 


Love, romance and sexual behavior


Because of heightened attractions towards opposite sex, the adolescents fall in love with each other. They become overwhelmed in infatuations and day dreams. They spend most of time, texting and love making. Adolescents often date with their class mates and same age group and tend to hang around in pairs in social gatherings. Holding hands and staring in each other's eyes, lip kissing, hugging are some romantic events in adolescents. Sexual intercourse is less pronounced and often some adolescents make a try to explore it.


Adolescent behavior

  There is difference between youth and adolescence. Youth fail to understand the implications of their behavior. Adolescents do not fully understand the implications of their behavior though they have some understanding of it. The adults can fully comprehend the implications of their behavior. In adolescence the judging power of the brain is not fully developed. Thus adolescent’s behavior may include inappropriate sexual, health and other decisions. Also, the fast maturing body may fail to fully control itself that may cause rash or inappropriate behavior.

Family, school and community provide the required support that may help the individual to successfully undergo the transition from childhood to adulthood. It is the duty of parents, guardians, teachers to provide key support to the young adults to learn right skill and acquire right knowledge during this period. 

Hormones involved in adolescent growth

The growth in humans is associated with many hormones. These include the pituitary growth hormone, the thyroid hormone, sex hormones viz. estrogen and testosterone and the sex gland stimulating hormones or the pituitary Gonadotrophic hormones.
Pituitary growth hormone or human growth hormone (HGH)
The pituitary gland is an endocrine gland located in the brain.  The gland secretes many hormones and the hormone that is concerned with growth is the growth hormone or Somatotrophic hormone.

The growth hormone or HGH has a molecular weight of approximately 21000 daltons. This hormone is a crystalline protein. The human GH consists of a straight chain polypeptide structure having 2 disulphide bridges. The human HGH contains around 188 amino acids. The human growth hormone has growth stimulating as well as lactogenic effects. 

Actions of human growth hormone or HDH

The hormone increases the length of the cartilage bones by stimulating the multiplication of epiphyseal cartilage.     The hormone exerts a direct influence of growth of tissues. It stimulates muscular growth and visceral growth. It also stimulates the growth of the thymus gland and stimulates secretion of milk during lactation period. It has no effect on growth of nervous system. If there is inadequate or no secretion of pituitary growth hormone then dwarfism occurs. Children who do not have HGH are fat. When HGH is secreted in them, they lose fat and gain height

The hypothalamic hormone GHRF or growth hormone releasing factor secreted by hypothalamus controls the secretion of HGH by the acidophil cells of the pituitary.  It has been found that a sharp lowering in blood sugar level results in secretion of GHRF due to which HGH is released. The secretion of HGH results in increased mitotic activity of the thymus which is mediated by cyclic AMP.

Thyroxine or T3 

The thyroid hormones are secreted by thyroid gland that is situated in the root of the throat. The thyroid gland secretes thyroxine hormone that is involved in both physical and mental growth. Thyroxine is also called T4 and chemically is an amino acid having chemical formula  3,5,3’,5’-tetraiodothyronine.Both HGH and thyroxine exert influence on the growth of tissues of body. The differentiation of growing is solely influenced by thyroxine. Thyroxine helps in the maturation and differentiation of epiphysis or skeletal muscles. Lack of thyroxine causes decreased muscular, gonad and brain growth which is called cretins.
 

Human nervous system

We comprehend, perceive and respond to the world around us because of the nervous system. Other unconscious functions of the body such as digestion, respiration etc are also controlled by the nervous system. The human immune system controls and unites the different body functions. It also keeps the internal body environment normal despite any kind of changes that may occur in the external environment. The nervous system acts because of certain regulatory reactions. All the actions and behaviors that are done by a human being depend on the nervous system as it receives, stores and releases sensory and  motor information.

The resultant actions can be as small as cellular in nature and as large as involving the whole body.The 2 main parts of nervous system are:The central nervous system, and The peripheral nervous system

Development of nervous system

The human nervous system develops from the primitive layer of ectoderm. Some areas of nervous system also originate from medullary plate or neural plate. The brain and its ventricles are formed of cephalic tube while the spinal cord and its central canal are formed of caudal end of the neural plate. The nerve cells or the neurons originate from neuroblasts and the neuroglia develops from spongioblasts.

Neural cells or neurons

As stated earlier, the neural cells are formed of neuroblasts.  During maturation of the neuroblasts, at first the neurofibrils appear in the cytoplasm of the cell. The neuroblasts then migrate away from the central canal. An axon appears at the periphery of the neuroblast. The axon containing neuroblast is bipolar in nature. It cuts off from the central canal. Dendrites then appear and multipolar neuroblasts or mature neurons are formed.

Glial cells or spongioblasts

Primitive spongioblasts get differentiated into astroblasts. The astroblasts connect with blood through protoplasmic extensions called end feets. They then branch further to form protoplasmic astrocyte. Some astroblasts have glial fibrils in cytoplasm. These glial fibrils containing spongioblasts form fibrous astrocytes.

The undifferentiated medulloblasts may proliferate and differentiate to form spongioblasts, neuroblasts and oligodendroglia.

Central nervous system or CNS

The central nervous system consists of brain and spinal cord. It is also called somatic nervous system. The CNS can be divided into 2 halves, 1 half mirroring the other. The spinal cord is located inside the vertebral column and the brain is located inside the cranial cavity. The brain and the spinal cord are divided by foramen magnum or the limiting line.

Spinal cord

The spinal cord lies in the vertebral column. The length of the spinal cord in males is 45 cm and in females is 43 cm. An adult spinal cord weighs approximately 35 gm. It is located between the foramen magnum and the lower border of lumbar vertebra adjacent to the 12th thoracic segment. The spinal cord is cylindrical in shape. There are 2 swellings in it. 1st is at the 6th cervical and 2nd is at the 3rd lumbar. The superimposed segments of spinal cord are arranged in series and form these segments from where the nerve roots originate. 31 pairs of nerves originate from the spinal cord. The 31pairs can be divided into 12pairs of thoracic, 8 pairs of cervical, 5 pairs of sacral, 5 pairs of lumbar and 1 pair of coccygeal nerves.

Brain

The brain can be divided into forebrain or prosencicephalon, midbrain or mesencephalon and hindbrain or rhombencephalon.

Hindbrain or rhombencephalon

The hindbrain or rhombencephalon

Cephalin connects the spinal cord with the rest of the brain. The hindbrain is composed of medulla oblongata, pons and 3rd to 12th cranial nerves. This portion of the brain governs the sub consciously done essential functions of the body through reflexes. The hind brain can be divided into myelencephalon or medulla oblongata and metencephalon which is comprised of pons and cerebellum Medulla oblongata or myelencephalon

It is situated in the posterior section of the cranial fossa. The cervical spinal cord continues from the foramen magnum to form medulla which extends upto the pons. The autonomic functional control of the body for lungs, heart and circulation lies in medulla. Medulla also governs the autonomic reflex actions like coughing, sneezing, swallowing, vomiting and gagging.

Pons- a part of metencephalon

Pons is situated above and in front of medulla. The transverse fibers collect here to form a bulge. The 4th ventricle separates the pons and the cerebellum. The inferior pontine sulcus separate the medulla and the pons. Superior pontine sulcus is present between the pons and cerebellum. Pons regulates respiration and some other vital functions of the body.

Cerebellum- a part of metencephalon

It is located in the caudal part of cranial cavity and is locate behind and above the medulla. Three peduncles or stalks join the cerebellum to other parts of the brain. The superior or rostral pair joins it to the midbrain, the middle pair of peduncles joins it to the pons while caudal or inferior pairs of peduncle join it to the medulla oblongata.The central part of cerebellum is called vermis which is surrounded by 2 lateral hemispheres. Cerebellum is concerned with smooth movements and therefore governs posture.

Midbrain or mesencephalon

The brain stem continues upward to form midbrain. The aqueduct of sylvius passes through it. Above the acqueduct (tectum) are located collection of nerve cells called superior colliculus and inferior colliculus. The third nerve nucleus and the red nucleus lie below and lateral to the aqueduct.

The underside of the aqueduct contains cerebral peduncle. The dorsal and ventral pats of this peduncle are divided by nerve cells (containing melanin) that are called substantia niagra. The ventral area is called pis and the dorsal area is called tegmentum. Nerve fibers pass through these areas. The midbrain governs vision and hearing. Substantia nigra produces dopamine that is vital for motivation.

Forebrain or procencephalon

The forebrain or procencephalon can be divided into 2 parts viz. the telencephalon and diencephalon. The telencephalon is composed of cerebrum, cerebral cortex, corpus striatum and rhinencephalon.  The diencephalon is composed of thalamus, hypothalamus, epithalamus and metathalamus. Speech, thought, sensory integration, control of voluntary movements and higher intellectual functions are controlled by forebrain. The forebrain can be divided into telencephalon and diencephalon.

Telencephalon

It consists of cerebrum, cerebral cortex, corpus striatum & rhinencephalon.

Cerebrum

It is the largest part of the human brain and accounts for 4/5 of its weight. The longitudinal divide splits it into 2 prominent hemispheres. The deep median fissure or the dividing line is also called cerebral fissure. Nerve fibers called corpus callosum connect these two hemispheres. Each of the 2 hemispheres is divided into frontal, parietal, occipital and temporal lobes. The fissures dividing them are central, parieto-occipital and sylvian. The right hemisphere receives sensory inputs from the left side of the body and provides motor function to it and vice versa. Thus each hemisphere controls the opposite side of the body. This arrangement is called contra lateral arrangement. Each hemisphere receives touch and vision input contra laterally while auditory input is common for both hemispheres.  In each individual one hemisphere is more dominant than the other. This dominant hemisphere controls analytical, mathematical, language and handedness functions. That is why there are right handed and left handed individuals. The non dominant hemisphere deals with facial recognition, emotions, auditory senses and spatial concepts.

Grey matter and white matter

The human brain has many levels of neurons that are connected to each other. These neuron collections are differentiated into grey and white matter. The grey matter is also called substantia griesa. The grey matter has no myelin blanket and is controlled by dendrites (short and branching filaments that carry the nerve impulses from the brain to rest of the body) and nerve cell bodies. The nerve cell bodies are those areas of the neuron that contain nucleus. The grey matter is grey because of the grey colored nuclei of the cells. Real information processing happens in the grey matter of the brain. It is approximately 40% of the whole brain. It also consumes 90% of the oxygen that reaches brain. The neurons of the grey matter do not have axons. The neurons of grey matter are connected by neurotransmitters. All the senses of the human body like hearing, speech, seeing, memory and feelings are controlled by the grey matter. The grey matter also controls the muscles and their movements.

The other name for white matter is substantia alba. The white matter neurons have axons or myelinated nerve fibers. The white matter is found between the cerebellum and the brain stem. The hypothalamus and the thalamus are composed of white matter neurons. The main function of the white matter is to relay all the information it receives from different parts of the body towards the cerebral cortex. It also controls sub-conscious functions of the body like blood pressure, temperature and heart rate regulation. Apart from these functions the white matter also controls food and water digestion, dispensing of hormones in different parts of the body and expression of emotions. Axons of the white matter neurons are covered and protected by myelin sheath. The myelin sheath gives the white matter its white color. The white matter composes around 60% of the total brain.

Cerebral cortex

The surface layer of cerebrum is called cerebral cortex. It is called the grey matter of the cerebrum. The cerebral cortex is made up of nerve fibers that are highly convoluted. The crest or the ascending area of the convolution is called gyrus (singular gyri). Two gyri are separated by a fissure called Sulcus (singular sulci). The sulci and gyri form patterns on the basis of which the cererbral cortex is divided into 4 parts viz. frontal, temporal, parietal and occipital lobes. These lobes are distinguished by 2 major sulci that are located on the sides of hemispheres. The frontal and parietal lobes are separated by fissure of ronaldo or central sulcus. The fissure of sylvius or lateral sulcus separates temporal, frontal and parietal lobes.

Frontal lobes- this is the largest among all the 4 lobes. The frontal lobe lies in front of the central Sulcus. The primary motor region of brain lies in this lobe and is formed of the precentral gyrus. An injury to this part of the brain may result in paralysis of the opposite side or contra-lateral part of the body. The bronco area, the region involved in speech is located in the inferior frontal lobe.

Parietal lobe- it lies at the back of the central Sulcus. It is divided in 3 parts viz. the post central gyrus, the superior parietal lobe and the inferior parietal lobe. All the sensory input from the opposite half of the body is received in the post central gyrus. The superior parietal lobe is located below and behind the postcentral gyrus and above the intraparietal sulcus. This area is not involved in sensory input processing and does not provide motor function. It is associative in function. Supramarginal and angular gyri compose the inferior parietal lobe. The inferior parietal lobe is concerned with the integration of sensory signals.

Temporal lobe-it is located behind the lateral Sulcus. The temporal lobe fills the middle hollow fosaa of the brain. The superior, middle and inferior gyri compose the outer area of the lobe. 2 transverse gyri are present at the margin of the lateral Sulcus which from the basic or primary auditory area of the brain. Different parts of this area represent different auditory frequencies. Near the ventral surface of temporal an inner protrusion called uncus is present. The uncus forms the larger part of the olfactory area.

Occipital lobe- it lies posterior to the parieto- occipital Sulcus that is joined by the calcarine sulcus. It is primary visual area of the brain and receives vision sensory input from the contra lateral or opposite part of the body. The rear areas of this portion of the brain represent central vision (the gaze sensory input) and the front areas represent peripheral vision (outside direct vision or gaze sensory input).

Limbic lobe- this lobe is composed of adjacent portions of temporal, parietal and frontal lobes which surround the corpus collasum.  Thus limbic lob is also called the synthetic lobe. This lobe relates with the autonomic and sensory activities. It receives sensory input from the thalamic nuclei that are connected with the hypothalamus.

Corpus striatum

The corpus striatum or striatum receives sensory input from the cerebral cortex. This part of the brain deals with motivation. It is composed of medium spiny neurons. This area of the brain releases acetylcholine.

Rhinencephalon

This is olfactory lobe of the brain. The olfactory nerves arise out of these lobes. Rhinencephalon is a paired structure and consists of 2 halves.

Diencephalon

The diencephalon section of for brain consists of thalamus, hypothalamus, epithalamus and metathalamus.

Thalamus-A pair of large and ovoid organs occupy majority of 3rd ventricle walls. Each singular part is called thalamus while the pair is termed as thalami. Various receptors provide neural impulses to the thalamus. The thalamus translates these impulses and transfers them to the cerebral cortex. Touch, temperature, pain and synaptic transmissions of resting stage form these sensations.

Hypothalamus

This region of the brain lies on the floor of 3rd cerebral ventricle and is located below the thalamus. This small cone shaped structure projects downwards from the brain and ends in the pituitary stalk which is tubular connection to the pituitary gland. The hypothalamus controls the majority of autonomous functions of the body and also affects the flow of endocrine hormones. The endocrine effect of the hypothalamus is because of its complex relationship with the pituitary gland.

Epithalamus- it consists of pineal body, habenular nucleus and  habenular striae.

Pineal body-pineal body is also called the pineal gland. It is a small endocrine gland situated in brain that produces melatonin. Melatonin is responsible for cincardian rhythm or sleep cycle of the body. The gland forms the root of diencephalon. The pineal gland is located between the 2 cerebral hemispheres and behind the 3rd cerebral ventricle. It is around 0.8cm long and has a weight of 0.1gm.adrenergic nerves join pineal gland. Pinealocytes and other supportive cells constitute the pineal gland.

Habenular nucleus- the habenular nucleus is formed of small group of nuclei. This nuclei group is located behind the thalamus. This region links the pineal gland with the rest of the brain. 

Habenular striae- It is located at the medial surface of thalamus. Habenular striae are formed of afferent fibers which originate from the septal nuclei.  Read more..

The peripheral nervous system or PNS

The peripheral nervous system consists of the cranio-spinal nerve system (composed of 31 pairs of spinal nerves and 12 pairs of cranial nerves) and visceral nerve system consisting of sympathetic and parasympathetic nervous system.The essential function of the PNS is to connect the CNS to other parts (limbs and organs) of the body. It forms the communication link between the CNS and the other extremities of the body. PNS is not enclosed in bones and hence is open to attack to toxins and other harmful elements/pathogens that may circulate in blood. PNS is composed of somatic nervous system and autonomic nervous system. Acetylcholine and noradrenaline are the 2 main neurotransmitters of the PNS.

Somatic nervous system

It is also called the voluntary nervous system as it is concerned with the skeletal muscles that have a voluntary control over body movements. It consists of afferent and efferent nerves. The afferent nerves relay sensory inputs towards the CNS while the efferent nerves stimulate the muscle contraction in skin and skeletal muscles. The somatic nervous system is made up of spinal, cranial and association nerves. The spinal nerves carry sensory and motor information from the spinal cord. The cranial nerves carry sensory and motor information towards and away from the brain stem and are associated with sensory inputs like vision, smell, taste etc. the association nerves integrate the sensory and motor functions of the body.

Autonomous nervous system or ANS

It is also called as involuntary or visceral nervous system. It influences the functions of the internal organs of the body. Thus, heart rate, urination, sexual arousal, digestion and other subconscious movements of the body are governed by ANS. The ANS also governs the fight or flight response of the body. The hypothalamus governs the autonomic functions of the body within the brain. ANS is divided into sympathetic and parasympathetic nervous system. The sympathetic nervous system is the “fight or flight” system. The “fight or flight” system is the body’s physiological reaction in situations of survival, harmful attack and event. The parasympathetic nervous system deals with the” rest and digest” scenarios.

Human immune system

  

Immunity

Immunity can be defined as the defensive mechanism of the body that acts against foreign proteins, toxins produced by the invading pathogens or microorganisms and micro organisms.Natural immunity can be defined as the immunity that is present since birth in a human to any of the pathogens, microbial proteins and toxins. We resist many of the diseases by our own immune system and need no external vaccine or drug for the resistance. This is so because we have the necessary antibodies for resisting such diseases.

Types of immunity found in humans
Immunity can be either generated inside the body or the body can also acquire the immunity from outside sources.

Active Immunity

Active acquired immunity is the immunity which we acquire later in life when we are exposed to some pathogen. When the body successfully resists an attack from the below given factors, then active acquired immunity is attained. The immunity provided by the T cells and B cells are types of active immunity. The factors are:

A particular microorganism

A closely similar micro organism

A modified toxin

A killed microorganism

A micro organism that has been artificially made less hostile

Artificially acquired active immunity

A vaccine can induce artificially acquired immunity. Vaccine contains the necessary antigen due to which the body creates specific vaccine for resisting a particular pathogen.

Passive immunity

If we inject the serum of any animal that has antitoxins (produced because as a result of active immunity) into a human being, then the antitoxins produce passive immunity for that particular organism inside the human being.

Artificially acquired passive immunity

There are some antibodies that the human body cannot produce. When these antibodies are directly injected through short term immunization vaccines, the body attains passive immunity for those diseases of which antibodies have been injected.

Naturally acquired passive immunity

This immunity is developed in the body during pregnancy when some antibodies are directly passed form the maternal blood to the fetal blood.

Terms of immunity for familiarization

Antigen

The foreign bodies, proteins or bacteria that enter into the blood are called antigens. An antigen’s body can divided into 2 parts viz. hapten and protein nucleus. Hapten can be a mono or a polysaccharide.

Antibody

When an antigen enters the body, the body reacts to this invasion by secreting antibodies or immune bodies. These antibodies are part of the plasma proteins and are classified under the gamma globulin fraction of plasma proteins. The antibodies can be classified in 4 groups. These 4 groups are given below.

Antitoxins- These are released when toxic substances enter inside the body.

Agglutinins- These antibodies clump together the antigens and then destroy them.

Precipitins- They cause the precipitation of foreign proteins.

Haemolysins or cytolysins- They cause the haemolysis of old RBC’s.

MALT

The abbreviation malt stands for mucosa associated lymphoid tissue. Some parts of the human body like skin, eye,  salivary glands, lung , breast, thyroid and gastrointestinal tract contain MALT which is diffuse system containing small amounts of lymphoid tissue. MALT contains high amounts of B cells, macrophages, plasma cells and T cells. These immunity cells resist and destroy the antigens that pass through the mucosal lining.

GALT

GALT stands for gut associated lymphoid tissue. GALT is found in esophagus, tonsils, adenoids stomach and other areas of the gut. GALT also contains T and B cells which resist and destroy pathogens.

Similarly, other areas of the body also have lymphoid tissues that contain numerous T and B cells for resisting attack of pathogens/antigens. These are:

BALT or bronchus associated lymphoid tissue,

NALT or nasal associated lymphoid tissue,

CALT or conjuctival associated lymphoid tissue,

LALT or larynx associated lymphoid tissue, and

MALT or mucosa related lymphoid tissues are found in the body.

Mechanism of human immune system

The human body has a 3 layer immune defense mechanism against the foreign pathogens and toxins. The primary layer is called the physical barrier, the secondary layer is called the innate human system and the tertiary barrier is the adaptive immune system.

The surface immune barrier or the primary barrier

This is the first barrier of the body against the harmful foreign pathogens and toxins and is made of skin. The human skin does not allow the intrusion of any foreign particle inside the human body. But it is not possible to completely cut off the body from all foreign exchanges. The air we breathe can carry many pathogens. Pathogens can also travel inside the body through contaminated water and food. Thus it is not possible to have a total shield for the human body through which no pathogen or toxin can penetrate. Fortunately the body has certain mechanism which helps in protecting the body against pathogens. Some of the primary barriers of the body to pathogens are given below.

Cilia or tiny hairs are found in the respiratory tract that causes sneezing and coughing. Sneezing and coughing helps to eject many foreign harmful pathogens outside the respiratory tract.

Urine and tears flush out many harmful pathogens outside the body.

The human gastrointestinal tract or the GI tract and the respiratory tract secrete many mucous elements that trap, entangle and then cause the destruction of pathogens.

Skin is a water proof mechanical barrier that stops pathogens form entering the body.

Stomach acid (Hcl) kills many harmful bacteria and pathogens.

Chemical components of the physical barrier

Human secretions have certain chemicals that serve as the primary defense barriers of the body. The enzymes lysosome and phospholipase that are found in tears, saliva and breast milk are bactericidal in nature. Beta defensins is an antimicrobial peptide secreted in the skin and respiratory tract

The human semen has defensins and zinc that can kill the pathogens. Many pathogens enter the body through the food we intake. The proteases enzymes and gastric acid destroy these pathogens. The human gut has the commensal flora or friendly microorganisms that help the GI tract and the gastro urinary tract to fight against the harmful foreign pathogens. The commensal flora may alter the ph balance of the gut to make it non inhabitable for the pathogen. It can reduce the availability of iron and can also compete with the pathogen for space and food inside the gut.

Inflammation

When the body tissues are infected by some foreign pathogens or are injured, then the tissues of that area become inflamed. The inflamed swollen, red area can be distinguished from other normal areas. These changes occur as there is enhanced blood flow to the infected or injured areas. There is pain and heat in the inflamed area.  The infected /injured cells release cytokines and eicosanoids.

 Cytokines are cell signaling proteins. They include interleukins that mediate communication between WBC’s, interferons (proteins) that act against pathogenic bacteria and chemokines that help cause chemotaxis or response of the organism to chemicals.

 Eicosanoids are oxidized fatty acids that also act as signalers. Prostaglandins are eicosanoids that cause the inflammed blood vessels to dilate. Prostaglandins also produce fever in the affected organism. Leukotrienes are eicosanoids that attract other leukocytes to the affected area.

Human Innate Immune System or the secondary barrier

Those pathogens and toxins that cannot be dealt by the primary barrier are subsequently engaged by the secondary barrier or innate immune system. The different parts of innate immune system are described below.
a.Complement System

The complement system is the first arm of innate immune system. A series of chemical reactions (also called biochemical cascade or complementary system) are initiated inside the bodies which result in the attacking of the surfaces of foreign cells. The complementary system is so called as it complements the antibodies in destroying foreign pathogens. The complementary system consists of 20 proteins. The component system is humoral in nature. Unlike the cell mediated immunity which acts by activation of T-Lymphocytes and phagocytes, the humoral immunity acts by the action of macromolecules like peptides, proteins and antibodies. These peptides, proteins and antibodies are also called as the components of the complementary system. The component attaches itself to the pathogen and kills them.

b.Cellular barriers

The second arm of human innate immune system consists of leucocytes or white blood cells. Neutrophils, Basophils, Eosinophils, Mast Cells, Macrophages, Dendritic Cells and Natural Killer Cells comprise the cellular barriers. These cellular barriers engulf and destroy the pathogens by a process called Phagocytosis. The cellular barriers may also cause activation of the adaptive immune system which is the third immune barrier.

c.Phagocytosis

Phagocytosis is the process in which the pathogen is engulfed by any one of the phagocytes. Phagocytes comprise neutrophils, macrophages and dendritic cells. The entrapment occurs in phagosome which is a vesicle. The phagosome combines with the lysosome enzyme and forms phagolysosome. The digestive enzymes kill the pathogen. A respiratory burst (release of oxygen species) can also kill the pathogen.

Macrophages and neutrophils flow in the bloodstream and are carried throughout the body by blood. They locate and destroy the pathogens in all parts of the body. Macrophages also destroy the debris and old cells and thus act as scavengers. The Dendritic macrophages are found in tissue of nose, lungs, skin, stomach and intestines. Many of them come in contact to external environment and are not carried in bloodstream.

Cells of human innate immune system

The different cells of innate immune system are described below.
1.Granular leucocytes or granulocytes

These cells have granular cytoplasm. They are formed in the red bone marrow after birth. They are of 3 types.
Neutrophil

They have a multi- lobed nucleus. Their strength is 60 to 70% and count varies from 3000 to 6000 per cubic mm of blood. They are around 10 to 12 µm in diameter. The number of nucleus lobes can be more than 7 in a neutrophil. They are also called polymorphs as they have multi-lobed nucleus. Generally 3 to 4 lobed nucleus cells are found. With maturity the number of lobes increases. The cytoplasms of these cells have neutrophilic granules. These cells are amoeboid and phagocytic in nature. The enzymes found in these cells are lipase, protease, nucleotidase, phosphates etc.  Also lutathione, glycogen, ascorbic acid etc are found. The enzyme and other content of the neutrophils help them in their phagocytic activity.

Eosinophil

They have 2 or 3 lobed nucleus. Their strength is 1 to 4% and count varies from 150 to 400 per cubic mm of blood. They have a diameter ranging from 10 to 12 µm. The cytoplasm has course granules and the nucleus has 2 lobes. The eosinophils do not show any phagocytic activity. They are amoeboid though. They contain histamine.

Basophil

 the nucleus is lobed and the diameter of the cells varies from 8 to 10µm. The cytoplasm contains granules like all other granulocytes. The cell’s granules contain 5 hydroxytryptamine and histamine.

Macrophages

They develop from monocytes. Monocytes vary from 5-10% in strength. Their count varies from 350 to 800 per cu. mm. They have larger diameter which varies from 16 to 18 µm. The young nucleus is round or oval. The older cells have convoluted nucleus that can be kidney or horse shaped. The cells have non granular cytoplasm and an eccentric nucleus. These monocytes turn into macrophages when they enter the damaged tissue through the endothelium.

Fixed leukocytes

Some of the WBC’s do not keep roaming in the blood but take a place in the tissues. These are called fixed leukocytes. The mast cells and dendritic cells fall under this category.

Mast cells

Mast cells are oval or round and are large cells. The cytoplasm of mast cells consists of course granules. If we study the mast cells under an electron microscope, we will find freely roaming RNA and some amount of granular cytoplasmic reticulum. There size is similar to the basophils.

Dendritic cell

The Dendritic cells mediate between the innate and adaptive immune system. They process the pathogen material and carry them to the helper T cells. They are formed in the red bone marrow.

Natural killer cells lymphocytes

Natural killer cells are those cells that do not destroy foreign pathogens. They destroy those cells of the human body that are infected with the pathogens. They therefore destroy the virus infected cells and the tumor cells. These natural killer cells are cytotoxic lymphocytes. Lymphocytes can be small or large. The lymphocyte strength is 1500 to 2700 per cubic mm of blood and the count varies from 1500 to 2700 per cubic mm.

The Adaptive Immune System

In response to invasion by a particular antigen/pathogen, a particular and specific antibody is created in the human body. This particular antibody destroys the specific antigen. The particular characteristic of adaptive immune system is that it remembers that specific antibody and whenever that antigen invades again, it has that particular antibody ready as an immunological response. The adaptive immune system has memory cells for the purpose. The adaptive immune system also has a stronger response for the pathogens than the innate immune system.

The adaptive immune system consists of specialized leukocytes called lymphocytes. The 2 special types of lymphocytes are B cells and T cells. These cells are produced in the haemopoietic stem cells of the red bone marrow. Lymphocytes can be small or large. The lymphocyte strength is 1500 to 2700 per cubic mm of blood and the count varies from 1500 to 2700 per cubic mm.

Development of lymphocytes

Thymus is the main source of production of lymphocytes though they are produced in the spleen and red bone marrow also. The reticulum cells of the germinal center (found in the central part of lymph node) multiply and form the lymphoblasts. The lymphoblast has a diameter of 15 to 20µm. They have a non granular cytoplasm and a round or oval nucleus. The lymphoblasts proliferate to form the large lymphocyte. These cells do not mature further and are found in circulation. Those which mature form the small lymphocytes.

T cells and B cells

The 2 specialized lymphocytes of the Adaptive Immune System are T cells and B cells.

T cells

 The T cells recognize the pathogen material once it has been processed and presented by dendritic cells. The T cells have receptors for recognizing the pathogen material. The antigen/pathogen material is combined with a receptor and the combination results in the formation of major histocompatibility complex or MHC molecule. The T cells are of 3 types viz. the Killer T cells, the Helper T cells and the Suppressor T cells.

Killer T cells

These cells recognize and destroy the class 1 MHC.  All cells of body can from class 1 MHC molecules. MHC is nucleated pathogens combined with cell receptors.  In the class 1 MHC are categorized some bacteria like reickettsia, mycoplama, bacterial l forms and viruses. The killer T cells are antigen specific like B cells. The T cell receptor binds to the MHC class 1 molecule. There is also another co-receptor on the killer T cells, called CD8 which helps in identification of the specific antigen MHC1 complex. After contacting with the MHC1-antigen complex, the Killer T cells secrete cytoxins such as perforin and granulysin.  These cytoxins form pores in the pathogens cell wall and pass toxins through the pore. This causes apoptosis or programmed cell death.

Helper T cells

They recognize MHC class 2 molecules and consist of cells of the immune system. Pathogen cells presented by B cells, dendritic cells, professional antigen presenting cells (APC’s) and macrophages come under this classification. The helper T cells regulate the immune response in both innate and adaptive immune system. The helper T cells do not have cytoxic activity and hence do not kill pathogens directly. They direct other immune cells of the body for killing the pathogens. The helper T cell receptors recognize the antigen MHC class 2 molecules. Another receptor on helper T cells that helps in recognition of MHC 2 molecules is CD4. The helper T cell then release cytokins that signal the other immune cells like killer T cells and macrophages to act on the pathogen and destroy them.

Suppressor T cells – They modulate immune response.

B cells

The B cells are a class of lymphocytes that have antibody on their cell surface. These cells are antigen specific and do not require any processing of the antigen material by any cells for their recognition. They recognize the whole pathogen on their own and destroy them subsequently.

The antibody of the B cells attaches to the antigen. The antigen antibody complex is then processed by the B cells by proteolysis. Proteolysis leads to formation of peptides. These antigenic peptides are then displayed on the cell walls of the B cell in the form of MHC class 2 molecules. The helper T cells recognize these MHC class 2 molecules and release lymphokines. The lymphokines signal other B cells to form more antibodies, which subsequently leads to the destruction of other antigens.

Humoral and Cell mediated immunity

A foreign pathogen invasion in the human body can occur in 2 ways. The pathogens/antigens like foreign cells, bacteria, fungus, parasites, other foreign molecules and viruses can freely circulate in the body or the antigen may be broken down into MHC. MHC’s are mostly formed of exogenous antigens, tumor infected cells and virus cells.

For freely roaming antigens, the body creates the humoral response. Humoral response is an antibody mediated response. In humoral response, B cells are activated which directly engulf the antigens and destroy them.

The cell mediated immune response of the bodyworks by the activation of phagocytes, T lymphocytes, and the release of cytokines in response to an antigen which is the MHC molecule and not freely roaming pathogen. Therefore cell mediated does not involve antibodies.

Human blood composition and clotting mechanism

Blood is a highly complex fluid that has two parts viz. the cellular part (WBC’s, RBC’s etc.)  and the intercellular part (plasma). The cells are submerged in plasma. The collective name for all blood cells is blood corpuscles. The cellular content of blood is 45% while the plasma content is around 55%. When we say whole blood, we refer to the cellular part of the blood and exclude plasma. We next see the proportions of all the constituents of whole blood.

Constituents of cellular content of blood or whole blood -

RBC’s or red blood corpuscles- these constitute around 45 % of whole blood. WBC’s or white blood cells- these constitute around 1% of the total whole blood volume. Platelets-these constitute around 0.7% o the whole blood.

Constituents of plasma

Water – the water content of plasma is 91 to 92%. Inorganic and organic constituents of blood (solids) - they constitute around 8 to 9 % of blood plasma.Inorganic constituents- they constitute around 0.9% of the solid content of blood plasma. The inorganic constituents of plasma are iron, copper, sodium, potassium, calcium, magnesium etc. Organic constituents- protein and non protein nitrogenous substances form the organic constituents of the solid content of blood plasma.

Proteins like serum globulin, serum albumin, prothrombin, fibrogen form around 7.5% of organic constituents.NPN or non protein nitrogenous substances like urea, uric acid, creatinine, creatine, ammonia, amino acids etc, fats like phospholipids, cholesterol etc, carbohydrates, other substances substances like antibodies, enzymes and coloring matter like bilirubin, carotene etc form the rest of the organic constituents of the solid part of blood plasma.

Haematocrit value of Blood

The plasma content of whole blood is more than the cellular content. Thus, plasma strength varies from 52 to 55% while the cellular strength varies from 45 to 48%. Males have more cellular content than females. Males have around 45% cellular content while females have 40% of it. The ratio of red blood cell or RBC content of whole blood to the plasma content is expressed as haematocrit value and is measured by an instrument called haematocrit.

Specific gravity of blood

The specific gravity of whole blood varies from 1.05to 1.06 at 15 degree Celsius. The average is 1.057 in males and 1.053 in females. Specific gravity of blood rises when water loss occurs from the body, when new fluid flows in serous cavities because of surgical operation or when inflammation occurs in tissues and cells or when there is low water intake. Specific gravity of blood falls when ever haemorrhage occurs, when there is large intake of water or when saline solution is injected into blood.

Origin of the plasma protein content of the blood

The mesenchymal cells produce plasma proteins in the embryo. In adults, liver produces all the major plasma proteins like fibrogen, albumin, prothrombin etc. Albumin is also formed by the reticulo-endothelial cells, by disintegrated red cells and from lymphoid nodules.

Functions of plasma proteins

Plasma proteins serve very important for some functions of the       body. The functions are listed below.They regulate the osmotic pressure of the blood and also regulate the fluid distribution between the cells and blood.They are required for clotting of blood. In the absence of plasma proteins, no blood coagulation will take place and total haemorrhage can occur.They maintain the blood pressure.They maintain the viscosity of bloodThey act as buffers in maintaining the acid base balance of the blood.They are the protein reserve depots of the body.They aid the process of respiration as they form carbamino proteins which carry carbon-di-oxide.They contain antibodies (gamma globulin) in nature which strengthen the immune system of the body.They transport enzymes and hormones, iron, copper etc from one place to another.

Viscosity of blood

Human blood is 5 times more viscous than the pure distilled water. Whole blood or the cellular part of blood is viscous because of the cellular content while plasma is viscous because of plasma proteins. The relative viscosity of plasma is 1.8 and that of whole blood is 4.7. Some pathological conditions like hypercalcemia, hyperglycaemia, diabetes mellitus etc. increase the viscosity of blood. Blood viscosity gets reduced during exercise, fever, malaria and lymphatic leukemia.

Coagulation of blood and its mechanism

When the blood comes out of the body, it loses its fluidity in a few minutes and turns into a semi solid jelly form. This phenomenon is termed as blood clotting or blood coagulation. When left outside for some more time, a straw colored fluid is secreted from the retracting clot which is called serum. The serum does not further coagulate. The RBC’s and WBC’s do not take part in blood coagulation though platelets take some part in it. Blood coagulate by actions of plasma. The RBC and WBC get trapped in the clot meshes and are subsequently removed.  Blood coagulation is a very important property of blood as in absence of it haemorrhage occurs.

The process of blood coagulation takes place with the action of platelets. When blood is shed from the body through any means, the blood platelets get shed on water wettable rough surfaces. The platelets disintegrate and release thromboplastin. The tissues of the damaged area also release the substance. This thromboplastin converts the prothrombin content of the blood into thrombin. Calcium ions aid thromboplastin in the process. The thrombin interacts with fibrogen to form fibrin. The blood clot is thus formed with fibrin formation.

There are 13 factors that are responsible for coagulation of blood.

These are: Fibrogen or Factor 1- this factor is globulin in nature but is a lot bigger than the serum globulin. Its molecular weight is 330000.It converts to fibrin during the process of clotting. Prothrombin or factor 2- it is contained in normal plasma and is protein nature. It has a molecular weight of 62,700. Vitamin K is essential for formation of prothrombin in liver. Thromboplastin or factor 3- it is contained in tissues as well as in blood plasma. Prothrombin is converted into thrombin during blood clot formation.  Thromboplastin and calcium ions aid the process of conversion.

Blood only clots on rough surfaces. That is why blood does not clot inside the body. But if the surface of blood vessels becomes rough because of any reason, body will coagulate inside the body.

1.Calcium or factor 4- calcium acts as a cofactor for blood coagulation.

2.Labile factor or factor 5-this protein is necessary for total conversion of prothrombin into thrombin.

3.Accelerin or factor 6- it is formed form proaccelerin. 

4.Stable factor or proconvertin or factor7- the factor is present in blood plasma. Antihaemophilic factor(AHF) or platelet cofactor1 or factor8- it is contained in blood plasma but disappears with the formation of clot. It aids the formation of intrinsic thromboplastin and also helps in the conversion of prothrombin into thrombin.

5.Christmas factor or platelet cofactor 2 or factor9- this cofactor is essential for formation of internal thromboplastin. 

6.Stuart factor or factor 10- it is similar to factor 8. Plasma thromboplastin antecedent or (PTA) or factor1-  the Hageman factor activates it. PTA helps in forming thrombin. Hageman factor or factor 12- this protein is activated only when blood gets in contact with a rough surface. With its activation the protein splitting enzyme kallikrein is activated which results in the formation of linins in plasma. Kinins cause dilation of blood vessels and increase their vascular permeability.

7.Laki-lorand factor (LLF) or factor 13- this factor, along with calcium, converts the soft fibrin clot to a fibrous and solid form.

Human Circulatory System

The human circulatory system comprises both cardiovascular system and  lymphatic system. The chief constituent of human cardio vascular circulatory system are heart, blood vessels and blood which carries oxygen, metabolic wastes, nutrients,hormones to all parts of the body. 

Cardio vascular system

The cardiovascular system is a part of human circulatory system. Its main function is to provide the necessary materials to all the cells of the body that are required for their growth, maintenance and repair. The cardiovascular system also removes the metabolic wastes away from the cells and tissues. The fluids and materials of cardio vascular system flow through pressure gradients. The heart acts as the central pump for the flow of fluids. Apart from heart, the cardio vascular system also includes other parts such as arteries, capillaries and veins. Circulation of blood is the main function of all the parts of cardio vascular system.
We can divide the cardiovascular circulatory system in 2 parts. These are:

The systemic circulation-it passes through all the tissues of the body.
The pulmonary circulation- it passes through the lungs.
The circulatory system also helps the body to maintain blood pressure levels.

Arteries

The 2 outer layers of arteries are very thick. The modification is because the arteries have to withstand high amounts of blood pressure. The three layers of human arteries are tunica adventitia or the outer layer, the tunica media or the middle layers and the tunica intima or the inner most layer.
The tunica adventitia is formed of white fibrous connective tissues. It runs parallel to the blood vessels and has an elastic membrane that is located very close to the tunica media. The outer covering of tunica adventitia is non elastic. The non elastic membrane prevents the layer to stretch and thereby prevents its rupture or distension. The layer has vasa vasorum or special kind of vessels that supply blood to these layers.
The tunica media is composed of smooth muscle cells that are arranged in circulation.
The tunica intima has a single layer of endothelium. This endothelium layer rests upon internal elastic membrane which is made up of elastic tissues. The internal elastic membrane has folds which prevent injury to the endothelial cells that may occur because of pulsation. The inner layer of the tunica intima has endothelial lining consisting of very delicate fibro elastic areolar connective tissue.

Arteriole

the smallest branch of artery is called arteriole. The walls of arterioles are thicker than the arteries and have narrow lamina. These vessels have adequate vasomotor innervations. They distribute blood by vasoconstriction and vasodilation. The lumen of arterioles is modified so that the pressure and flow of the vascular systems can be maintained to optimum level.
Arterioles also have 3 layers. The tunica intima is composed of an inner elastic membrane and endothelial cell lining. The tunica media has elastic fibrils and muscle cells. The loose connective tissues form tunica adventitia.

Capillaries

Capillaries connect the arteries to veins. Flat endothelial cells link the capillaries and form their walls. The basal lamina separates the capillaries from the supportive bed of connective tissues. The endothelial cells of the capillaries have an elongated or ovoid nucleus. These endothelial cells are stretched and taper at their ends. Reticular and collage nous fibers surround the capillaries. Capillaries are also surrounded by fibroblasts, macrophages, mesenchymal cells and other cells of neighboring nerve fibers. There are also rouget cells that have long branching processes which surround the capillaries.

       The diameter of human capillaries is around 7 to 9 µ. The capillaries can be grouped into 2 types. These are the muscular types and the fenestrated type. The muscular type of capillaries are found in lung, smooth muscles, cardiac muscles, central nervous system and other tissues. They consist of endothelial cell layers that lie uninterrupted and are of equal thickness. Slight budges are present that are formed because of cell nuclei.

    The fenestrated type capillaries are found in endocrine glands, renal glomerulus, intestinal mucosa and in other organs. Here the endothelial lining is not continuous and ahs numerous pores that have length of 300 to 500 angstrom.Electron microscopic studies have revealed that all capillaries are composed of small vesicles that lie on the basal and luminal surfaces of endothelial cells. These vesicles are called as pinocytotic vesicles. The pinocytotic vesicles help in transporting the fluid across cell membranes.

Veins

Veins have all three layers of arteries but they differ in size. The media and intima layers of the veins are thinner because they have less elastic and muscular components. Still the veins are very strong as they have connective tissue components. The endothelial cells of tunica intima are less elongated in veins. The tunica media of veins have more amounts of collagen fibers and lesser mounts of muscle and elastic tissue. The tunica adventitia of veins if highly developed and has elastic, collagen and muscle fibers, the cerebral veins, retinal veins, meningital veins and some other veins do not have smooth muscles.

Valve of veins- Almost all veins have valves especially the lower limb veins. These valves prevent backflow of blood. The intima folds to form semi lunar pockets which act as flaps or valves.

Blood

Blood is a specialized connective tissue. When fresh, blood is red, opaque, thick and slightly alkaline. The cells that are found in blood include red blood cells, white blood cells, other forms elements and the platelets. These cells are suspended in a liquid intercellular substance that is called as plasma. Approximate specific gravity of whole blood is 1.060.   Blood assumes around 7% of the adult human weight which is around 5 liters. 

Compositon of blood

Functions of blood

Higher forms of living organisms have more specialized cells. Thus, in addition to supply and excretion, blood also carries out many other functions. Some of the important functions of blood are listed below.

1. Transport of nutrition- Blood carries the digested nutritive substances for the intestines to other parts of the body. According to need, it also transfers these substances inside the body from one place to another. For example, the nutritive substances are transferred from the food storage depots inside the body to specific tissues where they are required by blood.
2. Transport of gases (oxygen and carbon-di-oxide) - blood carries oxygen from the lungs to all the tissues of the body and takes back carbon-di-oxide to the lungs for its removal from the body.
3. Transport of other essential elements- Blood carries vitamins, minerals, hormones, enzymes and other essential substances inside the body to the places they are required within the body.
4. Regulation of the temperature of the body- blood has abundant quantity of water which due to its specific heat helps in regulation of body temperature. Blood distributes the body heat to all the body parts through conduction. Excessive heat is also dissipated in the external atmosphere through the blood water evaporation from skin and from lungs.
5. Drainage of body wastes- The waste products of the body (waste generated during metabolic activities) are carried by the blood to their respective organs of secretion (for example kidney and lungs).
6. Coagulation- Blood has a property to coagulate. This property helps to prevent excessive loss of blood (haemorrhage)
7. Regulation of blood pressure- Blood regulates blood pressure by changing its viscosity and volume. It is termed as the haematocrit value of blood. 
8. A key pillar of the immune/defensive system of the body- In response to a foreign toxic agent the blood develops antibodies. Blood also contains WBC’s or white blood cells which engulf the pathogens and destroy them.
9. Maintains ion balance- Blood maintains ion balance of the body which is necessary for proper functioning of all the cells of the body.
10. Maintains water balance in the body.

Blood cells

Blood cells are classified into three types- erythrocytes, leucocytes and platelets.

Red blood corpuscles or erythrocytes

The adult human erythrocytes are circular, non nucleated and biconcave discs.These are most numerous cells in the blood.Immature RBC have all cell organelles. After maturation, organelles like nucleus, mitochondria, lysosomes, Golgi bodies,endoplasmic reticulum, ribosomes etc disappear.In some mammals like camel, Lama RBC have nucleus.In adulsts RBC are formed only in the marrow of long bones while in embryonic stages, they are formed in the liver and spleen. RBC have life span of 120 days. Old Red blood cells are destroyed mostly in spleen and to some extent in liver. Therefore spleen is called "Grave yard of Red blood cells".

 They are composed of proteins and lipids which form a meshy framework. Haemoglobin fills these meshes. They have a red color but under a microscope they show yellow or light brown color. A delicate outer membrane envelops the RBC’s. The constituents of this membrane are phosphatides, proteins and cholesterol. The middle RBC layer is composed of lipids while the inner and outer layers are made of proteins. The membrane of RBC is highly selective and does not give passage to cations like sodium and potassium and to bigger molecules. Anions like chlorine and crystalloids like urea can freely pass through.

Composition of RBC

Each cell is composed of an envelope that has no color.  Inside the envelope there is 65% water, 35% solids and a semi-liquid material. The solid content is composed of 33% haemoglobin which is bound to a stromal network. The stromal network consists of cholesterol, phospholipid, protein, neutral fact and cholesterol ester. There are organic constituents inside the RBC like urea, creatinine, amino acids, adenyl pyrophosphate etc. These are present in traceable quantities. The lipid content of RBC is composed of 60% phospholipid, 30% cholesterol and 10 % cholesterol esters and fats. It also contains salts like potassium phosphate etc. The normal average RBC count in an adult male is 5.4 million and in adult female is 4.5 million per cubic millimeter of blood.

Fate of RBC

The average life period of a mature RBC is 120 days after which it is engulfed by the phagocytic cells of the red bone marrow, liver and spleen. Its constituents are excreted or absorbed for further processing. The old and senile RBC’s become brittle. They become flask shaped and throw out poikilocytes. The RBC disintegrates by gradual breakage of these poikilocytes away from the cell. The reticulo endothelial cells absorb the broken fragments of RBC’s. The reticulo endothelial cells of spleen, liver etc engulf and digest the old RBC’s. Haemoglobin is released in the intracellular breakdown of RBC’s. The haemoglobin degrades to choleglobin. It is next decomposed to haem and protein. The protein content forms amino acid while the haem content gets stored in the body in the form of haemosiderin and ferritin. These constituents allow for the formation of new haemoglobin. The remaining part of haem molecule gets converted into bilirubin and biliverdin. Biliverdin and bilirubin circulate in the blood stream by combining with plasma alpha 1 globulin. Once they enter the liver, they decompose into monobilirubin and dibilirubin glucuronide. After further processing, some part of materials is absorbed while the rest is excreted.

Functions of RBC’s

1. To carry oxygen and carbon-di-oxide inside the body.
2. To maintain ion balance of the body
3. To maintain viscosity of blood
4. To provide new material like bilirubin, biliverdin etc after degradation.

Haemoglobin

The red pigment found in blood is called haemoglobin. Haemoglobin is a kind of chromoprotein and has two parts. The first part which forms 96% of haemoglobin is called globin or histone. The other part that forms 4% of the haemoglobin is a prosthetic group which contains iron. It is called haem. Haem consists of 4 pyrrole groups joined together to form a protoporphyrin compound. Hame can also be said to be metalloporphyrin compound where the metal is iron. Iron forms about 0.34% of hemoglobin. Around 3 gm of iron is present is adult human blood. Iron is in ferrous form. Globin aids haem so that it can contain the iron in ferrous state. It also helps the iron ion to combine with oxygen for its transportation inside the body. The molecular weight of haemoglobin is 68,000. Each haemoglobin molecule contains 8 atoms of sulphur and 4 atoms of iron.

White blood cells/corpuscles or leucocytes

Leucocytes are a type of blood cells. They are different from erythrocytes. They have abundant nucleoprotein. They also contain ascorbic acid, cholesterol, glycogen, lipids and many enzymes. There is lot of variation in WBC count in a normal human body and the cell count may vary from hour to hour as well. The normal WBC range is between 4000 to 11000 cells per cubic mm of blood. The WBC count is thus less than the RBC count. The ratio of WBC to RBC is 1:700.

Classification of WBC’s or leucocytes.

WBC are two types namely Granulocytes and Agranulocytes.

Granulocytes

These cells have granular cytoplasm. They are formed in the red bone marrow after birth. They are of 3 types viz Neutrophils, Eosionophils  and Basophils. 

a. Neutrophil-These cells can be stained with neutral dyes hence they are called so. Their count varies from 3000 to 6000 per cubic mm of blood. They are around 10 to 12 µm in diameter. The number of nucleus lobes can be more than 7 in a neutrophil. They are also called polymorphs as they have multi-lobed nucleus. Generally 3 to 4 lobed nucleus cells are found. With maturity the number of lobes increases. The cytoplasms of these cells have neutrophilic granules. These cells are amoeboid and phagocytic in nature. The enzymes found in these cells are lipase, protease, nucleotidase, phosphates etc.  Also lutathione, glycogen, ascorbic acid etc are found. The enzyme and other content of the neutrophils help them in their phagocytic activity.  

b. Eosinophil-These cells can be stained with acid dye called eosin hence they are called Eosinophils or  acidophils. They have 2 or 3 lobed nucleus. Their strength is 1 to 4% and count varies from 150 to 400 per cubic mm of blood. They have a diameter ranging from 10 to 12 µm. The cytoplasm has course granules and the nucleus has 2 lobes. The eosinophils do not show any phagocytic activity. They are amoeboid though. They contain histamine.  

c. Basophil- These cells can be stained with basic dyes- therefore they are called basophils.The nucleus is lobed and the diameter of the cells varies from 8 to 10µm. These cells are phagocytic in nature and play vital role in healing process.

Agranulocytes

These cells do not have any granular material in their cytoplasm.The nucleus in these cells is large.There are two types namely Lymphoctes and Monocytes.  

a. Lymphocytes- they can be small or large. The lymphocyte strength is 1500 to 2700 per cubic mm of blood and the count varies from 1500 to 2700 per cubic mm.  

b. Monocytes- they vary from 5-10% in strength. Their count varies from 350 to 800 per cu. mm. They have larger diameter which varies from 16 to 18 µm. The young nucleus is round or oval. The older cells have convoluted nucleus that can be kidney or horse shaped. The cells have non granular cytoplasm and an eccentric nucleus. 

Functions of white blood cells or leucocytes

1. Phagocytosis- the monocytes and the neutrophils engulf foreign bacteria and particles and destroy them.
2. Antibody formation- the lymphocytes play a vital role in the manufacture of beta and gamma fractions of the protein globulin which act as antibodies.
3. Fibroblast formation – the lymphocytes are converted to fibroblast in areas where inflammation occurs. They thus help in cell and tissue repair.
4. Heparin secretion- basophils secrete heparin that prevents clotting of blood inside the blood vessels.
5. Trephone manufacture- trephons are plasma proteins manufactured by leucocytes. Trephones aid growth, nutrition and repair of tissues and cells.
6. Antihistamine function-histamines are found in eosinophils.  They relive the body in allergic reactions.

Blood platelets

The platelets have no nucleus. They are round or oval in shape and are covered by a membrane. The average size of a platelet cell is 2.5µm. When seen through microscope, they appear in clusters. The cell has 2 parts viz. the hyalomere and the chromatomere.

Hyalomere- The hyalomere consists of homogenous fine granular material. It contains microfilaments and microtubules. Thrombosthenin is contained in the microfilament. The pigment is capable of contraction like the actin and myosin found in muscles.

Chromatomere- The chromatomere consists of alpha granules (oval granules with dia.0.2µm and length 0.3µm. The alpha granules have lysosomal function that is important during platelet aggregation, in clot resolution and in phagocytic activity. They also contain mitochondria, sydersomes (ferritin containing vesicles) and very dense granules (contain 5 hydroxytryptamine). Apart from these, glycogen granules, ribosome, tubules and vesicles are also present.

Platelets contain protein and phospholipids (mostly cephalin). The strength of platelets varies from 250000 to 450000 per cubic mm of blood.

Functions of blood platelets

1. To initiate blood clotting.
2. To repair capillary endothelium
3. To prevent haemorrhage.
4. Fasten clot retraction

Formation of blood cells

Blood cells are formed in the bone marrow. The other name of bone marrow is myeloid tissue. In later post natal life, bone marrow only consists of the marrows or inner cavities of bone. The cancellous spaces and the medulla cavities of the bone contain the cellulovascular bone marrow tissue. Red bone marrow is the active bone marrow and it only manufactures new blood cells. The yellow bone marrow or the inactive bone marrow does not produce blood cells. As age progresses more and more red marrow is converted in yellow marrow. The yellow marrow does changes into red marrow when there is urgent need of new blood cells. The red marrow constitutes around 3 to 6 percent of total body weight in an adult.

Haematopoietic function of bone marrow

The red bone marrow can form the red blood cells and also other blood cells. It produces myeloid elements for the formation of WBC’s. The red bone marrow thus forms all of the blood cells including RBC’s or erythrocytes, WBC’s or leucocytes (includes monocytes, lymphocytes, granulocytes) and platelets. The bone marrow also destroys the aged, damaged and defective RBC’s. The macrophages of bone marrow digest these imperfect RBC’s.