Human endocrine system

The endocrine system is composed of ductless glands that are also called the endocrine glands. The endocrine glands secrete substances (hormones) that are necessary for running the key processes of the body.These hormones play the role of chemical messengers, which help in coordinating the metabolic activities of different organs.Without these hormones, growth may not occur normally, We may not become sexually and physically mature and our body may not be able to carry out several functions like metabolism, excretion, regulation of blood glucose concentration etc. The endocrine hormones are secreted from endocrine glands. The endocrine hormones are carried by the blood and plasma to distant organs inside the human body. The hormones bring about some effect on a distant organ. The effect is necessary for the functioning of the organ. Thus, the hormones of the endocrine system are essential for proper functioning of many organs of the body.  They also maintain the growth of tissues of the body. They guide and control the differentiation, multiplication and metabolic activities of tissues.

 It is because of the hormones that are secreted by the endocrine glands that the correct physiological balance of the body is maintained. They also maintain the composition of the fluids of the body. They have numerous other metabolic, physiologic and other functions.Hormones are important for the functioning of the body and have a distinct and very important role to play, from the development of the fetus the moment of death.Endocrine hormones are also called as information transferring molecule which transfer information form one set of cell to distantly located another group of cells.Thus, the body has the endocrine system for slow and sustained functions. The fast functions are executed by the nervous system. The interrelation of endocrine (hormonal) system and nervous system plays a key role maintaining the body homeostasis. The two systems are interrelated and also correlated at the hypothalamic level.The organ that is targeted by the hormone is called the target organ. A trophic hormone is that hormone that targets an endocrine organ itself. The anterior pituitary secrets all the trophic hormones.

Characteristics of hormones

1. They act in very low concentrations.
2. Hormones are only stored in their gland of origin.
3. Hormones are destroyed and excreted as soon as their functions are over.
4. Hormones influence/govern the processes executed by the organs but do not initiate them.
5. Hormones also exert dual control on certain processes. Some hormones help the conduction of a process while others inhibit it (for example during regulation of blood sugar).
6. In their structures, hormones are glycoprotein, amino acid derivatives (steroids), proteins or peptides.
7. All the hormones are interrelated, interdependent and live like a harmonious body.
8. Hormones have interrelations with vitamins.
9. The hormones are easily soluble, have low molecular weight and are easily diffusible.

The major endocrine glands are as follows:

1. Pituitary
2. Thyroid
3. Parathyroid
4. Adrenals- cortex and medulla
5. Pancreas
6. Ovary
7. Testes
8. The pineal body
9. Thymus

Pituitary gland

The pituitary gland has a weight of 0.5-0.6 Gms in adults .It is a pea shaped gland.  It is located in the bottom of the brain (in the hypothalamus region). It is reddish grey in color. More specifically, it is located in the sella turnica section of the sphenoid bone. The stalk attaches it to the floor of the 3rd ventricle. The gland is larger in females by about .1gm and enlarges during pregnancy. Its dimensions are:

1. 10 mm anterioposteriorly.
2. 6 mm dorsoventrally, and
3. 13 mm laterally.

Anatomy of pituitary
Anatomically, the gland consists of 2 parts;
1. Anterior lobe, and
2. Posterior lobe
If we study the pituitary gland through microscope, it consists of 6 distinct parts. They are –

1. Pars interior or pars distalis
2. Pars intermedia
3. Pars tuberalis
4. Pars posterior/pars nervosa
5. Tuber cinereum, and
6. Pituitary stalk or infundibulum.

Development of pituitary gland is from two outgrowths

1. The Rathke’s pouch or the glandular diverticulum-it grows (upwards) from the ectoderm and meets a similar neural growth ( downwards) that protrudes from the floor of the 3rd ventricle. Out of the two, only the down growth remains to form the pituitary stalk or infundibulum. Rathke’s pouch gives rise to the pars distalis, pars tuberalis and pars intermedia. The anterior lobe of the pituitary gland is formed of the pars distalis and pars tuberalis. The posterior lobe consists of pars intermedia and infundibulum.
2. Blood supply to the pituitary-the superior hypophyseal arteries provide blood supply to the anterior lobe of the pituitary gland.
3. The anterior lobe is controlled through the nervous system also.

Functions of pituitary gland- the pituitary gland is also called the master gland of the body because the trophic hormones secreted by it influence the activities of other endocrine glands. The hormones secreted by the pituitary are as follows.

A.Somatotrophic hormone or growth hormone-this hormone is a protein having molecular weight of 21000 to 48000. It has a single straight chain polypeptide structure with 2 intramolecular sulphide bridges. It is composed of 188 amino acids. The hormone promotes skeletal growth, regulates body growth in general, and has metabolic effects as well.  It aids in the metabolism of minerals, synthesis of proteins etc. The regulation of the secretion of growth hormone is done by the hypothalamic hormone

B.Thyroid stimulating hormone (TSH) or thyrotropic hormone- this hormone is secreted by the basophil cells of the pituitary. It is glycoprotein with a molecular weight of 25,000. It controls the activity and growth of the thyroid gland. The hormone is necessary for formation of thyroxine in the thyroid gland. The secretion of thyrotropic hormone is governed by the amount of thyroxine in the blood and by the hypothalamus.

C.Gonadotrophic hormone (GTH) - In this category the hormones included are luteinizing hormone, follicle stimulating hormone, and prolactin hormone. The basophil cells of the pituitary secrete gonadotropins. The GTH controls the growth of sex organs and secondary sexual characters. In females, the ACTH controls important processes such as lactation, pregnancy and menstrual cycle.The luteinizing hormone and the follicle stimulating hormone are amino acids with a molecular weight of around 15000.The function of the FSH is to maturate the Graafian follicles and then prepare the follicles to ovulate. In males the FSH stimulates spermatogenesis.The function of LH in females -is to completely develop the ovarian follicles for secretion. The LH also causes secretion of oestrogen. The LH causes growth of corpus luteum and stimulates the production of testosterone, androgens etc.The ovulation in females occurs because of the combined action of LH and FSH.The function of LH in males is to develop and give functionality to Leydig cells. The Leydig cells produce androgen, the primary sexual hormone responsible for male sexual activity.

D.Adrenocorticotropic (ACTH) hormone- The polypeptide chain of TSH consists of 39 amino acids. The molecular weight is 4500. The ACTH hormone controls the growth of the adrenal cortex and regulates the synthesis of cortisol. Therefore ACTH is essential for life. The secretion of ACTH is controlled by the hypothalamus and other nerve structures. Other mechanism such as stress control and steroid feedback also exert influence on ACTH secretion.Secretion of gonadotrophin is under the control of hypothalamus and the sex hormones.

E. Prolactin hormone- The hormone is only secreted during pregnancy by the acidophil cells. The hormone prolactin is a peptide and is composed of amino acids tyrosine, cystine methionine and arginine. It also contains tryptophan and sulphur. The molecular weight is 25000. It has around 180-190 amino acids and has several disulphide bridges. The breasts of women are prepared by the oestrogen and progesterone, but the lactation occurs only because of prolactin. The prolactin hormone also completes the development of breasts. The hormone also helps in maintaining the secretion of progesterone.Other gonadotrophins and the sex hormones control the secretion of prolactin. The nervous system also regulates its secretion.

G. Vasopressin- The hormone is secreted by the posterior pituitary. It is an octapeptide  is a peptide hormone and is also called anti-diuretic hormone as it helps in retention of water inside the human body. The hormone is released in the blood when the body gets dehydrated. From the posterior pituitary the hormone reaches the distal convoluted tubule of the nephrons and makes them more permeable to water.H. Oxytocin- The hormone is produced in the hypothalamus and stored in the posterior pituitary gland. Oxytocin is an octapeptide with a molecular weight of 1000.The hormone helps in the regulation of neurons and is a neuromodulator. The hormone also helps in contraction of womb in women during labor.

Pituitary hormones  and site of their action

Hormone	Site of action	Function
ACTH	Adrenal  gland	Stimulates the adrenal gland to produce a hormone called cortisol. It is also known as corticotrophin.
TSH	Thyroid	Stimulates growth of thyroid gland, synthesis and secretion of thyroid hormone TSH is also known as thyrotrophin.
LH & FSH	Ovaries Testes	LH promotes secretion of oestrogen and promotes ovulation. FSH promotes growth and promotes secretion of oestrogen in females and testosterone in males
PRL (Prolactin)	Breasts	Stimulates milk flow after birth.
GH	All cells in the body	Responsible for growth and repair of tissues
ADH	Kidneys	Controls the blood fluid and mineral levels in the body by affecting water retention by the kidneys. This hormone is also known vasopressin.
Oxytocin	Uterus Breasts	Responsible for contraction of uterus during child birth and stimulates ejection of milk

Thyroid Gland

The location of the thyroid gland is at the root of throat. It has two lateral lobes (symmetrical). The dimension of each of the lobe is 5 X 2 X 2 cm. The lobes are located on either side of the trachea. The lobes are joined together by a thin layer of thyroid tissue also called as isthmus. The isthmus is also called the middle lobe. The left lobe is slightly smaller than the right lobe. From the isthmus a pyramidal lobe extends upwards. The thyroid gland is attached to the pretracheal fascia and extends upwards during swallowing. As with other endocrine glands, the thyroid has no duct. The weight of thyroid gland is 20-25 gm. The blood flows into the thyroid gland at the rate of 3.5 to 6ml per gram of thyroid weight per minute. The blood flow is crucial for the supply of iodine to the thyroid.

The thyroid develops from the foregut and in front of the thyroid cartilage. Here, the thyroid gland is formed from the fourth pharyngeal pouch. Some other elements combine to form the lateral lobes of the thyroid. If we study the thyroid gland under a microscope, we will find that it consists of many follicles or vesicles lined in a slate on the cuboidal epithelium. The thyroid follicles are 15 to 150 µm in diameter. They are made up of cuboidal cells. The vesicles or follicles consist of a protein called thyroglobulin.

Thyroid gland hormones

The thyroid produces three hormones. They are 1. Thyroxin, 2.Calcitonin and 3.Iodo Thyronine. The hormones secreted by the thyroid are important for the growth and other metabolic processes. Thyroxine and iodo thyronine are stored in the thyroid follicle. Calcitonin is secreted form the C-cells of the thyroid gland.

Proper amount of iodine is necessary for the functioning of thyroid gland. A normal adult’s iodine intake per day varies from 100 to 150 µg. This iodine gets broken into the GI tract as iodide, which circulates in the blood and is absorbed and accumulated by the thyroid gland. The process of iodide accumulation by the thyroid gland is known as trapping. By oxidation of the thyroid, free iodine or unstable iodine ion is produced in the thyroid gland. Next, the free iodine attaches to the tyrosine residues by a process called organification.

The hormones produced by the thyroid are transferred to distant organs where they influence the processes of growth and metabolism. The thyroid hormones are important for the energy producing process of respiration. The thyroid hormones also influence oxygen uptake by the cells. They have an anabolic effect on protein, lipid, calcium and phosphorus metabolism. The thyroid hormones are essential for mental growth.

Disease of thyroid gland (Goiter)

It is the non inflammatory enlargement of the thyroid gland. Goiters are following types

1. Colloid goiter- occurs because of iodine deficiency.
2. Diffuse parencymatous goiter- occurs because of hypertrophy of alveolar epithelial cells.
3. Nodular goiter- occurs because of nodular swelling.
4. Toxic goiter-occurs because of excessive secretion of thyroid hormones.

Hyperthyroidism is also a disease of the thyroid gland and is associated with excess secretion of thyroid hormones.

Mode of action/working of thyroid hormones – The thyroid hormones induce metabolism by activating citric acid cycle. They inhibit the phosphorylation of ATP and cause increased oxygen uptake by the cells (this also occurs in hyperthyroidism). The thyroid hormone stimulate the protein synthesis by the increasing the formation of ribosomal RNA.

The thyroid hormones and the gland itself are controlled by pituitary gland, CNS, hypothalamus, sympathetic and parasympathetic nervous system, stress and external temperature. They are also influenced by their relationship with gonadotrophins and adrenals.

Parathyroid Gland

It is formed by the combination of 4 small oval bodies with dimensions 6 x 3 x 2 mm. each of the 2 pairs are located vertically behind the posterior lobe of thyroid. Weight of parathyroid gland is 140 mg. Both the pairs are formed form the branchial pouch. The parathyroid gland is formed of epithelioid cells. These cells are of two types 1. Chief cells and 2 Oxyphil cells.

The parathyroid gland secrets Parathormone.

The parathyroid gland is vital for life. They regulate the amount of calcium ions in the body. When blood calcium gets lowered, the parathyroid hormone secretes parathormone which stimulates the osteoclasts that reabsorb the calcium ions and bone calcium to maintain the required concentration of calcium in the body. The parathyroid gland also regulates the amount of phosphate that gets excreted via urine.

The parathormone is a polypeptide hormone with a linear chain of 84 amino acids. The molecular weight of the hormone is 9500.

Working of parathyroid gland/parathormone- Adenyl cyclase stimulates the action of parathormone on bones and kidneys. The action of the hormone is dependent on the DNA and RNA. The secretion of parathormone is governed by a feedback mechanism which depends on the Ca+ concentration in the blood.

Diseases of parathyroid hormone

1. Tetany- Tetany is a neuromuscular hyperirritability and occurs when the parathyroid gland is accidentally damaged or removed. The calcium levels also fall and death may also occur.
2. Hyperparathyroidism- it occurs because of a tumor that gets formed in the parathyroid gland.

Pancreas

Pancreas are both endocrine and exocrine in nature. The exocrine cells of pancreas secrete lots of enzymes and are called pancreatic acinar cells.

The endocrine cells of the pancreas are called Islet of Langerhans. They constitute around 1 to 2% of the total pancreatic gland volume. They are three types of islet of langerhans cells

1. A2 cells or alpha cells- They secret glucagon. They are oxyphil, granular and large.
2. Beta cells- They secrete insulin. They are basophils and granular.
3. A1 cells- They secrete gastrin. They are membrane enclosed granules.

Insulin- The hormone insulin regulates the blood glucose. It is a protein hormone and is called as hypoglycemic antidiabetic factor. It deposits the excess glucose on the liver and muscles in the form of glycogen. Insulin also oxidizes glucose. The hormone is formed of 2 polypeptide chains A and B. The A chain has 21 amino acids and B chain has 30 amino acids. The molecular weight of the hormone chain is 5800. It is secreted as pro-insulin which is later converted to insulin in the granule package.

Functions of insulin include oxidation of sugar, formation of glycogen, prevention of formation of ketone bodies etc.

The secretion of the hormone is controlled by the anterior pituitary, the blood level of sugar, by thyroxine, by glucagon and to some extent by the adrenal cortex. Many other hormones and neural factors also have some influence on insulin secretion.

Diseases of pancreas

Diabetes Mellitus- It occurs because of decrease in insulin secretion. It can be juvenile (onset in childhood) or maturity (onset later in life).  In diabetes mellitus, hyperglycemia occurs because of unmetabolized sugar in the blood. This has ill effects on protein, carbohydrate and fat metabolism. It occurs when the beta cells of langerhans degenerate or do not function properly.

Adrenal glands

The adrenal glands are also called as suprarenal glands. In human body, they are 2 in number. One each is situated on the top of each kidney. They have a triangular shape. The dimensions of the adrenal gland are 50mm length, 30 to 40 mm breath and 10 mm height. The weight of 1 adrenal gland varies for 5 to 9 gm in adult. The outer part of adrenal gland is called adrenal cortex and the inner part is called adrenal medulla.

The adrenal cortex develops from the coelomic epithelium (mesodermal) from the mesentry. The cortex consists of two zones. They are the zona glomerulosa, the zona fasciculate and the zona reticularis

The adrenal cortex develops from the tissues which also give rise to sympathetic ganglia. Adrenal medulla is made up of chromaffin tissue

.

The adrenal glands are highly vascular and have a blood supply of 6 to 7 ml per minute per gm. The blood vessels enter them through their surfaces and form a vascular plexus inside the adrenal cortex. The efferent central vein passes out of the adrenal gland through the Hilus.

The adrenals secrete glucocorticosteroids, mineralocorticosteroids, the androgens and the steroids.

Secretions of adrenal cortex

Glucocorticosteroids (cortisone) - They are produced in the zona fasciculate region of the adrenal cortex. They stimulate the formation of glucose in the liver, increase protein catabolism to amino acids, enhance lipid absorption and protect the body against stress.

Mineralocorticosteroids- They is formed in the zona gromerulosa region of the adrenal cortex. They help retain water and NaCl in the body, regulate renal function and manage stress levels.

Sex steroids or androgens- These include androgen, oestrogen, progesterone and dehydropiandrosterone (DHEA).  They regulate the sex hormones, their actions and the development secondary sexual characters. These sex steroids are converted to fully functional sex hormones in the gonads.

Diseases associated with adrenal cortex

1. Addision’s disease- It occurs because of the tuberculosis of the adrenal gland.
2. Cushing’s syndrome-It occurs when excessive secretion of cortisol occurs because of adrenal hyperplasia or adrenal tumor.
3. Hyperaldosteronism- When a tumor occurs in the zona glomerulosa region of the adrenal cortex, then excessive secretion of aldosterone occurs.
4. Adrenogenital syndrome- A  tumor in adrenal cortex causes over production of adrenal androgens.

The cortical secretions are controlled by the pituitary gland, feedback mechanism, nervous system, blood volume, inorganic intake and glomerulotrophine (released by the pineal gland).

Adrenal medulla

It is made up of polyhedral granula cells and is surrounded by blood sinuses. These cells are also called chromaffin cells. The hormones secreted by the adrenal medulla include epinephrine and nor-epinipherine. The combination of these two hormones is called catecholamines.

Epinephrine or adrenaline- It is a tyrosine derivative. It is a kind of sympathomimetic hormone. It’s action include increase of heart rate and output, constriction of blood vessels, dilation of coronary  and skeletal vessels, increase of blood pressure, relaxation of bronchial muscles. Its secretion is controlled by a feedback mechanism which depends on blood sugar level, oxygen and carbon dioxide strength, sensations, sino aortic reflexes etc.

Norepinephrine- It is sympathetic transmitter. It more sympathomimetic than epinephrine.

Thymus gland

It is a partly endocrine gland and is located in the thorax on the back of the sternum. It has 2 flask shaped lobes. The lobes are made up of lobules. The lobules are made of small follicles of 1mm diameter.

It develops form the fourth branchial clefts. The endoderm gives rise to Hassall’s corpuscles. The thymus gland secretes thymosin or thymin hormone. The thymin is the main source of lymphocytes in the blood .It also helps in providing immunologic competence to the body.

The action of thymus glands are controlled by the pituitary gland (which stimulates the secretion of thymin) and gonads (which inhibit the secretion of thymin).

The pineal body gland

The pineal body gland is also known by the name conarium. It is located in human brain. It has a cone shape. It is 5 to 8mm in length and 3 to 5 mm in diameter. It is attached to the 3rd ventricle by a stalk. The cells of pineal gland have neural origin. The two main cell types found in pineal gland are the chief/parenchymal cells and the supportive/interestitial cells.

The hormone that comes from the pineal gland is called melatonin. It is synthesized from serotonin. Melatonin is N-acetyl-5-methoxytryptamine. Melatonin helps to maintain the day night cycles or the circadian rhythm. It is also a powerful antioxidant and helps to strengthen the immune system.

Testis

The testis are found inside the scrotum. They are oval shaped and 2 in human male. During early fetal life, they are located in the abdomen. They start to descend from the abdomen to the scrotum from the third week of gestation. The testes are covered by tunica albuginea which is compact, tough and compact capsule.  From the capsule, trabeculae descend and divide the testes in a number of pyramidal lobules. The lobules are formed of seminiferous tubules which are 500 mm in length. The seminiferous tubules combine and form straight tubule. The straight tubules combine to form the rate testis which again combines to form vasa efferentia. The vasa efferentia finally combine to form the duct of epididymis. The length of the epididymis is 6 meter. The epididymis turns into vas deferens. It is in the epididymis that the spermatozoa are stored until they are ejaculated. The vas deferens is joined by a duct by the name seminal vesicle. The secretions of seminal vesicles activate spermatozoa.The testis are the place where spermatogenesis takes place. The interstitial cells or leydig cells secrete testosterone.

Testosterone 

It is the principal male sex hormone and a steroid.It is responsible for the secondary male sex characters like beard,whiskers, body,facial,underarm,pubic hair and base voice.
It is also concerned win the growth and development of the accessory sex characters like penis, seminal vesicle and prostate glands.  It is responsible for sex urge and sexual behavior. If testes is removed (Castration) prior to puberty causes failure of secondary sexual characters. If castration is done after puberty causes reduction of sexual characters.

Disease of testes

Impotency- In this condition there is no ejaculation or lesser/no erection. Impotence can be of different kinds. They kinds/types are described below.

1. Constitutional impotence- When a male has defective sexual drive without having any other organic disease.

2. Organic impotence- Develops because of some pathological process or disease.

3. Psychogenic impotence- When there is no sexual drive because of psychological processes/diseases.

4. Cryptorchidsm- When testes fail to descend in the scrotum

Ovary

It is a female reproductive organ. They are two in number, one on each free end of the fallopian tube. They have a bean shape and have dimensions 4x2.5x1.5 cm. They hang from the mesovarium. Each ovary is composed of follicle sin various stages of development. In between the follicle cells, the interstitial tissue called stroma is present. In adult mammals  the ovary contains degenerating follicles formed after ovulation called Corupus luteum. The ovaries produce three types of female sex hormones viz. Estrogens , progesterone and relaxin.

Estrogen is collective name of four different steroid hormones which are closely related and secreted by the maturing ovarian follicle. They are estradiol, esterone, estriol and estradiol 17 beta. Estrogen secretion is periodically stimulated by FSH and of Pituitary gland. Estrogens are responsible for mature growth, development and maintenance of the female reproductive system. Brings about the development of uterus,vagina,breasts and menstrual changes. Flavors uterine growth during pregnancy.Inhibits lactation.

Progesterone

The hormone progesterone is secreted by the corpus luteum. The hormone is a sterol derivative. These hormones only work under the influence of oestrogens. It accelerates the movement of ovum through fallopian tubes. It prepares the uterine endometrium for implantation of fertilized ovum. Helps breast development during pregnancy,Prevents the ovary from releasing additional ova during pregnancy. Progesterone is also essential for the formation of the placenta. It also has other metabolic and pregnancy related functions.Under secretion of progesterone causes abortion of pregnant females.

Relaxin

It is polypeptide but not a steroid. It is produced by the corpus luteum. It causes the relaxation of pelvic ligaments at the time of child birth. It contributes to the development of mammary gland.

Diseases of ovary

1. Polycystic ovary syndrome- When the ovary has more follicles than usual.

2. Luteoma -  A specific kind of tumor that occurs during pregnancy.

3. Hypogonadism- diminished activity of the ovary.

4. Ovarian cancer- When a tumor occurs in the ovary.

Placenta

Placenta is a connection between body of mother and developing embryo acting as an organ of nutrition, respiration.It consists of maternal part and foetal part. It is evident that the placenta in higher animals secrete three hormones namely estrogens, progesterone and chorionic gonadotropin.