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.

Sexually transmitted diseases (STDs)

Sexually transmitted diseases, as we can deduce through the name, are class of infections that are transmitted most commonly through sexual contact. There are some other modes of infection through which the STD’s are transmitted, though rarely. These are infections that occur in lab workers because of their working mode, from pregnant women to their children, through unhygienic practices and through fomites. Fomites are the objects/materials that can carry infections like furniture, clothes, utensils etc.  It is not possible to totally contain STD’s. The factors that are contributing to the increase in STD’s are homosexuality, change in sexual practices, permissiveness, ignorant attitude etc.

Most of the STD can be completely cured through chemotherapy. AIDS is an exception and still remains a fatal disease. Also, there are certain strains of H.ducrei and gonococci (microorganisms) that have more resistance to the commonly used chemotherapy drugs. Still, all the strains of STD inducing microorganism still retain their sensitivity to chemotherapeutic drugs and are treatable. As more than one STD inducing microorganism can be present in an STD infected person, it is better to screen the person for all STD’s including syphilis, gonorrhea, Chlamydia and HIV.

Some of the major STD are:

    Acquired Immune Deficiency Syndrome or AIDS
    Gonorrhea
    Syphilis
    Non-Gonococcal Urethritis or GNU
    Chancroid or Soft Sore
    Granuloma Inguinale
    Vaginitis
    Lymphogranuloma Venerum (LGV)
    Veneral Warts

Acquired Immune Deficiency Syndrome or AIDS

AIDS or Acquired Immune Deficiency Syndrome is caused by a retrovirus by the name Human Immuno Deficiency Virus or the HIV.

Origin of AIDS

The first case of AIDS was reported in the USA in early 1980’s many gay men reported opportunistic infections that were not easily treatable.  Soon after, HIV was discovered.

The HIV belongs to the lenti virus group of virus. A lenti virus(slow virus) is that virus which take a long time to produce the infection symptoms in the human body. It attacks the immune system of the body. Lenti virus is part of a large group of virus called retro virus. A retro virus has RNA instead of DNA as a genetic material. A retro virus replicates itself in the host cell by a process called reverse transcription. Lenti viruses are found in number of organism including cat, cattle, horse, sheep etc. Another lenti virus, by the name Simian Immunodeficiency Virus or SIV affects monkeys and is found to be 32,000 years old.

Most scientists believe that the ancestor of HIV is SIV. This is so because investigations have revealed that the structure of SIV is similar to the HIV 1 and HIV 2 viruses. HIV 1 is the predominant virus found in humans. The HIV 2 strain has a close resemblance to the SIV that has been found in the smooth collard monkey found in Western Africa. The AIDS epidemic causing HIV 1 has been found to closely resemble the SIV found in chimpanzees. Still, there are certain differences between the 2 viruses.

Theories for HIV cross over to humans

Viruses are capable to transfer themselves from one species to another. The SIV found in apes is said to be transferred from other primates. There are many theories which suggest how the virus might have crossed over to humans. Some of these are listed below.

The hunter theory

It suggests that the killing/hunting and eating for apes led to the transfer of HIV 1 from apes to humans.

The oral polio vaccine theory

The theory suggests that HIV was transferred by medical interventions. It suggests that the virus was transferred to humans in the process of testing of polio vaccine in chimps and its subsequent transfer to humans during 1950’s.

Some of the earliest known HIV infections are-

1. A plasma sample of a native of Congo taken in the year 1959.

2. A sample of lymph node of a Congo adult takes during the year 1960.

3. Tissue ample of an American who dies in the year 1969.

It was deduced in the year 1998 that HIV 1 was introduced in humans in between 1940 and 1950. The finding was based on a plasma sample form the year 1959.

However a study in 2008 said that HIV originated between 1884 and 1924.

Most of findings suggest that the virus was transferred in humans in Africa. It spread through other continents through travelling humans and the sexual contacts they made. Other means, for example blood industry, also contributed to the spread of AIDS epidemic.

Cause of spread of AIDS infection

The most common means of spread of the AIDS causing virus HIV is sexual intercourse which can be homo or heterosexual in nature.  Contaminated needles, syringes, infected blood and blood products transfusion can also transfer HIV virus. The HIV virus can also be transferred through an infected mother to the fetus.

The HIV infection

The AIDS virus infects the brain cells, lymphocyte and other cells of the body. When the body is infected with the AIDS virus, the immune response of the body weakens. It weakens as the AIDS virus destroys the lymphocytes and replaces and replicates them with its own viral strands. The AIDS virus also impairs the functioning of other helper T cells, B cells and macrophages. The mean incubation period of AIDS virus is 4.5 years i.e. in 4.5 years a patient is fully affected by HIV. AIDS virus destroys the cellular arm of the immune system and the infected person cannot overcome and cannot recover from any opportunistic pathological condition.

The body develops antibodies for HIV in a mean time period of 6 weeks. The patients become carriers of the disease and may transfer the infected disease through the above mentioned modes. Some patients develop febrile illness. Next the patient develops Lymphadenopathy or abnormal lymph Nodes. In Lymphadenopathy, other pathological conditions such as Herpes Zoster or Oral Candidiasis may occur. In the final stage of HIV infection, the patient can suffer from some kind of opportunistic infection like pneumonia, candidiasis, tuberculosis, meningitis, neurological disorder, malignancy etc.

Life phases of HIV virus

Like other microorganism, the pathological HIV virus spreads inside the human body once it infects a person.  Inside the human body, we can divide the life phases of HIV infection in the flowing phases.

A person is affected by the HIV and the HIV virus starts to freely circulate in the blood.
The free HIV virus circulating in the blood attaches itself to a healthy T lymphocyte cell or CD4 cell through the cell’s receptors.
The HIV empties all its cellular contents in the infected cell.
The reverse transcriptase enzyme of the HIV starts building the HIV DNA inside the infected cell.
The HIV integrase enzyme inserts the HIV DNA into the chromosomes of the infected cells.
Now, when the infected cell reproduces the HIV DNA and HIV virus present in it produces raw material for the manufacture of new HIV cells.
The protease enzyme cuts the long HIV proteins into individual HIV proteins. Assembly of a new HIV virus occurs with the combination of these new short proteins with the genetic material of the virus.
The new immature HIV virus forms inside the infected cell from where it is slowly pushed into the extracellular environment by a process called budding.
The immature virus develops into a mature virus and is ready to infect a new healthy cell.
The cycle goes on and more HIV infected cells are formed in the body.

Stages of AIDS

When a person gets infected with HIV, he/she develops AIDS.  A person’s response to the HIV infection is based on certain factors like his/her genetic makeup, the HIV subtype that has infected the person, stress level, co-infection, nutrition, age etc. The spread of infection inside the human body has been divided into certain stages. The stages of AIDS have been described below.

Acute infection stage

This is the first stage of progression of AIDS. Most people who get infected with HIV develop flu like symptoms. As this point of disease progression, the lymphocyte count or the CD4 count of the body drastically reduces as more and more of the CD4 cells get replaced by the HIV viral strands.  Symptoms like swollen glands, muscle and joint aches, fever, head-ache, rash and fatigue develop in the patient. The amount of HIV virus increases to a point/quantity in the human body called as the viral set point. At this point, the amount of virus in the body is stable. When the amount of virus reaches the viral set point, the CD4 count in the body starts to increase. Still, the amount of CD4 count in body never again reaches the pre-infection levels. Start of Anti retro viral therapy or ART at this point benefits the person. At the viral set point, the fluids of the infected body contain high amounts of HIV and thus the person can transmit the disease to another person through sexual contact and by other means easily.

Clinical Latency Stage or CLS

In this stage, though the HIV virus develops into the human body, there are no HIV infection symptoms. Even if there are any symptoms, they are very mild. During the clinical latency stage, the HIV virus reproduces at a very slow stage. If a person gets ART at this point of time, he/she may live further for several decades. The CLS lasts for an average of 10 years in the persons who do not take ART.

AIDS or Acquired Immune Deficiency Syndrome

This is the last stage of HIV infection. At this point of time, the CD4 count of the body has fallen drastically. Thus the body becomes highly prone to infection related cancers and other infections that are collectively called opportunistic infections. The normal CD4 count of the body is between 500 to 1600 cells per millimeter cube of blood. At the stage of AIDS, the CD4 count has been reduced to 200 cells per cubic milliliter of blood. If a person does not take ART at this point of time, he/she lives for further 3 years. A dangerous opportunistic illness and infection can reduce life to 1 year. People who get ART early mostly do not progress to AIDS. They also enjoy a near normal life span. 

AIDS Prevention

The treatment of is still in an imbecile stage. Thus, it is more convenient to prevent occurrence of disease by taking necessary steps. The spread of AIDS can be controlled by:

Taking precautions during sexual intercourse) like use of condoms.
Not having multiple sexual partners.
Use of sterilized needles and syringes
Following good blood banking practices.
Avoidance by pregnancy of women who have HIV infection.

Factors responsible for in curability of AIDS

The HIV virus is a very potent microorganism. Drugs are not able to fully control the spread of microorganism inside the body.  Factors responsible for impotent treatment of HIV are:

HIV infects the brain cells.
HIV becomes a part of the cells that it infects.
Most of the antiviral drugs are not able to cross the blood brain barrier.
The virus becomes resistant to single drug.
HIV drugs are toxic to the human body and many other pathological conditions, for ex. Anemia develop during the course of treatment.

Anti Retroviral therapy

There is no complete of the HIV infection as the virus irreversibly damages the immune system. Also, there is no drug that can completely eliminate the HIV virus from the human body once it gets infected. The therapy of AIDS focuses on improving the quality of life of the patient so that he/she can live longer. Anti Retroviral Therapy is the use of combination of drugs for treating AIDS. Like most others serious pathological conditions, for example cancer, HIV is also treated with combination of drugs.

Drugs used in the treatment of AIDS can be classified as follows.

Nucleoside reverse transcriptase inhibitors- Azidothymidine, Didanosine, Zalcitabine,  Stavudine, and Lamivudine
Non nucleoside reverse transcriptase inhibitors-  Nevirapine, Delavirdine and Efavirene.
Nucleotide reverse transcriptase inhibitors- Adefovir
Protease reverse transcriptase inhibitor- Ritonavir, Saquinavir and Indinavir.

Mode of action of HIV curing drugs

 Different drugs have different mode of action. The drug may get incorporated in the DNA of the HIV virus. It then ends the chain synthesis of the HIV DNA. The drugs can also inhibit the functioning of HIV’s reverse transcriptase enzyme and inhibit its growth. The protease inhibitors block the infectivity of viral strands and arrest their maturation. An HIV drug can also inhibit the protease enzyme of HIV that causes its maturation.

The current way of treating HIV is through a combination of these drugs. Thus nucleosides, reverse transcriptase inhibitors and protease inhibitors are combines to fight AIDS.

Recent developments in AIDS control

The first national aids control program- NACO was launched in India in the year 1992. The global community gave due weight age for an effective program which can combat HIV spread in the United Nations Millennium Declaration in the year 2000. The first declaration by the member states of UN was done in 2001. There were also political declarations in the year 2006and 2011. The national aids control program’s goals of India are aligned with the goals of the 12th 5 year plan of the government of India. Now, the aids control programs of India aims at reversing the AIDS epidemic. The NACO program is in coordination with the UNAIDS program. A combined effort by the UN country team, NACO group, other UN agencies and other community and co-operative societies has been launched to control AIDS.

Through combined and sustained efforts. India has been able to reduce the spread of the epidemic by 50% between the years 2001 and 2012, As per the UNAIDS global report, the access of individuals to AIDS retro viral therapy has been improved. Thus, the figures of AIDS related deaths have come down. Still, India has the third largest number of AIDS patients. Some population groups and some geographical areas of India still show increase in AIDS related cases.

The new AIDS prevention efforts aim to:

Prevent dying of HIV infected mothers and providing them retroviral therapy so that there are lesser chances of the baby being infected with HIV.
Reduce the sexual transmission of the HIV virus.
Increasing access to retro viral therapy.
Prevent tuberculosis deaths that result because of HIV infection.
Providing HIV services to people in prisons and other closed areas.
Increasing awareness in drug users and abusers so that they are not infected with HIV infected syringes and needles.
Reform laws, policies and practice so as to control the spread of AIDS epidemic.
Address the issue of AIDS in all kinds of humanitarian gathering and emergencies.
Integrate HIV response with other food and nutrition programs to provide education for AIDS prevention and treatment.
To make the people more aware of the disease through awareness announcements on radio and television.
To mobilize the private sector for strengthening aids response.

2. Gonorrhea

Gonorrhea is caused by gonococci, a bacterium.  The disease is spread through sexual contact and by an infected mother to her child. Almost all the strains of gonococci are sensitive to penicillin. It is more difficult to treat gonorrhea in women than in men.

3. Syphilis

The causative microorganism of syphilis is the Spirochete bacterium Tresponema Pallidum. The microorganism can be transmitted through sexual contact or from mother to fetus during pregnancy. The most common drug and the drug preferred for treating syphilis is Penicillin. 90 percent of syphilis cases can be treated if it is diagnosed early and adequate treatment is provided.

The first four years of disease infection are termed as early syphilis. In the infectious stage of early syphilis, the patient develops surface lesions that are highly contagious. The latent phase of early syphilis characterizes a positive serological test though there are no lesions and the Cerebro Spinal Fluid of the CSF is normal. Late syphilis occurs after the early syphilis (after 4 years). Cardiovascular Syphilis, Late Neuro-syphilis and Late Latent Syphilis characterize this condition.

4. Non-Gonococcal Urethritis or GNU

Like all other STD except vaginalitis, GNU occurs in both men and women and is a sexually transmitted disease. The causative organisms of NGU include Trichomonas vaginalis, Chlamydia trachomatis, and some other urinary tract infection causative organism. In GNU, the urethra becomes inflamed.  The symptoms of NGU are frequent urination, a white discharge during urination, sensation of pain or burning during urination, itching, irritation etc. The common drugs used for treating GNU include doxycycline, erythromycin, tetracycline and azithromycin.

5. Chancroid or Soft Sore

 H.ducrei, a bacterium is the causative organism of Chancroid. The disease is characterized by painful sores in the genetalia. Lymphadnopathy can also occur in a Chancroid patient. The antibiotic preferred for treating the disease is erythromycin.

4. Granuloma Inguinale

The disease is caused by Donovania Granulomates, a bacterium. The disease is characterized by ulcerative lesions in the genital areas. Drugs used for treating GI are ampicillin, erythromycin, tetracycline and streptomycin.

5. Vaginitis

Vaginitis is caused by C.albicans (yeast), Gardenella vaginale (bacteria) and Trichomonas vaginale. The disease is characterized by pain, irritation and discharge.  Other symptoms include pain during intercourse and foul odor form the vagina. Different kinds of Vaginitis include sexual atrophic vaginitis, candida vaginitis, bacterial vaginitis and trichomonas vaginalis. Apart from sexual contact, irritant vaginitis can also be caused by allergic reaction to external substances like soaps, perfumes, condoms etc. Flucanazole, metrondiazole, clindamycin, tinidazole and clotrimazole are preferred antibiotics for treating the disease. The disease can be prevented by performing safe sexual practices, by maintain hygiene of the genital areas and by eating healthy diets.

6. Lymphogranuloma Venerum (LGV)

The disease is caused by Chlamydia and is characterized by infectious lymph nodes and lymph. Symptoms of the disease depend on the mode of entry of the Chlamydia pathogen inside the human body. When the bacterium enters through the mucous linings of the genitals then abscesses in the groin region occur. If the infection occurs through rectal mucosa then rectal syndrome arises that are characterized by inflammation of the colon and rectum. When pharyngeal tissue gets infected, then buboes in the neck region occur. In the early stages of LGV, genital ulcers occur that are painless. In the secondary stage, the infection spreads throughout the body via lymphatic drainage pathways. In this stage, different manifestations of the disease can be seen that depend on the pathway of infection. These are lymphangitis, lymphadenitis, proctisis, rectum/colonic mucosa inflammation, cervicitis, perimetrisis etc. Later the lymph nodes start enlarging and are called buboes. Tetracycline or doxycycline is preferred drugs for treatment.

7. Veneral Warts

A local virus infection can cause this disease. The wart growth is enhanced by moisture presence at the infected areas while dryness inhibits the growth. The newest drug for treating venereal warts is Imiquimoid.

Human lymphatic System

The lymphatic system circulates lymph in the human body. The cells of human body are surrounded by the lymph. Lymph is a part of extracellular fluid in which the human cells bathe. Apart from lymph the extracellular fluid also contains Plasma and Transcellular fluids like Ocular fluid, Joint fluid and Cerebrospinal fluid. The lymph is formed of interstitial fluid. An adult has around 10 liters of lymph in his/her body. The lymph consists of salts, sugars, amino acids, hormones,coenzymes, neurotransmitters, fatty acids and the metabolic waste products. The composition of lymph varies in different tissues. The major functions of lymph are:

1. Drainage- Removal of metabolic waste,

2. Transmittive- To provide for intercellular communication

3. Nutritive- To deliver required materials to the cells, and

4. Defense- To strengthen the immune system of the body (as the lymphoid tissues contain lymphocytes)

Circulation of the lymphatic system

Though the lymphatic system is a part of circulatory system, it has no central pump and does not have an enclosure system.The movement of lymph occur through peristalsis. Other factors that contribute to lymph movement are contraction of arteries and skeletal muscles and the operation of valves.

The blood does not come in direct contact with the cells and tissues of the human body. The constituents of the human blood first exit the blood vessels and then form the interstitial fluid (lymph). The lymph then carries out cellular exchanges. The lymph vessels drains into the lymphatic duct which combine and pour their materials into the sub-clavian vein.

Organs of lymphatics System

Like other body systems, the lymphatic system consists of organs and vessels. The major lymphatic system organs are listed below.

1. Lymphatics or lymph vessels
2. Lymph nodes
3. Spleen
4. Thymus gland

 

We will look at the functioning of the various organs of the lymphatic system one by one.

The Lymphatics or The Lymph Vessels

The lymphatics vessels are closed end vessels. They are also called lymphatic capillaries. The lymphatics are found in the intercellular spaces. The walls of the lymphatics are formed of the fibrous connective tissues. The smaller lymphatics join to form larger lymphatics. The lymphatics gradually increase in size when they pass through lymph nodes, as at lymph node more lymphatics tributaries join each other. The total lymph content of the body gets connected into two major channels which are the thoracic duct and the lymphatic duct. The thoracic duct opens in the left sub-clavian vein and the lymphatic duct opens in the right sub-clavian vein.

The right lymphatic ducts- The length of the right lymphatic duct is around 1.25 cm. It drains the lymph of the right side of body areas including the right neck, chest and right forelimbs.

The thoracic duct- The length of the thoracic duct can vary from 3.8 to 45. The diameter of the duct is 4 to mm. The thoracic duct forms out of the Cisterna Chili (a dilated sac found at the lower end of the thoracic duct). It drains lymph from the left side of neck, chest and limbs in the left sub-clavian vein. The lymph of the hind limbs and the alimentary canal is received in the Cisterna Chili.

The lymphatics have valves, which aid the lymph flow and provide it proper direction. Lacteals are those lymphatics that remain in the small intestinal villi. The lacteals contain Chyle, a white fluid that is similar to lymph except that it is high in fat content. The CNS does not contain lymph vessels. The other components of human body which do not contain lymphatics are eye ball, internal ears, spleen and epidermis. The lymphatics carry tissue fluid to veins. They also carry protein and water from the interstitial fluid to blood, when they return. The lacteals help is absorption of food materials (especially fats) during digestion.

Features of Lymph

Lymph is modified tissue fluid and is contained in lymphatics. The lymph appears water like. The lymph fluid is formed by the exchange of substances between the tissue spaces and the blood capillaries. The process of exchange is called Transudation.

Composition of the lymph fluid

• The components of lymph can be divided into cellular and non cellular material.
• The cellular component consists of lymphocytes. The strength of leucocytes in lymph ranges from 500 per cu.mm. To 75000 per cu.mm.
• The non cellular of components of lymph are water and solids.
• The solids non cellular component of lymph consists of proteins namely fibrogen, globulin, albumin and traces of prothrombin (2 to 4.5%), fats (5 to 15%), carbohydrates (132.2 milligram per 100 ml of lymph and other constituents (present in small amounts) like phosphorus, creatinine, calcium, enzymes and antibodies.

Factors responsible for formation of lymph

Lymph is formed out of the fluid of tissues inside the human body. Thus, any process or thing that increases the amount of tissue fluid in the body will also contribute to increase in lymph. The lymph formation is not dependent on any secretory process or gland. It depends on physical processes and factors. The factors which contribute to the formation of lymph are described below.

• Capillary pressure- With increase in capillary pressure, the formation of lymph increases.
• Capillary wall  permeability- With increase in capillary permeability, the lymph formation rate gets increased
• Rise in temperature- With increase in temperature, the permeability of capillary walls of tissue also increases. Thus, lymph formation is increased.
• Decrease in oxygen supply to body and blood- The permeability of the vessels increase with decrease in oxygen supply (due to damage in capillary endothelium). This causes a raise in lymph formation.
• Alteration in osmotic pressure- A reduction in the colloidal osmotic pressure of the blood will result in increased lymph formation. The osmotic pressure of the blood and lymphatics varies with the amount of inorganic and organic substances present in them.
• Increase in metabolic activity of an organ- When any organ functions at a high rate, then the metabolic wastes generated increases the osmotic pressure of the tissue fluid. The temperature of the area also increases. There is a lack of oxygen and enhancement of capillary pressure. These all factors contribute to increase in formation of lymph and flow of lymph to that body area.

Circulation of Lymph in the Human Body

As we know, the lymph is not enclosed in some specific vessels like blood nor has it any pump like heart. Then how does the lymph flow across the body?

The flow of lymph in humans depends on 3 factors/things. These are:

• Presence of Valves- The valves cause the lymph to the flow in the required singular direction.

•  Pressure Gradient- The pressure difference between the tissue and the lymphatics contribute to the flow of lymph in the required direction.

•  Muscular or Skeletal Action/Contraction- The contraction of muscles causes the contraction of Lymphatics. This results in the onward flow of lymph as the valves do not allow the lymph to flow in the backward direction.

•  Movements of body parts during respiration- When we breathe in (inspiration), the diaphragm descends. This causes a fall in the intra thoracic pressure in the thoracic region. Due to fall in pressure, a pressure gradient is created which causes the movement of lymph from the lymphatics into the thorax. Next, there is a rise in the intra-abdominal pressure which results in the compression of Cisterna Chili. This increases the flow of lymph through the Thoracic Duct.

The Lymph Gland or the Lymph Node

The lymph node has a bean shape. It is small and is enclosed in a connective tissue that forms a capsular covering over it. Form the capsule of the node, small strands made of tissues pass into the nodular substance. They are called trabeculae which branch extensively and repeatedly. The trabeculae combine with the nodular reticular tissues to form sinuses. These sinuses contain the lymphoid cells. We can divide the lymph node into two regions, namely the Cortex and Medulla.

The Cortex of Lymph Node- In the cortex of lymph node, the lymph tissues are not scattered. Here, they are found in follicles or the lymphoid nodules. The diameter of the follicle can vary from .35 to 1 mm. The central area of the cortex contains Lymphocytes and is called the Secondary or Germinal nodule. The peripheral area of the cortex is called the Primary or Cortical Nodule. Blood sinuses separate the trabeculae and the lymph nodule. New lymphocytes are formed in the germinal area of the cortex. When a pathological condition occurs, a large number of macrophages get formed in this germinal area which is called the reaction center at that time. Macrophages engulf and destroy the foreign pathological substances.

The Medulla of the Lymph Node- The medulla does not has any lymphatic nodules and is less dense than the cortex. The lymph cells found in medulla are scattered. Apart from lymph cells there are also different kinds of reticulo endothelial cells found in the medulla. The trabeculae are irregularly arranged. The lymph cords of the trabeculae run in the medulla and form the communicating divisions. The lymph cords and trabeculae are separated by the Lymph Sinuses.

Hilus or Hilum- Hilus is the name given to the depression that is found in the lymph node. At this juncture, the bean shaped capsule of the lymph node is thicker. So at this juncture, the medulla comes to the surface as the cortex gets thin. Three vessels enter or exit the Hilus. 2 of them are.

1. Artery- It enters the lymph node through the Hilus and breaks into smaller capillaries called arterioles.  Mass of lymphatic cells surrounds these arteries.
2. Vein- The broken arteries gather again to form Venules. Small Venules combine to form the main vein. The main vein leaves the lymph node through the Hilus.

Therefore, the blood vessels enter and exit the lymph node through the Hilus. But it is not the case with the lymphatics. The afferent lymphatic vessels enter the lymph node through the outer shell of the capsule by piercing them. Once the lymphatics enter the capsule, they branch extensively, carry out exchanges and regroup to form the efferent lymphatic vessels. This efferent lymphatic vessel leaves the lymph nodes through Hilus. This is the third vessel found at the Hilus.

Functions performed by the Lymph Nodes

• They produce lymphocytes which aid to strengthen the immune system.
• The lymph node screens the lymph and engulfs the harmful pathogens.
• They filter poisonous content of the blood.
• They may also stop the spreading of cancerous cells.

Spleen

The spleen filters blood and is the largest lymphoid tissue of the human body. A human adult spleen weighs around 150 gm. It is located behind the floor of the abdomens. The spleen is located beneath the diaphragm and above the descending colon and left kidney. It is a haemopoietic organ i.e. formation of blood vessels occurs in spleen.

Structure of Spleen

The spleen is covered by capsule made of connective tissue. The peritoneum, or the outer layer of the capsule itself, is a serous membrane. A zigzag line or indent, called Hilus, is present at the medial area of the spleen. The nerves, blood vessels and lymphatics enter or exit the spleen through Hilus. The inner surface of the spleen (under the capsule) consists of many trabeculae. The trabeculae divide the spleen in many compartments. These tiny compartments are called lobules. Splenic pulp is the name given to the masses of parenchymal tissues present inside the capsule. The Splenic pulp is of 2 types viz. red and white. The red pulp consists of atypical lymphatic tissue and the white pulp consists of typical lymphatic tissue. The white pulp surrounds the arteries of the plasma cells, lymphocytes, macrophages and some other types of cells. The red pulp consists of the splenic sinuses and the splenic cords. The region between the junctional region of red and white pulp is called marginal zone.

Nature of circulation in the spleen

Arteries enter the spleen through the Hilus. After entering, the arteries divide themselves into trabecular branches. The branches then enter the Splenic parenchyma. The branches then reduce into reticular tissues and are infiltrated with lymphocytes.

Thus, after leaving the trabeculae the arteries enter the white pulp. Here they start branching. The branches enter the red pulp. In the red pulp the branches are sub branched and are called penicillar vessels. The long portion of penicillar vessels (made up of smooth muscles) is called pulp arteriole. The middle portion (a thick sheath) is called sheathed arteriole, and the terminal portion is called arterial capillary.

Functions of spleen

Spleen performs various functions. The major function s of spleen are listed below.

• The spleen functions as haemopoietic organ in embryo where the formation of blood vessels takes place.
• Destruction of old blood (red and white) cells.
• Spleen acts as a reservoir of blood.
• Spleen is a chief site for immune cell formation.

Thymus gland

It is a partly an endocrine gland and partly a lymphoid structure. It 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.

Structure of Thymus Gland

The thymus gland has an outer capsule made of connective tissue. It has 2 lobes, each having numerous lobules. Each lobule has a dense and peripheral cortex and a loose medulla. The 2 thymic lobes are joined by central strand.

The capsular region of the thymus consists of white connective tissue having plasma cells, macrophages, granular leucocytes mast cells and fat cells. The connective tissue is initially continuous with the capsule. Later, it dips into the lobules and forms a septal or trabecular system. The lymphatic vessels, nerves and blood vessels penetrate the capsule.

Thymus cortex- The cortical region the thymus lobules lack primary follicles when compared to the other lymphatic tissues. The cortex consists of lymphocytes.

Thymus medulla- The medulla of thymus consists of branched band of thymic tissue which are broad. The branching of the medulla results in the formation of lobular patterns. The medulla consists of reticular or epithelial cells. The medulla contains lesser amounts of lymphocytes than the cortex. The medulla also contains mast cells, plasma cells, melanocytes and eosinophil cells. The characteristic feature of the thymus cells are the thymic or Hassall’s corpuscles. These thymic cells are formed of reticular cells. The internal thoracic and inferior thyroid arteries provide blood to the thymus.

Lymphatic functions of Thymus Gland

• Thymus is a source of lymphocytes as lymphocyte production occurs in thymus.
• Thymus controls and influences the immune system of the body.

 Some diseases of lymphatic system

• Oedema- swelling caused by excessive secretion of lymph
• Tonsillitis- infection found in the throat tonsils
• Hodgkin’s disease-a kind of cancer found in the lymphatic system

Glandular fever- a kind of viral infection that occurs in lymphatics.