UPSC Biology Body Fluids and Circulation / शरीर के तरल पदार्थ और परिसंचरण Body Fluids and Circulation

 Body Fluids and Circulation

1.           Blood Plasma

• Plasma is a straw coloured, viscous fluid constituting nearly \[55\] per cent of the blood.
• \[90-92\]per cent of plasma is water and proteins contribute \[6-8\]per cent of it. Fibrinogen, globulins and albumins are the major proteins. Fibrinogens are needed for clotting or coagulation of blood.
• Globulins primarly are involved in defense mechanisms of the body and the albumins help in osmotic balance. Plasma also contains small amounts of minerals like \[N{{a}^{+}},\,C{{a}^{++}},\,M{{g}^{++}},\,HC{{O}_{3}}-,\,C{{I}^{-}},\] etc.
• Glucose, amino acids, lipids, etc., are also present in the plasma as they are always in transit in the body.
• Factors for coagulation or clotting of blood are also present in the plasma in an inactive form. Plasma without the clotting factors is called serum.

2.           Red Blood Cells (RBCs)

• Erythrocytes, leucocytes and platelets are collectively called formed elements and they constitute nearly 45 per cent of the blood.
• Erythrocytes or red blood cells (RBC) are the most abundant of all the cells in blood. A healthy adult man has, on an average, 5 millions to \[5.5\]millions of RBCs \[m{{m}^{-3}}\] of blood.
• RBCs are formed in the red bone marrow in the adults.
• RBCs are devoid of nucleus in most of the mammals and are biconcave in shape.
• They have a red coloured, iron containing complex protein called haemoglobin, hence the colour and name of these cells.
• A healthy individual has \[12-16\]gms of haemoglobin in every 100 ml of blood. These molecules play a significant role in transport of respiratory gases.
• RBCs have an average life span of 120 days after which they are destroyed in the spleen (graveyard of RBCs).

3.           White Blood Cells (WBCs), Leucocytes

• Leucocytes are also known as white blood cells (WBC) as they are colourless due to the lack of haemoglobin. They are nucleated and are relatively lesser in number which averages 6000-8000 mm-3 of blood.
• Leucocytes are generally short lived.
• There are two main categories of WBCs - granulocytes and agranulocytes.
• Neutrophils, eosinophils and basophils are different types of granulocytes, while lymphocytes and monocytes are the agranulocytes.
• Neutrophils are the most abundant cells (\[60-65\]per cent) of the total WBCs and basophils are the least (\[0.5-1\] per cent) among them.
• Neutrophils and monocytes (\[6-8\]per cent) are phagocytic cells which destroy foreign organisms entering the body.
• Basophils secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions.
• Eosinophils (\[2-3\]per cent) resist infections and are also associated with allergic reactions.
• Lymphocytes (\[20-25\]per cent) are of two major types - 'B' and 'T forms. Both B and T lymphocytes are responsible for immune responses of the body.

4.           Platelets

• Platelets also called thrombocytes, are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains \[1,50,000-3,50,000\]platelets \[m{{m}^{-3}}.\]
• Platelets can release a variety of substances most of which are involved in the coagulation or clotting of blood. A reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body.

5.             Blood Groups (ABO)

• Various types of grouping of blood has been done. Two such groupings - the ABO and Rh - are widely used all over the world. ABO grouping is based on the presence or absence of two surface antigens (chemicals that can induce immune response) on the RBCs namely A and B. Similarly, the plasma of different individuals contain two natural antibodies (proteins produced in response to antigens).
• During blood transfusion, any blood cannot be used; the blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion to avoid severe problems of clumping (destruction of RBC).
• 'O' group individuals are called 'universal donors'. Persons with 'AB' group can accept blood from persons with AB as well as the other groups of blood. Therefore, such persons are called 'universal recipients'.

6.             Rh grouping

• Another antigen, the Rh antigen similar to one present in Rhesus monkeys (hence Rh), is also observed on the surface of RBCs of majority (nearly \[80\] per cent) of humans.
• Such individuals are called Rh positive \[Rh+\left( ve \right)\]and those in whom this antigen is absent are called Rh negative \[Rh-\left( ve \right)\]
• An \[Rh-\left( ve \right)\] person, if exposed to \[Rh+\left( ve \right)\] blood, will form specific antibodies against the Rh antigens. Therefore, Rh group should also be matched before transfusions.
• A special case of Rh incompatibility (mismatching) has been observed between the \[Rh-\left( ve \right)\] blood of a pregnant mother with \[Rh+\left( ve \right)\] blood of the foetus. Rh antigens of the foetus do not get exposed to the \[Rh-\left( ve \right)\] blood of the mother in the first pregnancy as the two bloods are well separated by the placenta.
• However, during the delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the \[Rh+\left( ve \right)\] blood from the foetus. In such cases, the mother starts preparing antibodies against Rh antigen in her blood.
• In case of her subsequent pregnancies, the Rh antibodies from the mother \[Rh-\left( ve \right)\]can leak into the blood of the foetus \[Rh+\left( ve \right)\] and destroy the foetal RBCs. This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby. This condition is called erythroblastosis foetalis.
• This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

7.           Coagulation of Blood

• Blood is clot or coagulam formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped. Fibrins are formed by the conversion of inactive fibrinogens in the plasma by the enzyme thrombin.
• Thrombins, in turn are formed from another inactive substance present in the plasma called prothrombin. An enzyme complex, thrombokinase, is required for the above reaction. This complex is formed by a series of linked enzymic reactions (cascade process) involving a number of factors present in the plasma in an inactive state.
• An injury or a trauma stimulates the platelets in the blood to release certain factors which activate the mechanism of coagulation. Certain factors released by the tissues at the site of injury also can initiate coagulation. Calcium ions play a very important role in clotting.

8.           Disorders of Circulatory System

• High Blood Pressure (Hypertension): Hypertension is the term for blood pressure that is higher than normal \[\left( 120/80 \right).\]In this measurement 120 mm Hg (millimetres of mercury pressure) is the systolic, or pumping, pressure and 80 mm Hg is the diastolic, or resting, pressure. If repeated checks of blood pressure of an individual is \[140/90\](140 over 90) or higher, it shows hypertension. High blood pressure leads to heart diseases and also affects vital organs like brain and kidney.
• Coronary Artery Disease (CAD): Coronary Artery Disease, often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle. It is caused by deposits of calcium, fat, cholesterol and fibrous tissues, which makes the lumen of arteries narrower.
• Angina: It is also called 'angina pectoris’. A symptom of acute chest pain appears when no enough oxygen is reaching the heart muscle. Angina can occur in men and women of any age but it is more common among the middle-aged and elderly. It occurs due to conditions that affect the blood flow.
• Heart Failure: Heart failure means the state of heart when it is not pumping blood effectively enough to meet the needs of the body. It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease. Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).

9.         Heart

• The heart is a muscular organ which is as big as our fist. Because both oxygen and carbon dioxide have to be transported by the blood, the heart has different chambers to prevent the oxygen-rich blood from mixing with the blood containing carbon dioxide. The carbon dioxide-rich blood has to reach the lungs for the carbon dioxide to be removed, and the oxygenated blood from the lungs has to be brought back to the heart. This oxygen-rich blood is then pumped to the rest of the body.
• The separation of the right side and the left side of the heart is useful to keep oxygenated and deoxygenated blood from mixing. Such separation allows a highly efficient supply of oxygen to the body. This is useful in animals that have high energy needs, such as birds and mammals, which constantly use energy to maintain their body temperature. In animals that do not use energy for this purpose, the body temperature depends on the temperature in the environment.
• Such animals, like amphibians or many reptiles have three-chambered hearts, and tolerate some mixing of the oxygenated and de-oxygenated blood streams. Fishes, on the other hand, have only two chambers to their hearts, and the blood is pumped to the gills, is oxygenated there, and passes directly to the rest of the body. Thus, blood goes only once through the heart in the fish during one cycle of passage through the body. On the other hand, it goes through the heart twice during each cycle in other vertebrates. This is known as double circulation.
• The force that blood exerts against the wall of a vessel is called blood pressure. This pressure is much greater in arteries than in veins. The pressure of blood inside the artery during ventricular systole (contraction) is called systolic pressure and pressure in artery during ventricular diastole (relaxation) is called diastolic pressure. The normal systolic pressure is about 120 mm of Hg and diastolic pressure is 80 mm of Hg.
• Blood pressure is measured with an instrument called sphygmomanometer. High blood pressure is also called hypertension and is caused by the constriction of arterioles, which results in increased resistance to blood flow. It can lead to the rupture of an artery and internal bleeding.

10.        Blood Vessels

• Arteries are the vessels which carry blood away from the heart to various organs of the body. Since the blood emerges from the heart under high pressure, the arteries have thick, elastic walls.
• Veins collect the blood from different organs and bring it back to the heart. They do not need thick walls because the blood is no longer under pressure, instead they have valves that ensure that the blood flows only in one direction.
• On reaching an organ or tissue, the artery divides into smaller and smaller vessels to bring the blood in contact with all the individual cells. The smallest vessels have walls which are one-cell thick and are called capillaries. Exchange of material between the blood and surrounding cells takes place across this thin wall. The capillaries then join together to form veins that convey the blood away from the organ or tissue.