Blood

Blood is a special connective tissue made of plasma and formed elements, and it is the main body fluid that carries nutrients, gases and wastes in humans. This page covers the composition of blood, the kinds and counts of blood cells, the ABO and Rh blood groups, coagulation, and lymph (tissue fluid), following the NCERT Class 11 chapter Body Fluids and Circulation. NEET often asks blood cell percentages and functions, donor and recipient groups, erythroblastosis foetalis and the order of clotting events.

1. Why Animals Need Body Fluids

• All living cells need a steady supply of nutrients, ${\mathrm{O}}_2$ and other essential substances.
• Waste or harmful substances made by cells must be removed continuously for tissues to stay healthy.
• So animals need efficient ways to move substances to and from the cells; different animal groups have evolved different methods.
• ★ Exam imp Simple organisms such as sponges and coelenterates circulate water from their surroundings through their body cavities, so their cells can exchange substances.
• More complex organisms use special fluids inside the body for this transport.
• Blood is the body fluid used by most higher organisms, including humans.
• Lymph, another body fluid, also helps to transport certain substances.

Examples to Remember

2. Blood and Plasma

2.1 Blood: a fluid connective tissue

• It counts as a connective tissue because, like other connective tissues, it has cells (formed elements) lying in a matrix (plasma).
• Plasma makes up nearly 55 per cent of blood; formed elements make up nearly 45 per cent.

2.2 Plasma

• Plasma: a straw-coloured, viscous fluid forming nearly 55 per cent of blood.
• ★ Exam imp Water forms 90-92 per cent of plasma, and proteins form 6-8 per cent.
• The major plasma proteins are fibrinogen, globulins and albumins.

• Plasma also has small amounts of minerals such as $\mathrm{N}{\mathrm{a}}^{+}$, $\mathrm{C}{\mathrm{a}}^{++}$, $\mathrm{M}{\mathrm{g}}^{++}$, $\mathrm{HC}{\mathrm{O}}_3^{-}$ and $\mathrm{C}{\mathrm{l}}^{-}$.
• Glucose, amino acids and lipids are present in plasma, because they are always in transit in the body.
• The factors for coagulation (clotting) are also present in plasma, in an inactive form.

3. Formed Elements

• Formed elements: the erythrocytes, leucocytes and platelets taken together.
• They form nearly 45 per cent of the blood.

3.1 Erythrocytes (red blood cells)

• Erythrocytes or red blood cells (RBCs) are the most abundant cells in blood.
• ★ Exam imp A healthy adult man has, on average, 5 to 5.5 million RBCs ${\text{mm}}^{-3}$ of blood.
• In adults, RBCs are formed in the red bone marrow.
• RBCs have no nucleus in most mammals and are biconcave in shape.
• They contain haemoglobin: a red, iron-containing complex protein that gives these cells their colour and name.
• A healthy individual has 12-16 g of haemoglobin in every 100 mL of blood.
• Haemoglobin plays a major role in the transport of respiratory gases.
• ★ Exam imp RBCs live for an average of 120 days; they are then destroyed in the spleen, the graveyard of RBCs.

3.2 Leucocytes (white blood cells)

• Leucocytes are also called white blood cells (WBCs), as they are colourless due to the lack of haemoglobin.
• They are nucleated and fewer in number: on average 6000-8000 ${\text{mm}}^{-3}$ of blood.
• Leucocytes are generally short-lived.
• Two main categories: granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes).
• Phagocytic cells: cells that engulf and destroy foreign organisms entering the body.

3.3 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 ${\text{mm}}^{-3}$.
• Platelets release a variety of substances, most of which are involved in the coagulation (clotting) of blood.
• ★ Exam imp A fall in platelet number can cause clotting disorders, which lead to excessive loss of blood from the body.

4. Blood Groups

• Human blood looks alike but differs in certain aspects, so blood has been grouped in various ways.
• Two groupings, ABO and Rh, are widely used all over the world.

4.1 ABO grouping

• ABO grouping is based on the presence or absence of two surface antigens, A and B, on the RBCs.
• Antigens: chemicals that can induce an immune response.
• The plasma of different individuals contains two natural antibodies: proteins produced in response to antigens.

• Before any transfusion, the donor's blood must be carefully matched with the recipient's blood.
• This avoids severe problems of clumping (destruction of RBCs).

4.2 Rh grouping

• The Rh antigen is similar to an antigen present in Rhesus monkeys, hence the name Rh; it is also called the Rhesus factor.
• ★ Exam imp It is present on the RBCs of the majority, nearly 80 per cent, of humans.
• Rh positive (Rh+ve): persons who have the Rh antigen. Rh negative (Rh-ve): persons in whom it is absent.
• An Rh-ve person exposed to Rh+ve blood forms specific antibodies against the Rh antigens.
• So the Rh group must also be matched before transfusions.
• A special case of Rh incompatibility (mismatching) is seen between an Rh-ve pregnant mother and her Rh+ve foetus.

1. First pregnancy: the Rh antigens of the foetus do not reach the mother's Rh-ve blood, because the placenta keeps the two bloods well separated.
2. Delivery of the first child: small amounts of the foetus's Rh+ve blood may reach the mother's blood.
3. The mother then starts making antibodies against the Rh antigen in her blood.
4. Later pregnancy with an Rh+ve foetus: the mother's Rh antibodies can leak into the foetal blood and destroy the foetal RBCs.
5. This can be fatal to the foetus, or cause severe anaemia and jaundice in the baby.

5. Coagulation of Blood

• Coagulation (clotting): the response of blood to an injury or trauma, which stops bleeding after some time.
• It is a mechanism to prevent excessive loss of blood from the body.
• Clot or coagulam: the dark reddish-brown scum formed at the site of a cut or injury over a period of time.
• A clot is formed mainly of a network of threads called fibrins, in which dead and damaged formed elements of blood are trapped.

1. An injury or trauma stimulates the platelets to release certain factors that activate the mechanism of coagulation. Factors released by the tissues at the site of injury can also start it.
2. These factors start a cascade process: a series of linked enzymic reactions involving many factors present in the plasma in an inactive state.
3. The cascade forms an enzyme complex, thrombokinase.
4. Thrombokinase is required to form thrombin from prothrombin, an inactive substance present in the plasma.
5. The enzyme thrombin converts the inactive fibrinogens of plasma into fibrins.
6. Fibrin threads form a network that traps dead and damaged formed elements: the clot.

• ★ Exam imp Calcium ions play a very important role in clotting.

6. Lymph (Tissue Fluid)

• As blood passes through the capillaries in tissues, some water and many small water-soluble substances move out into the spaces between the cells.
• The larger proteins and most of the formed elements stay behind in the blood vessels.
• This released fluid is called interstitial fluid or tissue fluid.
• Tissue fluid has the same mineral distribution as plasma.
• ★ Exam imp Exchange of nutrients, gases, etc., between the blood and the cells always occurs through this fluid.
• Lymphatic system: an elaborate network of vessels that collects this fluid and drains it back into the major veins.
• Lymph: the fluid present in the lymphatic system. It is almost similar to blood except for the protein content and the formed elements.
• Lymph is colourless and contains specialised lymphocytes, which are responsible for the immune responses of the body.
• Lymph is also an important carrier of nutrients, hormones, etc.
• ★ Exam imp Fats are absorbed through lymph in the lacteals present in the intestinal villi.

7. Exam Essentials

Pairs to Match

Numbers to Remember

• Plasma: about 55% of blood; formed elements: about 45%.
• Water: 90-92% of plasma; proteins: 6-8%.
• RBCs: 5-5.5 million ${\text{mm}}^{-3}$ (healthy adult man); life span 120 days.
• Haemoglobin: 12-16 g per 100 mL of blood.
• WBCs: 6000-8000 ${\text{mm}}^{-3}$.
• Neutrophils 60-65%, lymphocytes 20-25%, monocytes 6-8%, eosinophils 2-3%, basophils 0.5-1% of WBCs.
• Platelets: 1,50,000-3,50,000 ${\text{mm}}^{-3}$.
• Rh+ve: nearly 80% of humans.

8. Quick Revision

• Sponges and coelenterates move water through body cavities; higher animals use blood and lymph.
• Blood is a special connective tissue: plasma (about 55%) plus formed elements (about 45%).
• Plasma: 90-92% water, 6-8% proteins (fibrinogen, globulins, albumins), minerals, nutrients, inactive clotting factors.
• Serum is plasma without the clotting factors.
• RBCs: most abundant, 5-5.5 million ${\text{mm}}^{-3}$, red bone marrow, biconcave, no nucleus, 120 days, spleen.
• Haemoglobin: 12-16 g per 100 mL blood; iron-containing; carries respiratory gases.
• WBCs: 6000-8000 ${\text{mm}}^{-3}$, nucleated, short-lived; order N, L, M, E, B.
• Neutrophils and monocytes are phagocytic; basophils are involved in inflammatory reactions; eosinophils resist infections and are associated with allergic reactions.
• Platelets: megakaryocyte fragments, 1,50,000-3,50,000 ${\text{mm}}^{-3}$; low count causes clotting disorders.
• ABO: antigens A and B on RBCs; antibodies against the missing antigen; O donates to all, AB receives from all.
• Rh antigen on 80% of humans; an Rh-ve mother's antibodies can cause erythroblastosis foetalis in a later Rh+ve foetus.
• Clotting: cascade, thrombokinase, prothrombin to thrombin, fibrinogen to fibrin, with calcium ions.
• Tissue fluid leaves capillaries without larger proteins and most formed elements; lymphatic vessels return it to major veins.
• Lymph: colourless, lymphocytes, carries nutrients and hormones; fats absorbed in lacteals.

9. Solved Examples

10. Practice Questions

Common Mistakes to Avoid