Discuss the composition and functions of mammalian blood.

Blood is a specialized and circulating tissue composed of cells suspended in a fluid intercellular substance which circulates through a closed system of blood vessels (arteries, veins) due to pumping action of blood. When blood is centrifuged, it separates into two distinct fractions: the upper fraction is a clear yellowish (straw-colored) fluid called plasma. The remaining fraction, comprising less than half of the tube, is packed with formed cells, consisting of red cells, white cells and platelets.

Plasma is an aqueous solution containing substances of low or high molecular weight that make up 10% of its volume. Plasma proteins account for 7% of its volume. They have molecular weight > 50000 gm/mole. Glucose, lipids, amino acids, vitamins, hormones, sodium chloride, other electrolytes, inorganic mineral salts, urea, uric acid and creatinine make up the other fraction of substances with molecular weight < 50000. Horses and cows fed on green forage have bright yellow plasma. The plasma in most animals is milky white after a fatty meal. A red colored plasma results from increased fragility and rupture of red blood cells.

The main plasma proteins are albumin; alpha, beta and gamma globulins; lipoproteins and proteins that participate in blood coagulation such as prothrombin and fibrinogen. Albumin is the most abundant and is important in maintaining osmotic pressure of blood. Globulins serve as major sources of antibodies.

When blood clots, a colorless fluid portion oozes out. This is serum. It differs from plasma in that the fibrinogen portion is not present in the fluid portion of serum. Essentially serum is plasma minus the other clotting factors of plasma. Serum is useful in prevention and treatment of disease because it contains antibodies.

The formed elements of blood include red blood cells, white blood cells and platelets. Erythrocytes constitute 32% of the total amount of blood. Under a microscope they appear as biconcave discs, circular in shape, flexible and anucleate. Erythrocytes of birds, fish and reptiles are nucleated. Those of camels are also nucleated and the nuclei are oval in outline and biconvex. Red cells are formed in the red bone marrow of the proximal ends of certain long bones and also in ribs, sternum vertebrae and in the pelvis. Erythrocytes function in the transport of gases by the blood. Oxygen diffuses from blood to metabolizing body cells while CO2 diffuses from these cells to blood.

Hemoglobin consists of a protein called globin united with four non-protein groups called hemes. Each heme contains an iron atom that is able to combine reversibly with one oxygen molecule. When hemoglobin combines with oxygen its called oxy-hemoglobin. When not carrying oxygen its called reduced hemoglobin.

Although oxygen is carried in association with the iron of the heme groups of hemoglobin, CO2 is carried in reversible association with the protein portion of the hemoglobin molecule. The CO2-globin complexes are called carbamino-compounds. Carboxyhemoglobin is a stable compound formed when CO (present in exhaust fumes of cars, buses etc.) combine with hemoglobin. This compound is unable to carry oxygen and the animal essentially dies of suffocation.

Hemolysis is a breakdown of red blood cells so that the hemoglobin escapes into plasma. It may be caused by bacterial toxins, snake venoms, blood parasites etc. hemolysis leads to hemoglobinemia and it gives plasma a reddish color.

Leucocytes are colorless cells found along with red cells in blood plasma. They are larger and fewer than red cells. They are all nucleated and contain no hemoglobin, are capable of independent movement. Apart from being present in blood may leucocytes are also found in lymphoid tissue such as thymus, lymph nodes and spleen.

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marto answered the question on April 16, 2019 at 08:56