Blood (Function, blood groups, blood coagulation and Rh factor)
Blood is a highly complex, liquid tissue composed of loose cells or corpuscles (45%) and fluid, plasma (55%). Specific gravity of blood ranges between 1.05 to 1.06 and it is salty in taste.
A. Blood Plasma:
It is a complex sticky aqueous solution and contains a liquid called serum, a coagulant, fibrinogen and an anti-coagulant, heparin. Its specific gravity is about 1.027.
B. Blood-cells or corpuscles:
Blood contains the following three kinds of loose cells (corpuscles):
I. Erythrocytes (red blood cells):
Erythrocytes (R.B.C.) are nucleated in all vertebrates except mammals and contain haemoglobin as respiratory pigment. They are formed in the red bone marrow during childhood, adulthood and old age while in the liver and spleen during embryonic development. The average life of R.B.C. is about 120 days.
Due to the presence of haemoglobin.
R.B.C. performs the functions of (i) transporting O2 from lungs to tissues (ii) transporting CO2 from tissues to lungs (iii) maintenance of blood pH and (iv) maintenance of blood viscosity.
Ii. Leucocytes (white blood cells):
Leucocytes (W.B.C.) are devoid of any pigment, and therefore, are colourless. They are found in lesser numbers as compared to R.B.C. They are formed in red bone marrow and lymphatic glands. The average life span of leucocytes is about two weeks. Leucocytes are divided into two categories:
Cytoplasm is granular and nuclei are large with two or more lobes. They are divided into three types:
They constitute about 2% to 3% of the total W.B.C. and are stained with acid dyes such as eosin. They bring about destruction and detoxication of toxins of protein origin
They are about 0% to 4% of the total W.B.C.
They are commonly called polymorphonuclear leucocytes as they have many- shaped nuclei. They constitute 65% to 70% of the total W.B.C. and are the most active of all the W.B.C. in terms of mobility and ingestion of foreign particles.
(2) Agranulocytes: These corpuscles do not have any granular cytoplasm. Nuclei are simple without lobes. These corpuscles can be grouped into two heads:
(i) Lymphocytes: They constitute 20% to 30% of total leucocyte count. They produce antibodies.
(ii) Monocytes: They form 4% to 8% of total W.B.C. They are motile in nature and can engulf bacteria.
Iii. Thrombocytes (blood platelets):
These are small, colourless, non-nucleated cells probably formed in the red bone marrow. They contain thromboplastic substance which helps in the clotting of blood. Their number varies between 2, 50,000 to 4,00,000 per ml of blood. The life span of these corpuscles is only two to three days. They perform the most important function, i.e., blood coagulation and prevent excessive loss of blood during haemorrhage.
Blood coagulation :
When blood is shed, it loses its fluidity and becomes a semi-solid jelly. This phenomenon is called coagulation or clotting. Only thrombocytes (blood- platelets) take an active part during this process.
Blood-platelets become sticky and rapidly adhere to the damaged blood vessels by binding to connective tissues. They also adhere to one another and form a plug which stops the bleeding. Fibrinogen, prothrombin, thromboplastin and calcium ions play important roles during the process.
Blood groups :
Blood group was discovered by Landsteiner (1900) who classified blood into four well defined groups. Blood grouping is based on the presence or absence of specific antigens located on the surface of R.B.C. In human blood, two types of antigens namely A and B are found in the R.B.C. and two types of antibodies namely a (anti A) and b (anti B) are found in the blood plasma.
Based on the presence or absence of these antigens and antibodies, the human blood is divided into the following four groups:
1. Group ‘A’ has the antigen A and antibody b.
2. Group ‘B’ has the antigen B and antibody a.
3. Group ‘AB’ has both A and B antigens but no antibody.
4. Group ‘O’ has both a and b antibodies but no antigen.
Rh factor :
This factor was discovered by Landsteiner and Weiner (1940) in the R.B.C. of Rhesus monkey. If a person’s R.B.C. contains the Rh antigen, he is said to be Rh positive and if there is no Rh antigen, he is Rh negative. Normally, blood of an Rh negative person does not carry any antibody, but such bodies may be produced if Rh negative blood comes across Rh positive blood.
If Rh positive blood is transfused into the body of Rh negative person, the Rh positive R.B.C. serves as foreign bodies to the Rh negative person and so his body proceeds to produce antibodies called anti Rh- agglutinin which destroy the Rh positive R.B.C.
One transfusion is not serious but repeated transfusions may result in severe reaction and ultimately the death of the Rh negative person. Indian population is 93% Rh positive and 7% Rh negative.