Blood is known as fluid connective tissue. Its fluid matrix is the plasma in which blood cells or corpuscles float freely and are carried on alongwith bloodstream. Its color is red and it is slightly alkaline having pH 7.3 to 7.5. In an adult man about 6-7 litres of blood is present.
The straw or faint yellow coloured plasma (blood-corpuscles) which is about 60% of the total blood volume constitutes of the following:
(i) Water 90 to 92%
(ii) Inorganic compounds like chlorides, bicarbonates, phosphates, sulphates and iron constitute about 0.9%.
(iii) Organic constituents form about 8.1%.
(a) Plasma or serum proteins- 6.8 %.
Blood viscocity, osmotic pressure and blood volume are maintained by these proteins. Serum globulines, serum albumins, fibrinogen and prothrombin are the major types of plasma proteins.
Since blood serves the transport of nutritive materials plasma is laden with nutrients like glucoses, fats,and lipids , lactic acids, pyruvic acid, choletsterol, etc. it also has various enzymes and hormones.
Excretory substances like urea, uric acids, ammonia, creatine and creatinine are carried in the plasma.
Hormones, vitamins, enzymes, antigens, antibodies and various drugs in patients under treatment are present in the plasma.
Oxygen, carbon dioxide and nitrogen etc. gases remain in dissolve atate in plasma.
Three types of blood cells constitute about 40% of blood volume. They are as follows:
(i) Erythrocytes or Red Blood corpuscles (R.B.C.s):
These are circular biconcave anucleated disc-like cells measuring 7.8µ in diameter and 2 µ in thickness at periphery. The natures of erythrocytes are devoid of any types of cell organelles. Their cytoplasm is mostly occupied by haemoglobin. The normal Erythrocyte count in men is 5-5.5 million person cubic millimeter of million. Their lifespan is 120 days in human .due to their biconvex structure they have a tendency for rouleaux formation. Excess Erythrocytes are stored in the spleen (blood bank). Degeneration of old and worn out RBCs also takes place in spleen (graveyard of RBCs).
(ii) Leucocytes or white blood corpuscles (W.B.Cs)
These are large coulorless, nucleated cells. Their number is very less (7000-8000 person cubic millimeter of blood). In children their number is higher but among adults in pathological condition the count is also goes up. Their size ranges from 8µ to 15µ. These are phagocytic and amoeboid in nature. They are said to be the mobile bodyguard in the defence system of the body. These are formed in lymph nodes, spleen and thymus. Old and damaged leucocytes are degenerated in blood, liver and lymph. Their lifespan is very short.
Types of Leucocytes:
The leucocytes are broadly divided into granulocytes and agranulocytes depending on the presence or absence of granules in their cytoplasm and the shape of the nucleus.
These are characterized by granual cytoplasm and lobed nucleus. Depending on the type of dyes taken by granules, these are of following three types.
It has a multilobed nucleus and its cytoplasmic granules are stained with neutral dyes. 67% to 70 % leucocytes are neutrophils. By their phagocytic nature, they kill foreign bacteria.
Its nucleus is ‘S’ shaped and the cytoplasmic granules take up basic stains. Of the total leucocytes, 0.5% to 1% are basophils. They engulf carbon and dust particles.
(iii) Acidophil or Eosinophil:
These are large leucocytes with a bi or trilobed nucleus and large cytoplasmic granules, which have affinity for acidic dyes. 3% of total leucocytes are eosinophils. Toxins produced by bacteria are neutralized by them. They also destroy the particles formed due to the reaction of antigen and antibodies.
B. Agranulocyte or Lymphocytes:
These contain rounded nucleus but no cytoplasmic granules. These are two types of lymphocytes as follows:
(i) Small lymphocytes- Its size is 8µ in diameter. It has a large nucleus. They form about 25% of total leucocytes.
(ii) Large lymphocytes- Its diameter is about 11 µ and nucleus is large. About 3% of leucocytes are of this type.
The lymphocytes take part in the immune system of the body. They produce antibodies against antigens.
Having 15-µ diameter these are largest leucocytes and they form about 1.5% of the total leucocytes. The nucleus is large, oval and occupies one side of the cell. They are phagocytic in nature.
The leucocytes are formed in the liver and spleen in embryo while in adult they are produced in spleen, lymph glands and bone marrow.
C. Blood platelets:
Platelets are 2-4 µ in diameter, rounded biconvex, anucleated colourless cells produced from megakaryocytes of the bone marrow. Person cubic millimeter of blood about 2.5-3 lakhs of platelets are seen survive for 10-12 days. Their role in coagulation is very important.
Functions of Blood:
(1) Transport of oxygen – The haemoglobin of Erythrocytes carry oxygen from lungs to tissues.
(2) Transport of carbon dioxide – Carbon dioxide produced in the tissues passes being dissolved in plasma as well by haemoglobin to lungs for its diposal.
(3) It transports nutrients from alimentary canal to tissues.
(4) Excretory matters are carried to the excretory organs.
(5) Transports of vitamins and hormones to different parts of the body.
(6) It regulates body temperature.
(7) Blood loss due to injury is prevented by mechanism of blood coagulation.
(8) Protections against diseases- Leucocytes destroy the foreign germs by their phagocytic nature. They also form the immune system.
(9) Water balance in the plasma and the tissue cells is maintained by blood.
(10) Blood pressure in penis and nipples helps in their erection.