Blood is liquid connective tissue which connects all the parts of body and body cells acts as a principal circulatory medium of animals to bring about the humoral inter communications and transport of materials in the body.

Structurally blood is a liquid tissue in which the cells or corpuscles and platelets suspended in an inter-cellular matrix fluid called plasma. Freshly shed blood is a red, thick, opaque, slightly alkaline fluid with specific gravity ranging from 1.055 to 1.060.

Composition of Blood of Mammal:

Blood of mammal is highly complex fluid composed of a liquid part called plasma and solid part includes corpuscles or blood cells and platelets.

(I) PLASMA: Plasma is the fluid matrix of blood constitutes about 55% of total volume of blood. It composed of —


(i) Water – 91 -92%

(ii) Solids – 8 – 9% – It includes the following:

(a) Inorganic elements: (0.9%) – Sodium, Potassium, calcium, Magnesium, Phosphorus etc.

(b) Organic Compounds:


It includes –

Protein – 7.5% – Serum albumin, Serum globulin Fibrinogen, Prothrombin etc.

Non-proteins (0.5 -1%) – Urea, Uric acid, Creative, Amonia, Aminoacids etc.

Fats – Neutral fats, Phospholipids, Cholestrol etc.


Carbohydrates – Glucose, Gatactose, Lactose, etc.

Pigments – Bilurubin, Carotin, Xanthophylin

Others – Enzymes, Antibodies, Hormones

(II) Blood Corpuscles (Blood Cells)


There are three types of cellular element in blood suspended in plasma. It includes Red Blood Corpuscles (R.B.C.), White Blood corpuscles (W.B.C.), and Blood Platelets. These constitute the formed elements of blood.

(A) Red Blood Corpulces (Erytgorytes):

(i) Structure:

Red blood cells of mammals are circular or discold biconcave, non-nucleated structures. The edges are thicker than the centre. In foetus these are nucleated mature R.B.C. us soft and flexible. In the cytoplasm there is a meshwork of proteins and lipids which enclose the pigment haemoglobin. Cytoplasm enclosed by the pigment haemoglobin. Cytoplasm enclosed by highly selectively permeable phopholipid membrane.


There are average 4.5-5 millions of R.B.C per cubic millimeter of blood in adult mammals. Size ranges between 6-9 micron in diameter.

The average life span is 20-120 days.

(ii) Composition:

Cytoplasm of R.B.C. composed of 60-70% water 30-40% solid out of which, haemoglobin 29%, proteins 0.5% – 1% other organic substances 0.2%, inorganic substance 0.7%.


(iii) Origin of R.B.C:

In foetus R.B.C. are formed in are a vascular of yolk sac, in mid-embroynic stage liver and spleen produce erythrocytes, whereas in pos natal adult R.B.C are differentiate into erythrocycle and are called haemocytoblasts. The process is called erythropoiesis.

(iv) Function of Erythocytes:

– Oxygen transport from lungs to tissues.

– Carbon dioxide transport from tissues to lungs.

– Maintenance of blood pH as haemoglobin acts as a buffer.

– Maintenance of ion balance in blood

– Maintainn viscocity of blood.

– Disintegration of haemoglobin produces other pigments like bilirubin & biliverdin.

(B) White Blood Corpuscles (Leucocytes):

Structure: White Blood Corpuscles (W.B.C) is colourless non-pigmented, larger sized, spherical, nucleated blood cells. Their shape varies and appeared amoeboid during active stage of phagocyzosis. Their functions are absolutely different from the erythrocytes. Their number per cubic millimeter of blood is much less than the R.B.C.

There are several varieties of white blood corpuscles on the basis of the structure and staining properties. W.B.C is broadly classified as granulocytes and agranulocytes.

(i) Granulocytes: (Granular leucocytes).

These leucocytes have a granular cytoplasm. They are formed in bone marrow from time of birth and onwards. The granules are of differentially stained and granulocytes further classified into followings:

(a) Neutrophil:

The cytoplasm takes neutral strain; nucleus is multilobed with (2-7) lobes. Their number ranges from 3000-6000 per cubic mm & blood about 60-70% of total W.B.C.

(b) Eosinophil:

The cytoplasm contains coarse granules which takes acidic strain, eosin. The nucleus is bilobed o trilobed. Their number ranges from 150-400 per cubic mm of blood or about 2-3% of total W.B.C. The size oranges between 10-15 M in diameter.

(c) Basophil:

The cytoplasm contains granules of different size which takes basic stains and turned much deep in colour. The nucleus is lobed. Their number ranges from 0.100 per cubic mm of blood and about 0.4% of total leucocytes.


(ii) Lymphocytes:

These are leucocytes with non-granular cytoplasm. Smaller in size about 8 M to 1 M in diameter and constitute 20-30% of total leucocytes. Their number ranges from 1500-2700 per cubic mm of blood. They are produced in lymph glands.

(iii) Monocytes:

These are agranulocytes, comparatively larger sized leucocytes, with 12-20 M in diameter and constitute few in number about (340-700) and aout 4 – 8% of total W.B.C. in blood.

(III) Blood Platelets: (Thrombocytes)

These are small detached part of mega karyocytes and are colorless flat granular corpuscles much smaller than erythosytes. They are probably formed in red bone marrow.

These cells contain thromboplasin in the cytoplasm. These are non-nucleated, round or oval, biconvex, discs with average size of 2.5 M in diameter. There is a red-violet central part called chromomere and ale blue rim called hyalomere. Their number ranges from 25-50 millions per cubic mm of blood. They initiate blood clotting.

General Function of Blood

Most important functions of blood in mammals have been outlined as follows:

(i) Transport of oxygen and carbon dioxide: As blood contains haemoglobin it combines with oxygen forms oxy-haemoglobin and transport oxygen from respiratory surface to the body tissue and carbon dioxid from tissue as respiratory by-product transported by blood to respiration surface.

(ii) Transport of food: Digested food absorbed the blood and transported to each cell through circulation.

(iii) Transport of waste products: Nitrogen wastes and other waste products transported by blood to the organs of excretion for their elimination from body.

(iv) Chemical co-ordination: Hormones secreted by endocrine systems transported by blood regulates chemical co-ordination in body.

(v) Maintenance of pH: Plasma protein keeps the pH constant in blood.

(vi) Water balance: Blood maintains water balance constant.

(vii) Transport of head and temperature regulation: Blood allows the transfer of heat from tissue to tissue and maintains body temperature constant through out the body.

(viii) Defence against infection: Blood contains W.B.C. posses phagocytic activity kills the foreign antigens and neutralize the toxins produced by infectious agents.

(ix) Blood coagulation: Blood loss from injury is protected due to coagulation in clotting of blood.