Composition Of Blood – Blood is fluid connective tissue, which circulates throughout the body. It transports oxygen and nutrients to all parts of the body, down to the individual cells and brings back the waste products of metabolism from all over the body. It consists of a fluid portion, the plasma and the blood cells. Erythrocytes (red blood cells), leucocytes (white blood cells) and thrombocytes (platelets) are suspended in the plasma. Plasma minus fibrinogen (blood clotting protein), is called serum.

A normal human adult has 6-7 litres =1 litre blood, of which 55% is plasma i.e. 3.70 litres = 0.55 litres and 15% blood cells i.e. 2.55 litres = 0.45 litre.


Plasma is a very complex fluid, which consists of about 90% water, and 7-8% proteins (albumins, globulins &fibrinogen). 2-3% of plasma contains glucose, amino acids, lipids, enzymes, hormones, antibodies and traces of many organic and inorganic materials including dissolved gases.

The normal average number / mm3of erythrocytes in persons living at high altitudes i.e. hilly areas is much higher than those living in plain. The average number of erythrocytes in these persons is nearly 7 million/.mm3 Do you know why this is so? The reason is that at high altitudes the oxygen is lower than the plains. So in order to meet out the oxygen requirements of the body the number of erythrocytes becomes high.

Blood Cells

The blood cells are of three types.

(i)Erythrocytes or Red Blood Cells (RBCs)

(ii)Leucocytes or White Blood Cells (WBCs)

(iii) Thrombocytes or Platelets

Erythrocytes or Red Blood Cells

In a normal human individual their number on the average is approximately 5 million / mm3of blood. In birds and mammals, the erythrocytes are produced in bone marrow. In mammals the mature erythrocytes lose their nucleus, mitochondria, Golgi bodies, and endoplasmic reticulum. They have biconcave disc-like shape. In all other vertebrates, the erythrocytes are nucleated and have an oval or ellipsoidal shape. The primary function of erythrocytes is to transport gases. The erythrocytes contain hemoglobin which is the respiratory pigment. It transports oxygen, from the lungs to other tissue of the body, and transports carbon dioxide from the tissues to lungs.

Practical Work: Study the blood smear under microscope.

1. Place a drop of anticoagulant like 1% sodium citrate solution on the slide and add a drop of fresh blood of chicken in the anticoagulant.

2. Spread the blood smear on the slide with the help of another slide edge.

3.Dry the spread for few minutes and observe under the microscope.

4. Draw the blood cells seen and compare it with the cells that you have studied.

5.Do not use blood or cheek smear from students , it may lead to serious consequences like contraction of aids and hepatitis.

Leucocytes or White Blood Cells

These cells are colourless and larger in size than the erythrocytes. They are nucleated and motile cells. These cells are of several types such as neutrophils, monocytes, lymphocytes, eosinophil and basopphils, which differ in structure and also in function. White blood cells defend the body from disease by engulfing and destroying germs and by producing antibodies. The total number of all these kinds of leucocytes is much less than the erythrocytes. In human beings their number ranges from 5000 to 7000 / mm3 of blood.     

Thrombocytes or Platelets

In mammals (including man), the blood in addition to erythrocytes and leucocytes contain platelets. The platelets are not complete cells (Fig. 12.13). They are minute and non-nucleated cell fragments. Their number in the circulating blood ranges between 150,000 to 350,000  /mm3of blood. They play clotting and are essential for it.

Blood pressure

Blood pressure is the pressure per unit area, which the flowing blood exerts on the walls of the blood vessels. In human beings, the blood pressure is measured at the artery of the upper arm. The measurement of blood pressure indicates the higher or systolic blood pressure and the lower or diastolic pressure. In normal young adults the systolic blood pressure is 120 mm Hg and the diastolic pressure is 80 mm Hg. Thus the normal blood pressure is expressed as 120/80 mm Hg.