THE BLOOD

Blood is a specialized form of connective tissue consisting of formed elements, or blood cells, and fluid intercellular substance, the blood plasma. The volume of blood in the healthy adult human is about five litres, and quantitatively blood constitutes about eight per cent of the body weight.

The cellular or formed elements of blood are of three main types. These are:

1. red blood corpuscles or erythrocytes;

2. white blood corpuscles or leucocytes;

3. blood platelets.

Erythrocytes.The erythrocytes, or red blood corpuscles, are highly differentiated cells that functionally are specialized for the transportation of oxygen. In mammals the erythrocyte is a cell that has lost its nucleus and its cytoplasmic organelles during development. Each cell is shaped like a biconcave disc. The corpuscles are elastic and are capable of considerable distortion, as is evident in their ability to pass through capillaries of small calibre. They average about 8,5 micrometers in diameter and 1,9 micrometers in thickness.

The erythrocytes are much more numerous than any of the other formed elements of blood. In human males there are 5 to 5,5 million erythrocytes per cubic millimetre; in females, 4,5 to 5 million.

Each erythrocyte is bounded by a delicate plasma membrane which is a lipoprotein complex. Below the plasma membrane is a cytoskeletal complex of two layers: a vertical granular and a horizontal filamentous meshwork. The meshwork is mainly constructed of a contractile protein distinctive to erythrocytes, called spectrin. Spectrin maintains the biconcave shape and allows for efficient flow of oxygen and carbon monoxide within interior.

Chemically the content of the erythrocyte consists of a lipid and protein colloidal complex, principally hemoglobin. It is because of the presence of hemoglobin the erythrocytes (and blood as a whole) are red in colour. Hemoglobin plays an important role in carrying oxygen from the lungs to all tissues of the body. In healthy person there are about 15 grammes of hemoglobin in every 100 millilitres of blood.

Erythrocytes are formed in bone marrow from where they enter the blood stream. Each erythrocyte has a life of about one hundred to one hundred and twenty days.

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