Description
Blood is a fluid connective tissue constituting about 7% of our total body weight (about 5 liters in the human). The primary components are:
A. Plasma: The liquid in which peripheral blood cells are suspended. Composed of water, electrolytes such as Na+ and Cl, (0.9%), 7% plasma proteins (such as albumin, fibrinogen, globulins), hormones, fats, amino acids, vitamins carbohydrates, lipoproteins as well as other substances. The normal plasma volume is 40 ml/kg of body weight.
B. Formed Elements (blood cells):
- Erythrocytes (red blood cells or rbc): occupy about 40-45% of the total blood volume or 30 ml/kg body weight.
- Leukocytes (white blood cells or wbc) and Platelets: together make up about 1-2% of the total blood volume.
Freshly drawn blood is a red fluid (specific gravity of 1.052-1.064). It forms a jelly-like mass if allowed to clot. The clot is formed by the conversion of fibrinogen to fibrin which forms a meshwork that entraps the formed elements. If the gel is centrifuged, cell free serum can be obtained. If an anticoagulant is added, blood can be sedimented into three distinct layers due to different densities of elements: rbc > wbc > plasma. The lowermost layer (45% of the blood volume) consists of erythrocytes. Above this region is a grayish white layer (buffy coat) which represents the platelets, lymphocytes and granulocytes (about 1-2% volume). Plasma is the top layer.