Both white and red blood cells are formed in the bone marrow. Each red blood cell measures about 7.5 microns (thousandths of a milimetre) in diameter and is shaped a little like a doughnut. Red blood cells contain haemoglobin, which gives them their red pigment. There are 5 to 6 million red cells per cubic mm of blood. The red cell only survives about 120 days and the damaged and old cells are removed by the spleen and liver.
A white blood cell is not really white but almost transparent. It can change shape, push out folds and finger-like projections and crawling like an amoeba in a pond. These cells survive less than a week.
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Fact File: An average adult body has about 5 litres of blood. At any one time, about 1,250 ml are in the arteries, 3,500 ml in the veins and about 250 ml in the capillaries. The cells in blood flow through a capillary for only half a second before they move into the next type of vessels, small veins. |
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