Quantity of Blood in the Body

QUANTITY OF BLOOD IN THE BODY The body of an adult man contains about 5 litres of blood, usually rather less. The quantity in man may be measured in one of two ways. Each depends upon putting a known quantity of some substance into the body, and, after it has been distributed uniformly over the whole circulating fluid, measuring the degree of dilution which has taken place. The two substances used are (I) carbon monoxide and (2) vital red.

(I) Carbon monoxide to the extent of about 25ocu.cm. (the exact quantity being known) is inhaled from a special apparatus; the gas unites with the haemoglobin of the blood, producing a compound the concentration of which can be measured spectro scopically.

(2)

Vital red is a pigment for which it is claimed that its injec tion into the blood stream has no deleterious effects, that the quantity injected does not leave the blood for some time, but in its entirety remains dissolved in the plasma. A known quantity of vital red is injected into a vein, a sample blood is subsequently withdrawn and centrifuged, the plasma is tinted red with the dye and the depth of the tint is a measure of the degree to which the dye has been diluted.

Probably different species and indeed different individuals in the species differ considerably in the quantity of blood contained in their vessels, some being more full blooded than others. As a very rough estimate the blood volume is somewhere around i 7 of the body weight in man. It is believed to increase at great heights, under tropical conditions and during anaemia.

Considerable loss of blood may take place without serious ill effects, but if the volume decreases beyond a certain point the blood pressure commences to drop and a condition of shock supervenes. Besides the blood which actually circulates in the arteries, veins and capillaries, the body possesses reserves which can be mobilized. One such is known to be located in the spleen. On the onset of haemorrhage the spleen shrinks, squeezing blood as from a sponge into the circulation, thus some animals may lose a fifteenth of their blood without the volume of that fluid in circulation suffering any appreciable decrease, the spleen con tributing as much as is lost by the haemorrhage.

The blood may lose in volume from causes other than haemor rhage, i.e., through exposure or injury, especially to the visceral organs, and a condition may arise in which the capillary walls appear to become unduly permeable and dilated; the plasma then escapes into the tissue spaces, the number of corpuscles per cubic millimetre increases, they being left in the vessels when the fluid escapes and ultimately the volume of blood becomes so small that the arterial pressure cannot be maintained. The condition is known as "surgical shock" and can be closely simulated by the introduction into the circulation of the drug histamine.