Fact of the Day : In which year was it discovered that blood contained iron? (from The Oxford Companion to the Body)
haemoglobin Both the red badge of courage and the blue blood of the aristocrat are due to haemoglobin, the pigment that gives blood its colour. Take it away, by removing the blood cells, and the resulting plasma is a very pale yellow. Haemoglobin combines with oxygen, enabling blood to carry 70 times more than if the oxygen were simply dissolved. Animals that are physically active and larger than a pea could scarcely survive without it. ‘But for haemoglobin's existence, man might never have attained any activity which the lobster does not possess, or had he done so, it would have been with a body as minute as the fly's’ (J. Barcoft).
Haemoglobin, contained in the red cells of the blood and constituting the main site of iron in the body, is present in all vertebrate species. In the human adult it is synthesized in the developing red cells in the bone marrow. Many worms have haemoglobin, but others and also most molluscs have different and more primitive oxygen-carrying pigments, which have not survived into higher forms of evolution.
Haemoglobin not only distributes oxygen as it is required by the tissues but is also an important store of the gas. Healthy humans have about 150 g of haemoglobin per litre of blood, and this can bind with 200 ml of oxygen per litre. With the body at rest the tissues only remove about one-quarter of the available oxygen reaching them in arterial blood, the other three-quarters remaining in the venous blood returning to the lungs. This constitutes an important reserve of oxygen supply which can be called on in conditions of work and exercise. In a typical total blood volume of 5 litres, even though more than half is in the veins, we thus have about 0.75 litre of oxygen combined with haemoglobin in the blood, and we have about the same amount as gas in the lungs. If we stop breathing, for example by holding our breath, these stores will maintain the functions of the brain for at the most a few minutes — but without them brain function would cease almost immediately.
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