haemoglobin

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 15 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.

The amount of oxygen free in solution in the blood plays no important role in carriage of oxygen to the tissues. The amount depends on the pressure of the gas in the lungs (see figure). If we breathe pure oxygen the amount in solution rises almost seven-fold and it can become a significant contribution to the body. If we were to breathe pure oxygen in a chamber at a pressure of three atmospheres, all the oxygen we need could be carried in solution and we would not need haemoglobin. This treatment is used for some conditions when haemoglobin is seriously deficient, but there are significant hazards of breathing high-pressure oxygen.

Each haemoglobin molecule consists of four iron-containing parts (haems) and four protein chains (globins). The fact that blood contained iron was discovered in 1747 by Menghini, who showed that if blood was burnt to an ash, iron-like particles could be extracted by a magnet. Chemical analysis of haemoglobin began in the mid nineteenth century and culminated in one of the great early triumphs of molecular biology, when in the 1960s the full chemical structure of haemoglobin was worked out.

Each haemoglobin molecule can combine with four oxygen molecules, but with no more. The complete combination is called oxygen saturation. The degree of combination depends on the pressure of the gas; in healthy humans the pressure in the alveoli of the lungs is above that needed for saturation. If the alveolar oxygen pressure is increased, for example by breathing more deeply or by inhaling pure oxygen, the haemoglobin in the blood will not take up any additional oxygen (see figure). However in patients with arterial blood not saturated with oxygen, for example with lung or heart disease, stimulation of breathing or administration of oxygen should increase the oxygen carriage in the blood and be beneficial or life-saving.

The combination of oxygen with haemoglobin is not related linearly to the oxygen pressure, and this is crucially important in its function. As oxygen pressure reduces below that required for full saturation, haemoglobin is relatively little desaturated until and unless the oxygen pressure reaches about the level which blood normally encounters in the oxygen-using tissues: it then parts with it readily. Thus in breath-holding, or in disease, or at altitude, alveolar oxygen pressure can approach half its normal value before haemoglobin saturation declines steeply in the blood leaving the lungs; and saturation is not itself halved until the oxygen pressure is reduced by almost two-thirds. Thus the properties of haemoglobin defend the oxygen supply against interruptions of breathing or shortage of oxygen in the atmosphere, whilst promoting its off-loading around the body.

The combination of haemoglobin and oxygen is weak, and oxygen can be pulled from the blood if the surrounding pressure of oxygen is low; indeed a vacuum will extract all the oxygen from a sample of blood. When blood flows through the capillaries of tissues which are using oxygen for metabolism, the low oxygen pressure in the tissue cells draws oxygen from its combination with haemoglobin and the gas flows into the cells. The resulting venous blood contains less than its full oxygen saturation, and the haemoglobin is partly ‘deoxygenated’. Such haemoglobin does not have the bright red colour of saturated haemoglobin, but is more blue. Thus conventionally arterial blood is red and venous blood is blue. In cyanosis tissues are bluish because their blood is deficient in oxygen.

Haemoglobin can also combine with carbon dioxide to form carbaminohaemoglobin, and this is one way in which this gas is carried round the body. The two gases have a complex chemical interaction with haemoglobin. When in metabolizing tissues carbon dioxide enters the blood, its combination with haemoglobin results in a weaker affinity for oxygen, which is split off and enters the cells. The reverse happens in the lungs. Temperature has a similar effect: if local temperature rises oxyhaemoglobin splits more easily. Both mechanisms help to match gas exchange to changing activity.

Red cells also contain 2,3-diphosphoglycerate (DPG), a substance that increases the readiness with which haemoglobin gives up its oxygen. The DPG is increased in exercise and at high altitude, which facilitates the supply of oxygen to the tissues. Unfortunately this process takes several hours. Stored blood loses its DPG and is therefore less effective on transfusion than fresh blood, although there are ways to treat it that restore the DPG.

Although the haem is the essential part of the haemoglobin molecule to enable it to combine with oxygen, it is the four globin molecules which determine the amount of binding or affinity for haemoglobin and oxygen. The globins are identified by Greek letters, and a very large number have been discovered, many of them related to diseases of the blood. Healthy human adults have two a-globins and two b-globins. Fetuses have two a-and two g-globins. As a result fetal haemoglobin has a stronger affinity for oxygen than does the adult form. When maternal blood flows through the placental circulation, oxygen diffuses across the placental barrier into the fetus and, because of the difference between the two haemoglobins, the fetus extracts a proportionally higher amount of oxygen. This success in parasitism is clearly to the advantage of the fetus. After birth the fetal haemoglobin is slowly replaced by the adult version.

In healthy humans, haemoglobin is only found in erythrocytes, the blood red cells. The advantage in confining the haemoglobin in cells is threefold. First, if the haemoglobin were free in solution it would give the blood a treacle-like consistency, and the heart would be unable to force it fast enough through the capillaries. Second, the chemical environment in the red cell, including for example the presence of DPG, allows the haemoglobin to take up and release oxygen with greatest efficiency. And third, if the haemoglobin were free in solution it would be excreted and lost in the kidneys. Patients with red cell breakdown, for example in malaria, pass haemoglobin into the urine, where it is broken down to the brown pigment methaemoglobin; hence one form of malaria is called ‘black-water fever’.

A few animals — some of the worms mentioned earlier — do have free oxygen-carrying pigments in the blood, but their molecular sizes are 40 times that of haemoglobin, so they are not excreted. One species of antarctic fish is said to lack both red cells and haemoglobin, but it lives in a cold environment and its metabolism and oxygen requirement must be very low.

Human red cells live on average about 120 days in the bloodstream, and then they become fragile and are broken up, especially by scavenger cells in the spleen and liver. The haemoglobin is not released into the blood, but is immediately broken down into haem and globins. The haem is in turn split into iron, which forms chemical compounds as part of the blood iron pool available for future haemoglobin synthesis, and an amber pigment, bilirubin, which contributes to the pale colour of plasma. Bilirubin combines with albumin in the blood, and the large size of this combined molecule prevents it from being excreted in the kidneys. Instead it passes to the liver, where it is excreted in the bile, contributing to its colour. When it reaches the intestines it is acted on by bacterial flora, and forms the brown pigment stercobilinogen. Most of the stercobilinogen appears in the faeces, giving it its characteristic colour (but not its odour), and the rest is reabsorbed into the bloodstream. Here a proportion recirculates in the bile but most, now called urobilinogen, is excreted in the urine. Thus not only does haemoglobin provide the colour of the blood, but its breakdown products are largely responsible for the colours of plasma, bile, faeces, and urine. Jaundice is due to an excess of bilirubin in the blood and tissues.

There are many diseases caused by abnormal haemoglobins. In all of them it is the globin part of the molecule which is abnormal. Not only may haemoglobin be unable to combine normally with oxygen, but since the haemoglobin is an integral part of the structure of the red cell, these cells may be deformed. An example is sickle cell disease, where the red cells become rigid and deformed and break down more readily, leading to anaemia. Another common disease is thalassaemia, where there is a defect in the synthesis of the b-globin chains. Less common conditions are the persistence of fetal haemoglobin long after birth, and abnormalities in the enzymes associated with haemoglobin (e.g. DPG) that affect its affinity for oxygen.

John Widdicombe

See also anaemia; blood; blood transfusion; carbon dioxide; cyanosis; jaundice; oxygen; respiration.