Haemoglobin

I

Introduction

Haemoglobin, most prevalent of the special blood pigments that transport oxygen; it is present in all but the least complex of animals. Haemoglobin carries oxygen from the lungs or gills, where blood is oxygenated, to body cells. When saturated with oxygen it is called oxyhaemoglobin and is a bright red colour. After haemoglobin releases oxygen to the body tissues, it reverses its function and picks up carbon dioxide, the principal product of tissue respiration, for transport to the lungs, where it is expired. In this form, it is known as carboxyhaemoglobin and it is a purply-red colour.

The erythrocytes or red blood cells are ideally adapted for carrying oxygen. They contain haemoglobin, which gives them their red colour and is actively involved in oxygen transport. The shape of the cells means that they have a large surface area to volume ratio for the diffusion of gases, and having no nucleus means that there is the maximum amount of space available to pack in haemoglobin molecules. In fact, each red blood cell contains around 250 million molecules of haemoglobin, giving it the capacity to carry 1,000 million molecules of oxygen. To combine properly with oxygen, the red blood cells must contain adequate haemoglobin; this, in turn, depends on the amount of iron in the body. The organism derives its store of iron by absorption from the gut. The organism conserves and constantly reuses the supply of iron. A deficiency of haemoglobin caused by a lack of iron leads to anaemia.

II

The Chemistry of Haemoglobin

Haemoglobin is the key to oxygen uptake. It is a very large organic molecule made up of four globin polypeptide chains. Each chain has a prosthetic haem group which contains iron and gives the molecule its red colour. The shape of the protein chains and the way they are folded around the haem groups is vital to the way the molecule functions, and any changes in the chemistry of the blood that might affect the shape of these polypeptides could be lethal. Haemoglobin has a high affinity for oxygen—which means it takes up oxygen very readily. The oxygen is bound quite loosely to the haem groups to form oxyhaemoglobin.

Haemoglobin reacts with oxygen in a remarkable way. The first oxygen molecule to be attached alters the shape of the haemoglobin in such a way that it is easier for the next oxygen to be taken on. This in turn alters the shape and makes it easier for the next oxygen to be taken up, until the fourth and final oxygen molecule combines with the haemoglobin several hundred times more rapidly than the first. The same process happens in reverse when oxygen dissociates from haemoglobin: it becomes progressively harder to remove the oxygen. This has very important implications for the way in which oxygen is taken up in the lungs and released in the respiring cells.

An additional factor in the functioning of haemoglobin in the body is the effect of carbon dioxide. The way in which haemoglobin takes up and releases oxygen is affected by the proportion of carbon dioxide in the air. As the proportion of carbon dioxide increases, the haemoglobin curves move downwards and to the right. This is known as the Bohr shift. The effect of this Bohr shift is that in higher partial pressures of carbon dioxide haemoglobin needs higher levels of oxygen to become saturated. More importantly, it gives up oxygen more easily. Thus in active tissues with high carbon dioxide levels the haemoglobin releases the oxygen needed very readily. On the other hand, carbon dioxide levels in the lungs are relatively low, and so oxygen binds on to the haem groups very easily.

The haemoglobin of the foetus is slightly different from adult haemoglobin and it has a greater affinity for oxygen. This means that as the maternal blood flows through the placenta, the foetal haemoglobin takes the oxygen from the maternal haemoglobin and carries it to the tissues and cells of the developing foetus.

Haemoglobin does not only combine with oxygen and carbon dioxide. It combines so firmly with the carbon monoxide found in cigarette smoke, faulty gas fires and car exhaust fumes that it can no longer combine with oxygen. If the carbon monoxide levels are too high this causes asphyxiation and death.

Alterations in the structure of haemoglobin can lead to life-threatening illnesses. The most important of these conditions is sickle-cell anaemia, which involves a hereditary change in one of the amino acids that make up haemoglobin. The thalassaemias are a group of hereditary diseases of similar origin.

III

The Breakdown of Haemoglobin

After a life of perhaps 120 days, red blood cells are destroyed in the spleen or, in the course of circulation, their haemoglobin is broken into its constituent parts, including iron, which enters new blood cells formed in the bone marrow. When blood vessels rupture, as in an injury, the red cells are released and escape into tissue, where they are broken down eventually. The haemoglobin is converted into bile pigments, the colour of which is responsible for the appearance of bruises.