More information about the blood and circulation

The blood circulates every minute at rest, and much faster when we exercise. Besides the red blood cells, whose major function is the transport of oxygen to the tissues and the removal of carbon dioxide, followed by the exchange of carbon dioxide for oxygen in the lungs, there are other cells, such as the lymphocytes and granulocytes which are involved in our defence mechanisms, and platelets which are involved, amongst other things, in blood clotting. Besides the cells there are chemical messengers such as adrenalin, glucose that we need for energy, waste products of metabolism such as urea, salts and minerals essential for the functioning of all our cells, chemicals such as antibodies used in our defences and protein molecules which transport many other molecules, and which are intimately involved in the maintenance of the circulation itself.

The blood cells - both red and white, and the platelets - are made in bone marrow (in the foetus they are initially made in the liver and spleen). They come from cells called stem cells, which have the potential to produce a number of different cell types. The red blood cells contain haemoglobin. The haem group, which is a porphyrin ring containing iron, binds oxygen as the blood passes through the lungs, and releases it in the tissues. Each haemoglobin molecule has 4 haem groups, each attached to a protein chain. Foetal haemoglobin - called HbF - has a different structure to adult haemoglobin, with a higher affinity for oxygen. At birth, HbF makes up about 80 - 90% of circulating haemoglobin; by the age of 1 this has dropped to 5%. Besides iron, we need vitamins such as vitamin B12 and Folic acid to produce haemoglobin. The process is under the influence of a hormone called erythropoetin, made in the kidney and increased when there are low levels of oxygen, such as happens, for example, at high altitude.

The white blood cells have different functions: neutrophils attack bacteria, while lymphocytes are involved either in killing viruses (T - cells) or in producing antibodies (B - cells) as part of our defence mechanisms. There are other white blood cells such as monocytes and eosinophils which have more specialised functions, and macrophages, which are involved in both our defences by attacking tumours, and the removal of waste material which results from the activities of the other cells.

When the heart beats, blood is forced into the arteries, which have muscular walls. The contraction of the heart is transmitted to the muscle of the arteries, which is what we feel when we feel our pulse. The arteries branch out into smaller vessels called arterioles. The walls of the arteries and the arterioles are impermeable (nothing passes through), but the smaller vessels - the capillaries - to which the arterioles lead, are leaky, so there is an exchange of oxygen and carbon dioxide, the passage of cells, and the transfer of glucose and urea between the blood and the tissues. The capillaries drain into small vessels called venules, which join together to produce veins. These are the blue vessels we see under the skin. Blood flows back to the heart through these veins not because of its pumping action - the contraction does not reach beyond the capillaries - but because of the negative pressure produced in the chest when we breathe in, and the pressure on the veins caused by contraction of muscles in our legs. There are valves in the veins which ensure that the blood only flows one way.

Besides being subject to disorders which arise in the components of the blood itself, there are many changes which arise as a consequence of disorders in other tissues and systems of the body, which is what you would expect in a medium which has major functions of transport and communication.