Blood is not a passive watery substance pumped endlessly round the body. It is a complex, ever-changing fluid with many different functions. Vitally important in theare the three main kinds of cell: red , white and platelets. They are so small and numerous that each drop of blood contains millions of them. In a random sample of a thousand blood perhaps one will be a white cell, the majority will be red cells, and 50 or 100 will be platelets. Blood cells are made in the bone marrow. The marrow is a granular, fatty, red substance found inside bones. In adults the bone marrow is produced in the spine, pelvis, skull, ribs, sternum (breastbone) and the long arm and leg bones, although in infancy all bones contain marrow. The liver and spleen play an active part in storing blood cells and breaking them down at the ends of their lives. All blood cells are derived ultimately from the same cell type, known as stem cells. The stem cells are special in that they can perpetually renew themselves and therefore, in a y adult, maintain their numbers for continuous production of new cells throughout life. The ‘offspring’ of the stem cells develop while in the marrow through many intermediate stages into the three mature blood-cell types. If fresh blood is examined through a microscope only a very occasional immature cell will be seen. The bone marrow can, if necessary, increase its output of blood cells six to ten fold if a person has suffered from blood loss or has an infection.
The red blood cell (also called an erythrocyte or red corpuscle) is small even for a cell, measuring about 8 thousandths of a millimeter in diameter and 2 thousandths of a millimetre in thickness. It is unlike all other cells in the body in that it has no nucleus. This means it cannot reproduce and so cannot multiply by division and must be replaced when it dies. It is an extremely flexible, dished disc capable of squeezing through capillaries only half its diameter. There are about five million red cells per cubic millimetre of blood, and red cells make up about 45 per cent of the blood volume. Each cell normally has a lifespan of 120 days before breaking up and being filtered out of the circulation, mainly by the spleen. More than two million red cells are made every second to keep up with the loss, and on average each cell makes one journey around the body in one minute. Healthy production relies on adequate supplies of iron, vitamins – particularly folic acid and vitamin B12 – plus thethyroxine, Cortisol and erythropoietin.
The prime function of red cells is to carry oxygen from the lungs to the, and it is the red pigment haemoglobin in the cells that enables this to happen.
Small as the red cell is, it contains approximately 640 million haemoglobin molecules. Haemoglobin consists of four iron complexes (haem groups), each attached to a protein chain (globin), with the four protein chains precisely interwoven. When the red cell comes into contact with dissolved oxygen in the capillaries of the lungs, each haemoglobin molecule picks up four oxygen molecules. The oxygen is then held tightly by the minute chemical forces produced by the alignment of the four protein chains. When the red cells arrive at oxygen-requiringthey encounter an acid environment. This is the result of the carobonic acid that is formed in metabolically active cells. This acid causes the shape of the haemoglobin molecules to change, slightly releasing their hold on the oxygen atoms and allowing them to pass out of the red cells into the tissues.
Lack of red cells results in a reduced oxygen-carrying capacity of the blood. Such a condition is generally known as anaemia. The causes may be a chronic blood loss (because of a haemorrhage), an impaired production – because of a lack of iron or vitamin Bu -or an enhanced destruction of red blood cells – for example because the haemoglobin that is produced is abnormal, as in sickle-cell disease. Red-cell formation is partly regulated by a hormone called erythropoietin produced by the kidney. In response to anaemia the hormone is released and stimulates red-cell production in the marrow. Similarly, if the body is starved of oxygen, as happens in the rarer atmosphere at high altitudes or in someone with lung disease, erythropoietin is released and more red cells are produced.
The white cells, known collectively as leucocytes or white corpuscles, are larger but not so numerous as red cells. There are about 10,000 per cubic millimetre of blood during childhood, and about 7,000 in adult life. There are three main types: granulocytes, lymphocytes and monocytes. The granulocytes – so-called because their cell fluid contains minute grains – can be subdivided in neutrophylic, eosinophylic and basophylic granulocytes.
All the white blood cells have different functions to perform. They are all derived from special stem cells in the bone marrow. The lymphocytes have to undergo special transformations in the lymph nodes before they can perform their function. The neutrophils are the most numerous of the white cells, making up between 40 and 75 per cent. Their main function, as part of the body’s defence mechanism, is the destruction of microbes. The bone marrow is capable of rapidly producing large numbers of neutrophils to cope with an infection.
After a few hours circulating in the blood they enter the tissues ready for action. There they survive four or five days unless called to defend the body, in which case they are used up very rapidly. If a blood sample is examined when you are infected, an increase in the number of white cells will be recorded. Around an area of inflammation or infection, various substances are released that act as ‘signals for help’, and neutrophils answer the call and migrate to the site. Once there they recognize their prey because it has an antibody stuck to it. The antibody encourages the neutrophil to engulf its target. Once this occurs enzymes in the white cell destroy the organism.
The general area becomes filled with dead white cells and other debris – this is the pus of infection. Eosinophils are usually present only in small numbers, 1 to 6 per cent, and their function is not well understood. They are involved in allergic reactions, such as those that occur inand hay fever. Basophils, at less than 1 per cent of total white cells, are also involved in allergy, although how is not clear. Lymphocytes make up 15 to 40 per cent of the white cells and are an essential part of the body’s immune system. They are responsible for the production of antibodies, which recognize invasion by a substance such as a virus or a transplanted organ, and try to destroy it.
Immunologists calculate that any one person has a potential repertorry of more than 100 million different antibodies to cope with almost every imaginable invading substance. It is because of the lymphocytes and their production of antibodies that in general you do not get a disease such as measles for the second time. The first time the body is infected, it takes some days before enough antibodies are produced to cope with the virus. During those days the symptoms of the disease manifest themselves and examination of the blood reveals abnormally large numbers of lymphocytes. When the body gets reinfected at a later time, the lymphocytes ‘recognize’ the virus at once and immediately start to produce special antibodies. In this way the virus is dealt with at an early stage. Symptoms of the disease do not develop: the body is said to have become immune against the virus in question.
Lymphocytes themselves cannot distinguish between unwanted foreign bodies and wanted ones such as transplanted organs. Modern anti-rejection (or immunosuppressive) drugs damp down the lymphocytes’ reaction to foreign tissue so that a transplanted organ has a better chance of survival. Monocytes, forming between 2 and 10 per cent of white cells, act as scavengers, scouring the tissues in their search for the debris of broken-down cells and micro-organisms which they consume.