COMPOSITION
BLOOD consists of a suspension of cells in an aqueous solution. In an adult man there are five to six litres of blood in the body.
Cells
Red cells (erythrocytes) Minute, biconcave discs. the red cells consist of spongy cytoplasm in an elastic membrane. They have no nuclei. In their cytoplasm is a red pigment, huemmoglobn, which is a protein with iron in its molecule. it has an affinity for oxygen and readily combines with it in conditions of high oxygen concentration. It forms an unstable compound called oxy-haemoglobas which, however, Conditions of low oxygen concentration readily breaks down and releases the Oxygen. This property makes it most efficient transporting oxygen from the lungs to the tissues.
The red cells are made in the red bone-marrow of the short bones such as the sternum, and and vertebrae. There are about five-and-a-half million in a cubic milimetre of blood. A red blood cell lasts for about four months, after which it breaks down and is disintegrated in the liver or spleen. About 200.000.000.000 are formed and destroyed each day, which means that about I per cent of the total is replaced daily.
White cells (leucocytes) There are about 600 red cells to every white cell. The actual numbers vary between 4000 and 13,000 mm3. Various kinds of white cell occur: they are made n the bone marrow, the lymph nodes or the spleen. Some are irregular in shape, can change their form, and all have a nucleus. Most of them are of a type called phagocytes which can move by a Flowing action of their cytoplasm and can pass out of blood capillaries by squeezing between the cells of the capillary wall. They ingest and destroy bacteria and dead cells by flowing round, engulfing and digesting them
They accumulate at the site of an injury or infection and devour invading bacteria and damaged tissue, so preventing the spread of harmful bacteria as well as accelerating the healing of the infected region.
Platelets are tiny cells without nuclei budded off from special, very large cells in the red bone marrow and they play an important part in the clotting action of the blood. There are about 400,000 of them in a cubic millimetre of blood and they appear as tiny, round or Oval structures.
Plasma. The liquid part of the blood is called plasma, which is a solution in water of many compounds, Some of the most important of these compounds are sodium chloride. sodium hydrogencarbonate, glucose, amino acids and proteins including albumin, fibrinogen and the globulin antibodies hormones, urea and other nitrogenous compounds. In the plasma, digested food, carbon dioxide and excretory products are carried round the body
Serum is blood plasma from which the fibrinogen has been removed.
FUNCTIONS OF THE BLOOD
It will be convenient at this point to distinguish between (a) the functions of the blood as the agent replenishing the tissue fluid surrounding the cells, and (b) the circulation of blood
(a) Homeostatic functions of the blood
All the cells of the body are bathed by a fluid, tissue fluid, derived from plasma, which supplies them with the food and 0xygon necessary for their living chemistry, and removes the products of their activities which, if they accumulated, would poison the cells
The composition of the blood plasma is very precisely regulated by the liver and kidneys so that, within narrow limits, the living cells are soaked in a liquid of unvarying composition This provides them with the environment they need and enables them to live and grow in the most favourable conditions y delivering oxygen and nutrients to the tissue fluid and removing the excretory products, the blood fulfils a homeostatic function maintaining the constancy of the internal environment.
(b) Circulation
The movement of the blood in vessels round the body constantly changes the fluid surrounding the living cells so that fresh supplies of oxygen and food are brought in as fast as they are used up and poisonous end products are not allowed to accumulate, The following account is concerned principally with the circulation as a transport system, rather than with the chemical properties of blood fluid as an internal environment On the average, a particular red cell would complete the circulation of the body in 45 seconds.
1. Transport of oxygen from lungs to tissues. When exposed to the relatively high oxygen concentration in the lungs the haemoglobin in the red cells combines with oxygen forming oxy-haemoglobin. Oxy-haemoglobin decomposes when it reaches an active tissue where oxygen is being used up, and sets free oxygen which reaches the cells. Oxy-hacmoglobin is bright red, while haemoglobin is a dark red. The combination of oxygen with haemoglobin as soon as it enters the red cell, effectively removes Oxygen from solution so that its concentration as a dissolved gas inside the cell is kept very low. Thus a steep diffusion gradien is maintained between the source of oxygen and the red cell and, as a result, the rate of diffusion of oxygen into the erythrocyte is rapid.
2. Transport of carbon dioxide from the tissues to the lungs. Carbon dioxide produced from actively respiring cells diffuses through the capillary wall and dissolves in the plasma. Some of it enters the red cells and some of it forms sodium hydrogencarbonate in the plasma. In the lungs it is released, diffuses into the air sacs, and is expelled.
3. Transport of excretory material from the tissues to the kidneys. Many of the chemical activities of the body form by- products or end-products that are poisonous. These substances diffuse into capillary or lymphatic systems and are carried off in the plasma. When they eventually reach the kidneys a large proportion of them is removed and excreted
4. Transport of digested food from the ileum to the tissues.
The soluble products of digestion pass into the capillaries of the villi lining the ileum. They are carried in solution by the plasma and after passing through the liver enter the general circulation. Glucose and amino acids diffuse out of the capillaries and into the cells of the body, Glucose may be oxidized in a muscle, for example, and provide the energy for contraction; amino acids will be built up into new proteins and make new cells and fresh tissues
5. Distribution of hormones, Hormones are chemicals which affect the rate of vital processes in the body They are carried in the blood plasma, from the glands which make them all round the body. When they reach certain organs such as the heart they affect the rate at which these organs Work.
6. Distribution of heat and temperature control Muscular and chemical activity release heat These processes occurs more rapidly in some parts of the body than others; for example chemical activity in the liver and muscular action in the limbs
The heat so produced locally is distributed all round the bod by the blood and in this way an even temperature is maintained in all regions. The diversion of blood to or away from the skin also plays a part in keeping the temperature constant.
7. Formation of clots. When a blood vessel is cut open, or its lining damaged, the blood platelets and damaged tissue produce chemicals which help to convert the protein fibrinogen to fibrin. This makes a network of fibres across the wound within which red cells become entangled, forming a clot which stops further loss of blood and prevents entry of bacteria and poisons.
The platelets also adhere to the damaged area and help to form a plug before causing the fibrin to precipitate. The dried clot eventually becomes a scab which protects the damaged area while new skin is forming.
8. Prevention of infection. (a) INFECTED WOUNDS. Normally the skin provides a barrier to the entry of any bacteria. The layer of dead cells on the skin provides a mechanical barrier while the mucus and chemicals of the alimentary canal offer a chemical defence. If the skin is broken, however, and bacteria enter the cut, certain of the white cells migrate through the capillaries in that region and begin to engulf and digest any bacteria that have invaded the tissues. Many dead white cells and selfdigested, dead tissues may accumulate at the site of infection and form pus. In this way, and as a result of clot formation which prevents free circulation, the site of the infection is localized and most of the bacteria are destroyed before they can enter the general circulation. Those which escape into the lymphatic system are trapped by stationary white cells in the lymph nodes or in the spleen and liver.
Certain virulent strains of bacteria cannot be ingested by the white cells until they have been acted upon by chemicals called antibodies, made in the blood by special white antibodies are not already present in the blood or are not made quickly enough, the virulent bacteria or their products will invade the whole body and give rise to symptoms of disease.
(b) DISEASE AND IMMUNITY. Many diseases are caused by the presence of bacteria or viruses in the body, and the symptoms may be due to one or more of the following: (a) foreign proteins of the bacteria themselves ; (b) the poisonous chemicals (usually proteins) called toxins, which are produced by the bacteria; (c) the breakdown products of the infected tissue.
Recovery from the disease and subsequent immunity depend to a large extent on the production in the blood of antibodies. These antibodies are proteins released into the plasma and they may affect bacteria or their products in a number of ways:
(a) opsonins adhere to the outer surface of bacteria and so make it easier for the phagocytic white cells to ingest them,
(b) agglutinins cause bacteria to stick together in clumps; in this condition the bacteria cannot invade the tissues,
(c) lysins destroy bacteria by dissolving their outer coats, and
(d) anti-toxins combine with and so neutralize the poisonous toxins produced by bacteria.
The substances which stimulate the production of antibodies are called antigens.
When the organism recovers from the disease the antibodies remain for only a short time in the circulatory system but the ability to produce them is greatly increased so that a further invasion by bacteria or viruses is likely to be stopped at once and the person is said to be "immune to the disease. People may possess this immunity from birth, they may acquire it after recovering from an attack as in measles, or it may bee induced in them by vaccination or inoculation. Natural or acquired immunity may occur because disease bacteria are present in the body without being sufficiently numerous or suitably placed to produce disease symptoms.
A vaccine is a collection of killed disease bacteria or viruses, or forms of these treated in such a way as to prevent their reproduction. When these are injeced into the blood stream the organism undergoes a mild form of the disease and its cells manufacture an excess of antibodies. In this way immunity is artificially acquired. The period of immunity, during which antibodies can be produced rapidly, varies from a few months to many years, according to the nature of the infection.
Serum. The blood of a person or animal which has recently recovered from a disease will contain antitoxins and antibodies. If the cells and fibrinogen are removed from a sample of this blood a serun is obtained which, when injected into other people, temporary immunity or cure them it they already have the disease. Sera for treating tetanus and snake biles and prepared from borse's blood. The horse is injected with dilated poison which stimulates the formation of antitoxins in the blood. Samples of the blood are then taken from the horse, and sera prepared from the samples is used to treat cases.
CIRCULATORY SYSTEM
The blood is distributed round the body in vessels, most but 9O all of them tubular, and varying in size from about I cm to 0.001mm in diameter. They form a continuous system, Communicating with every living part of the body. Blood Sows in them, always in the same direction, passing repeatedly through the heart, the muscular contractions of which maintain the circulation.
There are three types of blood vessel, arteries, veins and capillaries, connected to form a continuous system.
Arteries are fairly wide vessels which carry blood from the heart to the limbs and organs of the body. They are thick-walled, muscular and elastic and must stand up to the surges of high pressure caused by the heart-beat. The arteries divide into smaller vessels, called arterioles, which themselves divide repeatedly until they form a dense network of microscopic vessels permeating between the cells of every living tissue. These final branches are called capillaries.
Capillaries are tiny vessels with walls often only one cell thick. Although the blood seems to be physically confined within the capillary walls, the latter are permeable, allowing water and dissolved substances, other than proteins, to pass in and out. Through these thin walls, oxygen, carbon dioxide, dissolved food and excretory products are exchanged with the tissues round the capillary. The capillary network is so dense that no living cell is far from a supply of Oxygen and food. In the liver every cell is in direct contact with a capillary. Some capillaries are so narrow that the red cells are squeezed flat in passing through them. Eventually, the capillaries unite into larger vessels, venules, which join to form veins and these return blood to the heart.
Veins return blood from the tissues to the heart. The blood pressure in them is steady and is less than in the arteries. They are wider and have thinner walls than the arteries. They also have valves in them which prevent blood flowing away from the heart. Contractions of skeletal muscles during activity compress the veins, so forcing blood along in a direction determined by the valves. This assists the return of blood to the heart. The blood in the veins will usually contain less oxygen and food, and more nitrogenous waste and carbon dioxide, while the arterial blood has a higher concentration of Oxygen and dissolved food.
Exceptions to this are the pulmonary artery which carries deoxygenated blood to the lungs, the pulmonary vein which returns oxygenated blood to the heart, the hepatic portal vein to the liver from the alimentary canal which carries blood rich in glucose and amino acids, and the renal vein from the kidney where some water, salts and urea have been eliminated.