| The heart is really two pumps, each composed of an upper and lower chamber and two valves, one between the upper and lower chamber and one at the exit from he lower chamber. The right side of the heart is the less muscular of the two pumps; it receives oxygen-depleted blood from the venous system and sends it to the lungs for oxygenation. The other half, the left side, receives oxygen-rich blood from the lungs and sends it shooting up through the aorta and around the vhole body.
Branching off the aorta are arteries and arterioles; their walls are muscular and elastic, designed to transmit the pumping force of the heart to the farthest reaches of the body. Blood is collected from every part of the body by venules and veins, which are thin-walled and have valves in them so that blood can flow one way only - back to the heart. The pumping action of muscles helps to push blood though them.
Connecting the smallest tributaries of the arterial and venous system are the capillaries, blood vessels so small that in major organs and muscles there are thousands of them per square millimetre. Oxygen-carrying red blood cells are pushed through them by the pulsing pressure of the heart. Blood pressure depends on three things: the tone of the artery walls, the pumping force of the heart, and the volume of blood in circulation. Generally speaking, blood pressure creeps up with age. This is because the kidneys become slightly less efficient at regulating the water content of the blood, and because arteries tend to become less elastic, forcing the heart to work harder.
The arch enemy of arteries is atheroma, a fatty deposit which roughens their smooth linings, reduces heir elasticity, and slowly narrows their diameter. Choked or furred up arteries spell strokes, heart attacks, thrombosis, embolisms, and aneurysms.
Blood is the body's universal transport system. It carries everything the body needs - oxygen, sugars, fats, proteins, hormones, minerals, clotting factors, antibodies - as well as everything it does not need carbon dioxide, other dissolved gases, urea, hostile micro-organisms and their toxins, general debris. A grown man has about 5 litres of blood in him, about 8 per cent of his body weight. Plasma, a straw-coloured fluid which is mostly water, accounts for 55 per cent of blood volume; red and white blood cells and platelets make up the rest. Red cells and platelets - the red cells carry oxygen bound to an iron-containing pigment called haemoglobin, and the platelets prevent blood loss by initiating the clotting process - far outnumber white blood cells, though during infections the latter multiply enormously. White blood cells are part of the immune system, protecting the body against disease organisms. At the end of their useful life, which is seldom longer than three months even in the healthiest of us, red and white blood cells are destroyed and recycled by the liver and spleen. Destruction must of course keep pace with creation; in a healthy individual the bone marrow produces as many red blood cells as the spleen and liver destroy.
The lymph glands, the glands which swell up during infection because they are busy producing a variety of white blood cells, are part of a secondary transport network, the lymphatic system. The spleen, tonsils, bone marrow, and thymus are also part of the lymphatic system, whose vessels ramify to every part of the body. Lymph vessels have much thinner walls than veins, but like veins they have one-way valves in them and depend on the pumping action of muscles for their circulation. They transport lymphocytes to sites of injury and infection, collect emulsified fats from the small intestine, and drain fluid from the spaces between cells - this fluid contains proteins and other valuable substances which cannot be actively absorbed by the blood vessels. The contents of the lymphatic network eventually empty into the bloodstream through two ducts, one on either side of the neck at the junction of the subclavian and internal jugular veins. |
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