Human body
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Blood
All the tissues and organs that make up the body must be continuously supplied with food and oxygen. The job of transporting these essentials is done by the blood. Pumped by the heart, blood picks up dissolved food from the intestines and oxygen from the lungs, then delivers them to all parts via the arteries. Veins carry blood containing waste carbon dioxide from the tissues back to the heart to be pumped to the lungs. With about 5 litres (9 pints) flowing around an adult’s body, blood also clears away waste, clots to prevent leaking where blood vessels are damaged, protects against invading bacteria and viruses, and helps keep the body at a steady 37°C (98.6°F).
What blood is made of
Seen under a microscope, blood is made up of millions of tiny cells floating in a yellowish, watery fluid called plasma. Nutrients and other substances needed by the body’s cells are dissolved in the plasma. The plasma picks up waste carbon dioxide and returns it to the lungs. There are red cells, used for carrying oxygen; white cells, which fight any infection by invading bacteria or viruses; and platelets, which make the blood clot when a vessel is damaged, so sealing the wound.
One tiny drop of blood the size of a pinhead contains about 5 million red blood cells, 9000 white cells and about 250,000 fragments of cells, called platelets. Nutrients are dissolved in the plasma. About 90% of blood is water.
Red and white cells
Red cells carry oxygen from the lungs to all parts of the body. They are doughnut-shaped “sacks” of haemoglobin, a substance that combines very readily with oxygen. Different kinds of white blood cells work together to protect you from disease: these include T cells, which travel around the bloodstream identifying invaders; B cells, which make deadly proteins called antibodies that surround and disable the invaders; and phagocytes, which swallow up the disabled invaders and destroy them.
Making blood cells
Red and white blood cells and platelets are made in the marrow, a jelly-like substance found in the insides of some of our bones. In children, whose bones are growing quickly and contain a large amount of marrow, about 200 billion blood cells are produced every day. The new cells enter the bloodstream through the blood vessels that run through the bones. Red blood cells have an average lifespan of 120 days. They are finally transported to the spleen and liver, where they are destroyed and recycled.
Arteries and veins
The arteries and veins in your body look like a page in a road atlas. There are motorways—the main blood-carrying tubes or vessels, which lead out from the heart to the limbs and the head—as well as lanes and tracks, tiny vessels called capillaries that reach all the cells in the body. Altogether, your blood vessels are about 100,000 kilometres (60,000 miles) long, or more than twice the distance around the world.
There are three main types of blood vessels: arteries, veins and capillaries. Arteries carry blood from the heart to the body’s tissues, while veins do the return journey. Arteries have much thicker walls than veins, with broader elastic and muscle layers, that enable them to resist the high-pressure surge of blood produced by every heartbeat. You can feel this pumping action by placing your fingers on the inside of the wrist, a point where a main artery lies close to the surface of your skin.
Veins carry blood containing carbon dioxide and other wastes from the tissues towards the heart. Their walls are thinner than those of arteries because blood flowing through them is at a much lower pressure. The larger veins have valves inside them that open and close rather like canal locks. The valves ensure that the flow of blood is constant and that it cannot leak back.
Capillaries
Arteries divide into smaller branches called arterioles, which in turn divide into tiny tubes called capillaries. Each capillary is only one hundredth of a millimetre wide. The walls are extremely thin, enabling vital substances in the blood—including oxygen and nutrients—to seep through to the body’s cells, while carbon dioxide and other waste can pass the other way. Capillaries link up to form venules, which in turn link up to form veins, which lead back to the heart. While the cherry-red blood in arteries is rich in oxygen, the dark red blood in veins contains less oxygen and more carbon dioxide.
Blood groups
People belong to one of four blood groups, or types: A, B, AB and O. Which group a person belongs to depends on the presence or absence of proteins called antigens—A and B—on the surface of red blood cells.
Group A blood has A antigens on red blood cells, while group B blood has B antigens. Group AB blood has both A and B antigens, while group O blood has neither. If someone has a blood transfusion, it is important that they are given blood of the same group as their own. Your blood group is determined by genes inherited from your parents.
In addition to these groups, everyone either has or does not have D antigens on the surface of their red blood cells. If your red blood cells have D antigens, your blood is described as Rhesus positive; if you do not, your blood is Rhesus negative. Rhesus positive or negative is normally written with + or - after the name of the blood group (for example, A+ or O-). This information is also used to supply the correct blood for blood transfusions.
Hormones
Two systems help the body’s parts and organs work together. One is the nervous system. The other is the hormonal or endocrine system, based on body chemicals called hormones.
There are more than 50 different hormones. Each is made in a gland. Hormones flow around the bloodstream and affect certain cells, tissues and organs. They may cause them to work faster or slower, or release their products. For example, adrenaline from the adrenal gland makes the heart beat faster and more blood flow to the muscles, so the body is ready for action. The pituitary gland near the brain makes hormones that control other hormonal glands.
Consultant: Richard Walker