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Пособие.Увлекательный мир биологии.doc
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The heart

Each pump in the heart has two chambers: an entry chamber called an atrium (plural atria) and a pumping chamber called a ventricle. A one-way valve between each atrium and ventricle prevents blood being pumped the wrong way. There is also one-way outlet valve from each ventricle to stop blood flowing back. The ventricles have thick muscular walls. When these contract, the pressure inside increases and blood is forced into the arteries. This causes your pulse. When the muscles relax, the ventricles expand and blood flows into them from the atria.

Your heart beats about 70 times every minute, day and night. To do this, the heart muscles must have a good supply of oxygen. This is supplied through a system of arteries spread over the heart wall. These are called the coronary vessels.

Diseases of the circulation system

As the heart pumps blood into the arteries, there is an increase in pressure. This pressure on the walls of our arteries is called blood pressure. If the blood pressure is too low, blood circulates around the body too slowly. If the blood pressure is too high, there is greater strain on the heart and on the walls of the arteries. The walls of the arteries may be stretched so much that they burst. If an artery in the brain bursts, the leakage of block will damage cells in the brain. We call this a stroke. This may result in paralysis (loss use) of parts of the body and speech difficulties. The causes of high blood pressure are not fully understood. However, it is more likely to occur when someone is old, overweight, under stress or a heavy smoker.

As we get older, our arteries become thicker and harder. This is due to a layer of fat forming on the inside of the vessels. The arteries become narrower and the flow of blood slows down. An artery may become blocked by a clot of blood. This is called a thrombosis. If a thrombosis occurs in one of the coronary vessels (called a coronary thrombosis), the heart muscle is deprived of oxygen and may stop beating. We call this a heart attack. Hardening of the arteries is thought to be made worse by eating too much animal fat, by drinking too much alcohol and by smoking.

Tobacco smoke contains several poisonous substances, including nicotine and carbon monoxide. Carbon monoxide affects our blood because it combines with haemoglobin in red blood cells about 300 times more readily than oxygen. This means that the blood can carry less oxygen to our body tissues. In order to improve the supply of oxygen, the heart pumps faster, putting strain on itself and increasing the blood pressure.

People with heart problems in which their heart beats too weakly can be helped by pacemakers. Pacemakers contain very small nuclear batteries with a long life. The battery is placed inside the chest near the heart. It produces tiny regular shocks to keep the heart beating steadily.

Exercises

1. Continue the sentences based on the text:

  1. Blood looks like ordinary… .

  2. …transports food, waste products and other chemicals.

  3. …carry oxygen around the body.

  4. …help protect us against disease.

  5. Phagocytes are cells which … by engulfing them.

  6. Lymphocytes are cells which… .

  7. …help the blood to clot.

  8. Fibrinogen can be removed from… .

  9. …is stored in hospitals for transfusions.

  10. Blood carries …, …, … .

  11. Blood helps to keep our body temperature constant by… .

  12. The main organ involved in blood circulation is… .

  13. …carry blood from the heart.

  14. …carry blood back to the heart.

  15. …divide from the arteries and them rejoin to form veins.