Sistema circulatorio en ingles.

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- Why does our heart beat faster during exercise? Why doesn’t the heart return to its normal rate of beating as soon as you finish exercising?

Answer: Our heart beats faster during exercise because it is supplied by nerves which maintain a faster rate which can be adjusted to meet the body’s needs for oxygen. The heart doesn’t return to its normal rate because there is still the hormone “adrenaline” during exercising, which causes the heart rate to increase (and it takes some time for the adrenaline to leave the circulatory system.

- Complete the table

FUNCTION

TYPE OF

BLOOD

STRUCTURE OF THE WALL

REASONS FOR THE STRUCTURE

Veins

Return blood from tissues to heart.

Deoxygenated, less food, more carbon dioxide

(Exception of pulmonary veins).

Walls thinner, less elastic, and less muscular.

Contraction of body muscles compress thin-walled veins (valves in these veins prevent blood flowing backwards, so they can be compressed in this way).

Arteries

Carry blood from the heart to the limbs and organs of the body.

Oxygenated (exception of pulmonary arteries).

Elastic tissue and muscle fibres in their thick walls. Larger arteries have more elastic tissue.

Arteries need elastic tissue to stand surges of high pressure caused by the heartbeat.

Capillaries

Exchange of oxygen, food carbon dioxide and waste products between tissue fluid and living cells.

Both oxygenated (before exchange with living cells) and deoxygenated (after living cells exchange).

Thin vessels (one cell thick).

Thin capillaries wall allow some liquid to pass through.

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- What is the PACEMAKER? Explain location and function

Answer: The heart beat is initiated by the pacemaker, a small group of specialized muscle cells at the top of the right atrium. The pacemaker receives two sets of nerves from the brain. One group of nerves speeds up the heart rate and the other group slows it down. These nerves originate from a centre in the brain that receives an input from receptors in the circulatory system that are sensitive to blood pressure and levels of oxygen and carbon dioxide in the blood. If blood pressure rises, nervous impulses reduce the heart rate. A fall in blood pressure causes a rise in the rate. The heart rate is adjusted to meet the needs of the body at times of rest, exertion and excitement.

- What is BLOOD PRESSURE? Why does it vary?

Answer: The pumping action of the heart produces a pressure which drives blood round the circulatory system. In the arteries, the pressure fluctuates with the heartbeat, and the pressure wave can be felt as a pulse. The millions of tiny capillaries offer resistance to blood flow and, by the time the blood enters the veins, the surges due to the heart beat are lost and the blood pressure is greatly reduced.

Although blood pressure varies with age and activity, it is normally kept within specific limits by negative feedback. The filtration process in the kidneys needs a fairly constant blood pressure. If the blood pressure falls significantly because, for example, of loss of blood or shock, then the kidneys may fail. Blood pressure consistently higher than normal increases the risk of heart disease or stroke.

- Explain the BLOOD CLOTTING process

Answer: When the tissues are damaged and blood vessels cut, platelets clump together and block the smaller capillaries. The platelets and damaged cells at the wound also produce a substance which acts, through a series of enzymes, on the plasma protein called fibrinogen. As a result of this action, the fibrinogen is changed into fibrin, which forms into a network of fibres across the wound. Red cells become trapped in this network and so form a blood clot. The clot not only stops further loss of blood, but also prevents the entry of harmful bacteria into the wound.