Nature of the Heart Beat. The cardiac Output

Heart beat does not require nerve impulses. It is rhythmic: the ventricles relax fully before the next contraction. The period of time during which the muscle is not responsive to a stimulus is called the refractory period. When the heart muscle is stimulated, it will contract but will not respond again to that stimulus until it has relaxed. Even the heart forced to beat rapidly maintains a perfectly rhythmic beat with a little rest period in between.

The heart is a double pump; the volume expelled by the right ventricle is the same as that expelled by the left. When exercise is suddenly undertaken, the amount of «venous return» suddenly increases.

Cardiac output refers to the volume of blood which the left ventricle forces into the aorta per minute of time. It must be noted that this term refers to the output of the left ventricle only. The reason is that the left ventricle does a much greater amount of work than the right one. The total output is twice as much.

Cardiac output is the product of two factors: heart rate (the number of beats per minute) and stroke volume (the volume expelled per beat).

The heart rate is normally controlled by a balance between impulses reaching it over the vagus and the sympathetic nerves. Thus, inhibition of the vagus centre speeds up the heart. And inhibition of the sympathetic centre slows down the heart.

The stroke volume depends upon the «venous return». The normal heart is capable of a considerable degree of enlargement; after the venous return is increased the chambers of the heart are able to supply the additional blood. The walls of right atrium and the great veins are thin and stretch readily; therefore the heart rate is increased.

Задание 3. Ответьте на вопросы:

1. Whatis called the refractory period?

2. When does the amount of «venous return» suddenly increase?

3. What does cardiac output refer to?

4. What is cardiac output?

5. What is the heart rate normally controlled by?

6. What speeds up the heart?

7. What slows down the heart?

8. What volume depends upon the «venous return»?

9. What does the stroke volume depend upon?

10. When are the chambers of the heart able to supply the additional blood?

Задание 4. Заполните пропуски подходящими по смыслу словами, данными ниже:

1. … is a double pump.

2. The left ventricle does a much greater … than the right one.

3. Cardiac output is the product of … and stroke volume

4. Inhibition of the vagus centre … the heart.

5. … relax fully before the next contraction.

6. Heart beat does not require … .

7. Inhibition of the sympathetic centre … the heart.

8. Cardiac output refers to the volume of blood which the left ventricle forces into … per minute of time.

9. The period of time during which the muscle is not responsive to a stimulus is called … .

10. When the heart muscle is stimulated, it will … .

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nerve impulses, the aorta, the refractory period, contract, speeds up, the heart, the ventricles, slows down, heart rate, amount of work

III. Работа над грамматикой. Past Perfect/Future Perfect Active.

Past Perfect

Время Past Perfect обозначает действие, которое завершилось до некоего момента в прошлом:

The doctor on duty had begun his morning before the professor came.

He had finished his experiment by 6 sharp.

Случаи употребления:

Past Perfect употребляется для выражения:

1. Действий, которые начались и закончились до момента в прошлом. Момент может быть обозначен временем, другим действием или ситуацией.

The students had finished their laboratory work by then. – К тому времени студенты закончили лабораторную работу.

This patient felt wretched. He hadn’t slept well for two nights. – Пациент чувствовал себя разбитым. Он плохо спал две последние ночи.

After everybody had left, he began packing hurriedly. – После того, как все ушли, он начал в спешке упаковывать вещи.

2. Последовательных действий, предшествовавших моменту в прошлом.

Suddenly I realized that someone had opened the door and had put a letter.

Future Perfect

Время Future Perfect обозначает действие, которое закончится до определенного момента или начала другого действия в будущем или будет продолжать длиться после него.

They will have written the theses before he comes.

I shall have examined them by 5 o’clock p.m.

Случаи употребления:

Действие, которое начнется и закончится до определенного момента в будущем:

You will have spent much efforts before you can run a marathon.
Ты потратишь много усилий, прежде чем сможешь пробежать марафон.

время форма	Present Perfect	Past Perfect	Future Perfect
утвердительная	I have consulted a doctor.	I had already consulted a doctor by 4 o’clock.	I shall have consulted a doctor by 4 o’clock tomorrow.
вопросительная	Have you consulted a doctor?	Had you consulted a doctor by 4 o’clock?	Will you have consulted a doctor by 4 o’clock tomorrow?
отрицательная	I have not consulted a doctor yet.	I had not consulted a doctor by 4 o’clock yesterday.	I shall not have consulted a doctor by 4 o’clock tomorrow.