The Cardiac Output

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 yentrickfonly, and that the total output is twice as much. The reason that the output of the left ventricle is given this special name is that it supplies the entire body (except the lungs) with the blood. Another reason is that it does a much greater amount of work than does the right, and consequently is more likely to fail.

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

1. The heart rate is normally controlled by a balance between impuls­es reaching it over the vagus and over the sympathetics. Thus, inhibition of the vagus centre speeds up the heart. And inhibition of the sympa­thetic centre slows down the heart. It seems that in the human most of the effect is achieved by inhibition of the vagal centre of the sympathetic region.

2. The second factor affecting cardiac output is the stroke volume, that is, the amount of blood which the left ventricle ejects per beat. 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 - as it is in exercise — 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.

The increased venous return in exercise is brought about in the fol­lowing manner: 1) after muscles contract, they exert a «milking» effect on the blood vessels which they contain. With each contraction, blood is squeezed out¹ into the veins; it cannot be squeezed back into the arteries because the arterial pressure is high - and with each relaxation the blood vessels of the muscle again fill up with blood; 2) in exercise, breathing becomes deeper. The heart lies within the thorax; when the thorax expands, blood is «sucked² into» the heart.

The two factors, working together, lead to the increase of the blood amount returned. First the right side of the heart, and within a few beats the left, are dilated and take bigger «bites» of blood. Thus the stroke volume is increased.

Notes

1. to squeeze out просачиваться

2. to be sucked всасываться

Lesson 7 ^ 109

Часть III

Контрольно-обобщающие упражнения к уроку 7