THE CIRCULATORY SYSTEM: THE COMMON CARRIER FOR THE BODY 95 



In response to variations in the circulatory needs of the body, the normal heart 

 is capable of making great adjustments in rate and power of beat and in the volume 

 of blood discharged at each stroke. While the body is at rest, the amount of blood 

 discharged from each ventricle averages about 50 cc. per stroke; at maximum 

 capacity, the heart can increase this amount to about 150 or 200 cc. per stroke. 

 This is not, as might be thought, a contradiction of the all-or-none principle 

 already described as governing the contraction of the heart. Under a given set of 

 conditions, it remains true that each beat of the heart is as strong as every other 

 beat and that the strength of the beat is independent of the strength of the stimu- 

 lus. But in heart muscle, as in skeletal muscle, the muscle fibers (within limits) 

 contract the more powerfully the more they are stretched when contraction 

 begins. If for any cause the volume of blood entering the auricles is increased, this 

 stretches the heart muscles, and the resulting contraction is more powerful and 

 expels a greater volume of blood. Increased muscular activity, accompanied by 

 more rapid and powerful respiratory movements, is one of the more common 

 conditions that will cause an increased flow of venous blood into the heart, auto- 

 matically bringing about an increase in the strength of the heart contraction. 



The mechanism controlling the rate of heartbeat is quite complex, involving 

 physical, chemical, and nervous stimuli. 1 We shall be able only briefly to consider 

 some of the factors involved. First of all, we may recall our earlier statement that 

 the beating of the heart is "automatic," produced by an "inner stimulus," and 

 that it goes on even in the absence of outside stimulation. The heart is, however, 

 subject to influences that modify its independent activity. It is connected with the 

 central nervous system by two nerves 2 — the vagus and the cervical sympathetic. 

 Cutting these nerves does not stop the action of the heart; their function is merely 

 regulatory. Impulses from the vagus slow the heartbeat or, if sufficiently intense, 

 may cause it temporarily to cease. 3 Impulses from the cervical sympathetic nerve 

 accelerate its rate. Both these nerves possess "tone"; i.e., they both transmit 

 impulses more or less continuously, and the rate of the heartbeat is in large part a 

 resultant of the balance between the stimuli received from the two nerves. The 

 vagus may be thought of as the "brake" upon heart action; the cervical sym- 

 pathetic nerve, as the "accelerator." Increase in the inhibitory impulses from the 

 vagus may be brought about reflexly by stimulation of certain sensory nerves, 

 including those of the viscera, skin, and sense organs. In this connection, the 

 sensory nerves of the aorta, the venae cavae, and right auricle are of particular 

 importance. For instance, sensory stimuli caused by increase of blood pressure in 

 the aorta affect the vagus, slowing the heartbeat and thus automatically lowering 

 the pressure and safeguarding the heart and arteries against excessive strain. 



1 The pulse rate is generally more rapid (1) in the female than in the male; (2) dur- 

 ing and immediately after muscular activity; (3) during digestion or mental excite- 

 ment; and (4) after sudden changes in position. The rate also changes with age; at 

 birth it may be 130 to 150 beats per minute; in adult life, it averages about 72 per 

 minute; and in old age it falls to about 67 per minute. 



2 Actually, a right and left vagus nerve and a right and left set of cervical sympa- 

 thetic nerves. 



3 Under exceptional circumstances, vagal stimulation may cause death from cessa- 

 tion of circulation; but normally, after a certain period of inhibition, the heart escapes 

 from the control of the vagus and resumes its beat. 



