96 THE INDIVIDUAL ORGANISM 



Heartbeat rate commonly varies inversely as the blood pressure, although since 

 the controlling factors are numerous and are not the same for the two phenomena, 

 the relation is naturally not a fixed one. As an illustration of a case in which this 

 relationship does hold, the heartbeat is rapid immediately following a severe 

 hemorrhage, when blood pressure is very low. In this condition, no great force of 

 heartbeat is possible or necessary, the heart chambers not being much distended; 

 but the small amount of blood left in the vessels must be distributed as quickly 

 and used as often as possible. 



The emotions also have a pronounced effect upon the heart. Strong emotion, 

 by stimulating the vagus, may slow the heart sufficiently to cause fainting. An- 

 other factor influencing the rate and power of the heartbeat is the presence of 

 varying amounts of chemical substances in the blood. Thus increase in carbon 

 dioxide and lactic acid concentration in the blood, as a result of muscular activity, 

 tends to dilate the arteries and capillaries and by thus causing an increased flow 

 of blood back to the heart affects the rate and power of its beat. One of the most 

 important chemical effects is that produced by the hormone adrenalin, a substance 

 produced by the adrenal glands and discussed more fully in the later section on 

 the endocrine glands. This substance causes a rise in blood pressure, accompanied 

 by strengthening of the heartbeat. In the absence of nervous control, it also 

 increases the rate of heartbeat, but when the nerves are intact, a reflex inhibition 

 slows the heartbeat rate and thus prevents the blood pressure from rising too high. 

 Adrenalin also has numerous other effects on the circulatory system, including 

 constriction of the arteries and capillaries of the skin and viscera and dilation of 

 those of the heart and skeletal muscles. 



The efficiency of the heart as a pumping organ is strikingly shown by 

 the rapidity with which the blood is circulated. According to Best and 

 Taylor, the time required for a given blood corpuscle to make the pulmo- 

 nary circuit (heart to lungs and back to heart) averages about 1 1 seconds 

 in man. The length of the different systemic circuits varies so much that 

 the time spent in traversing this part of the system cannot be accurately 

 stated. It takes about 4 seconds for the blood to go from the heart to 

 the capillaries of the forearm and about 6.6 seconds to return to the heart. 

 It is calculated that the time required for the complete double circuit, 

 from the right side of the heart to the lungs, back to the left side of the 

 heart, out to the systemic capillaries, and back once more to the heart, 

 probably averages 25 or 30 seconds. Blood going to the foot would, of 

 course, take much longer than this to complete its circuit. 



The amount of blood pumped by the heart is also of interest. For a 

 heart with a stroke volume of 75 cc, a rate of 70 beats per minute, and a 

 total volume of blood in the body of 5,000 cc, a volume of blood equal 

 to all that contained in the body will pass through a single chamber of 

 the heart in a little less than 1 minute. 1 This does not mean, of course, 



1 70 X 75 cc. = 5,250 cc. 



