2O4 ESSENTIALS OF PHYSIOLOGY. 



The compensatory power of the heart enables it to adapt itself to 

 transient variations in arterial blood pressure. When the work of the 

 heart is -permanently increased by a continuously high blood pressure, 

 the heart wall hypertrophies, just as skeletal muscles enlarge as the 

 result of exercise. The effort of prolonged increase in the work done 

 by the heart is seen in the hypertrophy observed in athletes (athlete's 

 heart). 



When the heart beats rapidly the diastolic pause is shortened, less 

 blood enters it between the beats, and its output per beat is diminished. 

 Hence its output in a given time (e.g. a minute) is not necessarily 

 greater when the heart is beating rapidly than when it is beating slowly, 

 The increased rate of the heart, however, which usually accompanies 

 increased diastolic filling, is of advantage, since by preventing over- 

 distension during diastole it enables the heart to contract under the 

 most favourable conditions as regards its mechanical efficiency. 



Work of the Heart. Since the work done by the heart is measured 

 by its output multiplied by the arterial pressure (Q x R), it will be 

 altered either by a rise or fall of arterial pressure, or by variations in 

 the output of the heart, or by these two factors varying simultaneously. 

 Whenever the arterial blood pressure rises the heart does more work, 

 since the output remains unchanged. Again, increased filling of the heart 

 brought about by any of the causes already mentioned, by increasing 

 its output, will add to its work ; and the larger output from the heart 

 tends in itself to raise the arterial blood pressure, and thus to increase 

 still further the work done. In muscular exercise there is both a rise 

 of arterial blood pressure and a greater diastolic filling of the heart. 



In man, during exercise, the output of the heart per beat may be 

 90 to 100 c.c., and since the heart is beating much more rapidly, its 

 output per minute may be two or three times as great as during rest, 

 and its work is enormously increased. These facts indicate how im- 

 portant it is that persons, whose hearts have become less efficient owing 

 to disease, should be kept at rest. 



SECTION III. 

 THE PULSE. 



If the arterial system consisted of a series of rigid tubes, the blood 

 forced into it from the heart would, in accordance with the laws of hydro- 

 statics, cause an instantaneous rise of pressure throughout the whole 

 system, and an equal quantity of blood would at once escape from the 

 distal end of the system. Owing to the fact that the arteries are dis- 

 tensible, only a fraction of the blood entering the arterial system at 



