PHYSIOLOGY OF TO-DAY 



three hundred years ago, Harvey wrote: "Meanwhile 

 I know and state to all that the blood is transmitted 

 sometimes in a larger amount, other times in a smaller, 

 and that the blood circulates sometimes rapidly, sometimes 

 slowly, according to temperament, age, external, or in- 

 ternal causes, normal or abnormal factors, sleep, rest, food, 

 exercise, mental condition and such like." Harvey was 

 here referring, of course, to the quantity of blood which 

 circulated around the body in a given time, which is the 

 same quantity as that pumped by the heart in the same 

 time. The problem of the cardiac output is here clearly 

 stated, and a qualitative expression of the main factors 

 which influence the output is given. 



Since Harvey's time, attempts have been made in various 

 ways by a long line of investigators to determine the cardiac 

 output of man and the variations of this function under 

 different conditions. It is obvious, of course, that some 

 indirect method must be devised; the output of the human 

 heart cannot be measured directly. At first, guesses were 

 made as to the amount of blood pumped out by the heart 

 of man based on a measurement of the total capacity of 

 the heart as determined on the cadaver. But, when it was 

 later found from experiments on animals that the heart 

 does not completely empty itself at each beat, the value 

 of such estimates became doubtful. After the development 

 of methods for keeping the isolated heart alive by perfusion, 

 accurate direct measurements of the output of such a heart 

 could be obtained. Based on the relative size of man and of 

 the animal from which the heart was taken, a calculation 

 could be made for the human cardiac output. However, 

 although this line of investigation has added enormously 

 to our knowledge of the function of the heart, it tells us 

 rather what the heart can do than what it actually does 

 in a normal man or animal. 



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