BIOLOGY IN HUMAN AFFAIRS 



Many other quantitative data of great interest have 

 also been obtained by the use of these methods for de- 

 termining the output of the human heart, but there is 

 still need for more information concerning variations of the 

 normal under different physiological conditions, not to men- 

 tion the application of the methods in disease. One of the 

 most surprising conclusions, however, from the results 

 already obtained is that the heart rate is no indication of 

 the amount of blood which the organ is discharging per 

 minute. Thus, a fast pulse may be accompanied by a de- 

 creased, and a slow pulse by an increased, output per 

 minute. 



Concerning the regulation of the cardiac output of the 

 human subject, much remains to be determined. We know 

 with certainty that ingestion of food, changes of external 

 temperature, psychic disturbances, lowered oxygen tension 

 in the inspired air, and muscular exercise all change the 

 cardiac output, but we are at present far from understanding 

 the mechanism by which these changes are brought about. 

 It is, of course, obvious that the output of the heart 

 cannot exceed its input; hence, where the cardiac output 

 is increased, the input must be increased. But whether 

 this increased venous return to the heart is brought about 

 primarily by changes in the cardiac mechanism, by altera- 

 tions in the blood vessels, or by both, cannot be stated 

 with certainty at present. 



The above brief description of how the problem of the 

 cardiac output of man has been attacked presents an in- 

 stance of what may be called quantitative descriptive 

 physiology; as yet the explanatory or theoretical side 

 has just begun. Let us now take another example from the 

 different field where the trend of recent work has been 

 more on the explanatory than on the descriptive side. 

 We say that we can explain a mechanism or a process in 

 the animal organism when we can express it in terms of 



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