PHYSIOLOGY OF TO-DAY 



— the latter has proven the most satisfactory. The subject, 

 after a period of rest or exposure to some special condition 

 under which the determination is to be made, has his 

 oxygen consumption determined by one of the usual meth- 

 ods. He then rebreathes from a rubber bag a mixture of 

 acetylene and air for a period sufficiently long to bring 

 about mixture in the lung-bag system (fifteen to eighteen 

 seconds) and a sample of gas is taken from the system. 

 After five seconds more, another sample is taken. An 

 analysis of these two samples, the barometric pressure, 

 and the oxygen consumption give all the data necessary 

 for calculating the output of the right side of the heart, 

 and hence also of the left side. The technical details cannot 

 be discussed here. 



What results have been obtained by the use of these 

 cardiac output methods on man? In the first place, it has 

 been found that a man in the basal condition (eighteen hours 

 after food and at complete rest) has a cardiac output of 

 2.2 liters for each square meter of body surface. This means 

 that, depending on the size of the individual, his heart 

 pumps each minute from three to six liters or from nearly 

 a gallon to a gallon and a half of blood. Moreover, at 

 each beat of the heart a little over two ounces of blood 

 is discharged. The cardiac output of any individual under 

 standard conditions is a very nearly constant quantity, 

 varying only slightly from day to day. Many influences, 

 however, modify the amount of blood being circulated 

 around the body. Thus, the ingestion of food increases 

 the cardiac output, a large meal having a greater and more 

 lasting effect than a small one. The emotional state of 

 the subject is also of importance, for emotional disturbances 

 may raise the value above the basal normal. In muscular 

 exercise, the output of the heart may be enormously in- 

 creased, and, of course, this is necessary to supply the active 

 muscles with oxygen. 



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