THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 233 



mobile exhaust, these may be regarded as unnatural states. 

 The problem is best presented and is best met during 

 vigorous muscular exertion. The oxygen requirement of a 

 man of average size may be only 0.25 to 0.30 hter (quart) 

 per minute when he is at rest; but very rigorous exertion 

 may raise the requirement to 15 liters per minute or more. 

 Even in most favorable circumstances hov^ever, the maximal 

 intake of oxygen is at a rate less than 4 Kters per minute. 

 Thus during highly strenuous effort the intake may be 

 from 10 to 12 times what it is during rest and yet be far 

 short of what is needed. When this situation arises the 

 lactic acid which attends muscular contraction is not 

 burned to carbon dioxide and it accumulates in the muscles. 

 Contraction can continue, but with decreasing efficiency 

 because of increasing concentration of the acid. Thus an 

 "oxygen debt" is incurred; and even though activity 

 ceases, extra oxygen must be delivered to the muscles to 

 burn in part the lactic acid, until the debt is paid and the 

 resting state is restored. Various and complex adjustments 

 of the respiratory and circulatory systems are made, each 

 tending to supply an amount of oxygen sufficient to meet 

 the need of the laboring parts or to pay the oxygen debt 

 if the need has not been met during the period of labor. 



The respirations, first of all, are deeper and more frequent. 

 This change occurs at the very start of a muscular effort, 

 too soon to be caused by any other agency than the nerve 

 impulses which initiate the effort itself. Thereafter the 

 greater volume of breathing, which we have all noted when 

 exercising vigorously, is due to an increase of carbon dioxide 

 (and perhaps lactic acid in addition) in the blood. As this 

 increase develops, the portion of the brain which governs 

 the respiratory movements becomes more active and by 

 ampHfying and accelerating these movements it brings 

 about a much greater pulmonary ventilation than before. 

 This carries away the carbon dioxide which is given off 

 from the circulating blood into the myriads of Httle sacs or 

 alveoh of the lungs. At the same time the greater ventilation 

 maintains the. percentage of oxygen in these alveoh. By 

 this double process the blood unloads its volatile waste 

 (carbon dioxide) and is promptly loaded with oxygen for 



