THE VASCULAR MECHANISM. 203 



more strongly during systole, and the output of the heart again 

 becomes as large as it was at the lower arterial pressure (fig. 72). 

 The capacity of the ventricle to maintain its normal output in 

 spite- of a greatly raised arterial pressure is spoken of as its "power 

 of compensation," and is of the utmost importance. In its relation to 

 the body as a whole the output of the heart is one of the fundamental 

 facts of the circulation, since it determines the supply of oxygen to 

 the tissues ; and if the output were diminished whenever the blood 

 pressure rose, for example during exercise, the high blood pressure 



FIG. 72. Output of heart. ( Knowl ton & Starling.) 

 A, volume of ventricle : B, arterial pressure ; C, output of left ventricle ; D, time in seconds. 



would necessarily involve a smaller and possibly inadequate supply of 

 oxygen to the tissues. 



If the arterial pressure becomes very high, the dilatation of the 

 left ventricle during diastole is so great as to diminish its efficiency 

 when it contracts, the output of the heart falls, and the left auricle 

 empties itself less completely into the already distended ventricle. 

 In consequence, the left auricle contains more blood, the pressure 

 within it rises, and blood passes less readily from the lungs into the 

 auricle and accumulates in the pulmonary veins and capillaries. This 

 accumulation of blood in the lungs is spoken of as their " reservoir " 

 action, and helps to prevent excessive dilatation of the left side of 

 the heart. 



