954 PHYSIOLOGY 



The normal filling of the heart during diastole can be prevented by 

 anything which hinders its expansion, such as the presence of fluid in the 

 pericardial cavity. The same effect may be produced experimentally. If oil 

 be allowed to flow into the pericardium, when the pressure of the oil rises 

 to about 60 mm., the pressure of the vena cava rises to a height just above 

 that obtaining in the pericardia! cavity. On increasing the pressure, a 

 point is finally reached at which no more blood can be driven from 'the veins 

 to the heart, so that the arterial blood pressure falls to zero and death ensues. 



In order to maintain the arterial pressure it is necessary that the amount 

 of blood, driven into the arterial system by the contraction of the left ventricle, 

 should be exactly equal to that leaving the arteries to pass into the capillaries 

 during the period which elapses between each systole. 



Over-filling of the heart is prevented to a certain extent by the resistance 

 of its walls. The danger of over-filling is therefore most marked in the 

 right ventricle. An important part is played moreover by the pericardium 

 in this regard. Even when beating normally, the heart during diastole 

 tends to protrude through a slit made in the pericardium, and Barnard 

 has shown that the right auriculo- ventricular valve ceases to be entirely 

 efficient when the pericardium has been freely opened, the closure of this 

 valve being dependent on the support afforded to the heart by the 

 pericardium. 



SYSTOLIC OUTPUT OF THE HEART 



The amount of blood which passes through the whole body and is avail- 

 able for the metabolic exchanges of all the tissues depends on the amount 

 of blood which leaves the heart each minute. The height of the arterial 

 pressure also depends on the relation between the amount of blood leaving 

 the arterial system by the capillaries and that entering from the heart. 

 The determination of the output of the left ventricle is therefore one of 

 the most important problems in physiology. The output of the right ventricle 

 must be equal to that from the left ventricle, otherwise the blood would 

 accumulate on one or other side of the heart and bring the circulation to 

 a standstill. It is therefore immaterial on which side of the heart the 

 output be determined. 



The methods which have been devised for determining the cardiac 

 output fall into two classes. In the first class it is sought to determine 

 the total volume of blood leaving the right or left ventricle in the course 

 of a given time, say one minute. If this amount be divided by the number 

 of heart beats' in the same time, the output of each ventricle per beat is 

 at once obtained. A second method consists in the determination of the 

 volume changes in the ventricles at each beat of the heart. During diastole 

 the ventricles are receiving Mood and increase in volume, during systole 

 they expel blood and therefore diminish in volume. The change in volume 

 ;it each beat gives therefore the combined output of riujht and left ventricles 

 and must be divided by half in order to '/ive the output of cither ventricle 

 separately. 



