910 PHYSIOLOGY 



diminution of intrathoracic pressure is due the * cardiao -pneumatic ' movements. 

 These are recorded by attaching one nostril to a delicate tambour by means of a tube, 

 while the other nostril and the mouth are kept closed. If a carotid pulse tracing be 

 taken at the same time it will be found that there is a fall of the lever attached to the 

 nasal cavity synchronous with the rise of the pressure in the arteries, due to the expulsion 

 of blood from the heart. 



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

 thing 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 under pressure, 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 pericardial 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 determina- 

 tion 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 



