THE MECHANISM OF THE HEART PUMP 909 



Although the pulse frequency may undergo considerable variations 

 according to the condition of the individual, being higher during activity 

 or under conditions of mental excitement, the greater part of the difference 

 in duration of the cardiac cycle thereby induced falls upon the diastohc 

 period. Thus to take wide limits the pulse-rate may vary between 32 and 

 124 beats in the minute, while under the same circumstances the period 

 occupied by the systole only varies between 0-382 and 0-190 sec. and these, 

 of course, are extreme limits. Variation therefore in the time occupied by 

 each cardiac cycle is determined mainly by variation in the time occupied 

 by the diastole. 



FILLING OF THE HEART IN DIASTOLE 



Since the heart is perfectly flaccid during diastole it is unable to exert any 

 suction force on the blood in the veins. Its rilling during diastole depends 

 on the existence of a positive pressure within the veins, or at any rate 

 of a pressure greater than that in the auricles and ventricles ; the greater 

 the pressure within the large veins the more rapidly will the blood enter 

 the heart during diastole and the larger the amount of blood in this viscus 

 when it begins its contraction. In this process an important part is 

 played by the mechanical conditions existing in the thoracic cavity. Owing 

 to the elasticity of the lungs and the fact that they are constantly 

 tending to contract, the pressure in the thorax is less than that of the 

 external atmosphere by the amount which is required to distend the lungs 

 to fill the cavity. 



At the end of expiration this difference amounts to about 5 mm. Hg. 

 rising to 9 mm. Hg. at the end of inspiration and to 30 mm. Hg. at the end of 

 a forced inspiration. On the other hand, the veins outside the thorax are 

 exposed to a pressure which is a little above that of the atmosphere. When 

 the thorax is at rest the veins and auricles are therefore expanded and the 

 flow of blood into them rendered more easy. The respiratory movements, 

 by causing an alternating suction on the walls of the great veins, act like an 

 accessory pump and cause an aspiration of blood into the veins of the 

 thorax with each inspiration. 



It is evident that if the pressure within the thorax be sufficiently raised so 

 as to cause a positive pressure on the big veins and auricles, the return flow 

 of blood to the heart must come to an end. Thus during extreme muscular 

 efforts the glottis is fixed and a positive pressure is produced in the thorax. 

 The deficient circulation and the deficient aeration of the blood thereby 

 induced are shown by the engorgement of the superficial veins and the blue- 

 ness of the surface. Weber showed that by a forcible expiration, with the 

 glottis closed, the pulse might disappear at the wrist and the circulation be 

 brought for a time to a standstill, so that even loss of consciousness might 

 supervene. 



Since the heart during its systole diminishes its own volume by the expulsion of 

 blood from the thorax, it becomes smaller, and the space thus provided in the chest 

 cavity is taken up by an expansion of the veins, auricles, and lungs. To this systolic 



