I 



THE MECHANISM OF THE HEART PUMP 953 



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

 occupied by the systole varies only 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 filling 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 vthat 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 blueness 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 

 diminution of intrathoracic pressure is due the ' cardio-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 and due to the 

 expulsion of blood from the heart. 



