1024 PHYSIOLOGY 



then suddenly relaxes, the period of relaxation occupying about 

 0*5 sec. ; the plateau of the endocardiac pressure curve on the average 

 lasts about 0*18 sec., and according to the condition of the heart 

 and the peripheral resistance may present a gradual ascent or descent. 

 Directly relaxation commences the ventricular pressure falls below 

 the aortic pressure or the pulmonary pressure and the semilunar 

 valves close, giving rise to the second sound. Systole is now at an end 

 and the diastolic period of filling recommences. The first sound is 

 synchronous with commencement of the ventricular contraction, 

 and the same event is signalled by the occurrence of the apex beat. 



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 diastolic 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 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 ; 

 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 procees 

 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 



