176 CIRCULATION OF THE BLOOD 



6. TIME RELATIONS OF THE CARDIAC EVENTS 



The duration of the auricular systole is very short. In man, as in the 

 horse and dog, it may amount to 0.1 second. 



The duration of the ventricular systole shows but slight variations, not- 

 withstanding considerable differences in the pulse frequency. Thus in varia- 

 tions between 32 and 124 beats per minute the time occupied by the systole 

 varies only between 0.382 and 0.190 second (man). In animals also where 

 the pulse frequency is made to vary between wide limits by stimulation of the 

 inhibitory and accelerator nerves (cf. 11) the systolic time varies only a 

 little, whereas the diastolic time presents considerable variations. We may 

 say therefore that the variations of pulse frequency in the same individual 

 as well as in different individuals of the same genus are due in the main to 

 variations in the length of diastole. On the average the length of ventricular 

 diastole in man may be estimated at about 0.4 second. 



7. FILLING OF THE HEART IN DIASTOLE 



The most important factor in the filling of the heart during diastole is 

 the impetus which the heart has given the blood in systole. But since the 

 blood meets with great resistance in its passage through the vessels, the 

 residual driving power is relatively small, and 'the accessory mechanisms play 

 an important part. 



One important mechanism is the suction of the thoracic cavity (cf. Chapter 

 IX). Almost the entire air pressure takes effect on the great veins outside 

 the thorax. A small part of it of course is borne by the skin, etc. ; but it 

 may be assumed as certain that the air pressure on the extrathoracic veins is 

 greater than the pressure which is exerted on the organs inside the thorax by 

 the lungs. Consequently in the static position of the thorax the intrathoracic 

 veins and the heart are to a certain extent expanded. 



In expiration the negative pressure inside the thorax decreases and both 

 the intrathoracic veins and the auricles become less distended: hence the 

 return flow of the blood into the thorax becomes more difficult. Quite other- 

 wise is it with inspiration. The intrathoracic pressure decreases in direct 

 proportion to the depth of inspiration and to the degree of expansion of the 

 lungs; and since this decrease of pressure is continuous throughout the entire 

 act of inspiration, a continuous expansion of the intrathoracic veins and of 

 the auricles must result, and therefore a direct suction of the blood from the 

 veins to the heart must take place. Under certain circumstances this suction 

 is felt even by the farthest veins. 



The return flow of the blood is favored likewise by a static inspiratory 

 position of the chest wall; but in this instance direct suction cannot result, 

 because this would presuppose that the auricles are being continually ex- 

 panded by extraneous forces, which is not the case unless the chest wall is 

 actually moving. 



If the air pressure inside the chest be raised sufficiently the return flow of 

 the blood to the heart is hindered; the circulation stops, and death may 

 result, if the abnormal increase in pressure in the thorax is too long continued. 



