150 



THE CIRCULATION OF THE BLOOD 



is a period during which, the ventricle by its contraction is getting up a 

 sufficient amount of pressure in the fluid contained in it to force open 

 the semilunar valves against the resistance of the pressure in the aorta, 

 and it has been popularly called "the period of getting up steam," or, 

 in physiological language, the isometric, or the presphygmic, period. We 

 shall use the last-mentioned term in our further discussion here. 



After the aortic valves have been opened, it will be observed that the 

 pressure in the ventricles is just a little above that in the aorta, and that 



Fig. 34. Superimposed pressure curves from aorta, ventricle and auricle, along with electrocar- 

 diogram and phonocardiogram. A, aorta. V, ventricle. Aur, auricle. E, electrocardiogram. P, 

 phonocardiogram. 



it continues so during the whole of ventricular systole. When diastole 

 sets in, the pressure in the ventricles quickly falls, and a point is soon 

 reached at which equality of pressure in ventricle and aorta is again 

 attained. This corresponds to the moment of the closure of the semi- 

 lunar valves. The pressure in the ventricle, although now rapidly fall- 

 ing, takes a little time before it has fallen low enough to permit the 

 auricular valves to open. Here again, then, the ventricle is a closed cavity, 

 and we have what is known as the postsphygmic period. 



