198 



ESSENTIALS OF PHYSIOLOGY. 



When the systole ends at 3, the intra-ventricular pressure rapidly 

 falls ; a short distance down the descending part of the tracing, namely, 

 at the point c, the pressure in the ventricle falls below that in the aorta, 

 and the semilunar valves close ; their closure does not alter the form 

 of the intra-ventricular record, though it causes a series of small 

 oscillations in the aortic pressure. At the point 4, the pressure in the 

 ventricle falls below that in the auricle, the mitral valve opens, and 

 blood flows into the relaxed ventricle. 



Auricular Pressure. A record of the pressure changes in the 

 auricle presents three main waves (fig. 69). The first corresponds with 



Aorta 



Left 



ventricle 

 Left \ 



auricle. ) 



FIG. 69. Simultaneous record of the changes of pressure in the aorta, left ventricle 

 and left auricle. (From Piper.) 



the auricular systole, the second with the sudden closure of the auriculo- 

 ventricular valves, and the third, which occurs toward the end of ventri- 

 cular systole, is due to the filling of the auricle with blood while the 

 auriculo-ventricular valves are still shut. 



The maximum pressure in the left ventricle of the dog is usually 

 from 140 to 160 mm. Hg, and in the right ventricle from 25 to 30 

 mm. Hg. 



THE OUTPUT AND WORK OF THE HEART. 



When the ventricles contract, they force blood into the aorta and 

 pulmonary artery against the blood pressure in these vessels. In so 

 doing the heart performs work, the amount of which may be deter- 

 mined by the formula W = Q x R, where W is the work done, Q is the 



