182 



ESSENTIALS OF PHYSIOLOGY. 



The pressure in the large veins may be determined in a similar 

 manner, except that, as the pressure is low, the whole manometer is 

 usually filled with sodium sulphate solution. Observations made in 

 this way show that whereas in a systemic artery the mean blood 

 pressure may vary from 100 to 140 mm. Hg and alters slightly with 



FIG, 56. Blood-pressure tracing. 



each heart beat and respiratory movement, the venous pressure amounts 

 only to a very few mm. Hg and is not affected by the heart beat. It 

 is found that the arterial pressure is highest in the aorta, rather less 

 in the medium-sized arteries, and that there is an abrupt fall of pressure 

 in the arterioles ; in the capillaries the pressure is low, and filially there 



BP 



H 



LV 



FIG. 57. Scheme of blood pressure. (Starling's Principles of Physiology.} 



LV, left ventricle ; A, arterioles ; C, capillaries: V, veins; RA, right auricle ; 

 00, line of no pressure ; BP, blood pressure. 



is a steady fall of pressure in the veins, until, in the large veins near 

 the heart, it may actually be negative, that is, less than the atmospheric 

 pressure. These differences of pressure are diagrammatically repre- 

 sented in fig. 57. The pressure in the pulmonary artery varies from 

 20 to 25 mm. Hg, and on the average is about one-sixth of that in 

 the aorta or its main branches. 



We may now consider the factors which are concerned in this 

 distribution of pressure, and in the conversion of the jerky flow 



