CIRCULATION OF BLOOD 69 



weight by the height to which the weight is carried. The 

 weight lifted is that of the amount of blood sent by a single 

 systole of the heart (pulse volume). The amount of blood 

 thus lifted is about 66 cc., and its weight 0.07 kg. The 

 height to which this is raised is equal to the blood pressure 

 in the aorta or in the pulmonary artery. In the aorta the 

 pressure is about I 50 mm of mercury or about two metres 

 of blood ; in the pulmonary artery, the pressure is about one- 

 third of that in the aorta. During one contraction, the left 

 ventricle, therefore, does the work of 0.07 X 2 or 0.14 kilo- 

 grammetre, the right ventricle 0.047. I n a ^ tne heart in 

 twenty-four hours does about 18,000 kilogrammetres of work. 



The pulse volume is estimated by many authors as much greater 

 (up to 1 80 cc) and then the work done is correspondingly greater. 



In this calculation, no account is taken of the work which the 

 heart does in imparting velocity, i.e. kinetic energy, to the blood 

 (about 0.3 m per second). But this work is very little compared 

 with that done to overcome the blood pressure; the former not 

 being more than \% of the latter. 



3. CIRCULATION OF THE BLOOD IN THE VESSELS 



i. The blood pressure in the vessels. The blood pres- 

 sure is the pressure of the blood upon the walls of the 

 vessels, this determining the tension of the walls. 



The blood pressure in the larger vessels is measured by inserting 

 a canula into the vessel and connecting this canula with a register- 

 ing manometer. The pressure can also be determined without 

 any operation in many blood vessels of man, it being equal to the 

 pressure necessary to close these vessels. An artery is closed when 

 no pulse is felt peripheral to the compression. The capillary 

 pressuie is found by pressing upon a glass plate, placed on the red 

 part of the skin, till the skin becomes pale. 



The blood pressure in different parts of the vascular system 

 varies greatly. The difference in pressure is produced by 

 the activity of the heart and is the cause of the movement of 

 the blood. Each particle of blood is forced from a place of 

 higher to a place of lower pressure. 



The blood pressure constantly decreases as we proceed 



