236 



HUMAN PHYSIOLOGY 



were not at all involved. In other words we have treated 

 the subject as though the heart and all the vessels were 

 in the same horizontal plane. This can never be exactly 

 the case and it is very far from the condition in man with 

 his erect posture. In one sense gravity does not in- 

 fluence the circulation: there is always as much blood 



_ Subtract 



50 mn, 

 mercury 



H.L. 



FIG. 49. To suggest how the weight of the blood-columns adds to 

 local pressures below the heart and subtracts from those above its 

 level. There are obscure factors which somewhat temper the extreme 

 variations. 



falling as there is rising in the system and we have to do 

 with balanced columns. The heart does not permanently 

 lift any weight from a lower to a higher level. But 

 gravity does have a marked influence upon local pressure. 

 All blood below the heart presses harder upon the 

 vessels than it would at the heart's own level. All blood 



