The Circulatory System 175 



Medium-sized artery (during systole) 250 mm./second 

 Capillaries 0.5 mm./second 



Vein (large one) 100 mm./second 



This difiference in rate of flow is inversely proportional to the total 

 cross section of the vessels. It is easily noted that the slow rate of 

 flow in the capillaries facilitates the exchange of gases, foods, etc. 



The flow of the blood back toward the heart through the veins is 

 made possible by a number of different factors. The first of these is 

 that the numerous capillaries gradually join to form the venuoles, and 

 a slight pressure is built up as the cross-sectional area of the vessels 

 suddenly decreases. Back flow is prevented by the valves in the walls. 

 Muscular movements are another important factor in maintaining venous 

 flow; and finally there is the fact that the thoracic cavity has a slight 

 negative pressure and thus actually pulls the blood toward the heart. 



A complete circuit of the body is possible in one minute in man. 

 Of course, this is subject to variation, depending upon the point to 

 which the blood goes. Obviously it takes longer for blood to go to and 

 return from the foot than to take a round trip to the head region. 



If an artery is cut, the blood flows out in spurts. This is due to the 

 difference in pressure of the blood during systole and diastole of the 

 heart. A comparative constancy of flow is maintained through the 

 arteries by the elasticity of the walls. At each systole, the walls expand 

 slightly taking up much of the extra push; thus the blood vessels are 

 not emptied between systoles. 



The pressure of the blood within the arteries can be measured in 

 various ways. The commonest method is that of using a rubber arm 

 band which shuts off the arm artery when the band is inflated. The 

 pressure of the band is gradually released, and the point at which the 

 pressure of the blood is sufficient to flow through is noted. A typical 

 blood pressure for an individual of about 20 is 120/80 mm. of mercury. 

 The 120 refers to the systolic pressure, the 80 to the diastolic pressure. 

 The difference between these two is the pulse pressure. With increasing 

 age, the elasticity of the arterial walls lessens, thus the walls offer greater 

 resistance to blood flow. In such a person, the blood pressure usually 

 rises, and often there is danger of rupturing the small vessels and 

 capillaries in the brain. 



