THE CIRCULATION 283 



well enough be three times the average, while in the opposite direction 

 the speed rapidly lessens perhaps to one three-hundredth of the average. 

 In the capillaries then it has been estimated that the blood's velocity 

 is not over 0.5 mm. per second, which is about one mile in thirty-seven 

 days. The capillaries average in length about 0.5 mm., so that the blood- 

 flow through the capillaries requires about one second. In this 0.5 mm. 

 alone and in this one second alone, the blood is in practical contact 

 functionally with the tissues and performs promptly all its varied func- 

 tions. If one compares this mile in thirty-seven days with the mile in 

 three hours which the blood moves on the average in the arteries, one 

 has somewhat of a rough measurement of the effects of the great friction 

 in the capillaries, despite the exceedingly smooth surface of the endothe- 

 lium in the tubes. 



FIG. 149 



The blood's pressure as it varies in different regions of the circulation, indicated by the graphic 

 method. Abscissa o, o, indicates the regions and zero pressure, while the ordinate o, h, suggests 

 the blood-pressures in millimeters of mercury. The pressure then at the heart, h, is about 160 

 mm. Hg., falling at first slowly then rapidly in the arteries to about 35 mm.; ranging thither 

 to about 15 mm. in the capillaries; while in the course of the veins the pressure falls to about 

 9 mm. Hg. less than zero, the suction of the heart in diastole. (Yeo.) 



In the veins the velocity increases rapidly from the capillaries to the 

 heart. Probably the average blood-speed here is less than that in the 

 arteries. 



The Circulation-time is the period in which a given erythrocyte, for 

 example, if unimpeded, can go from the heart to the feet and back to 

 the heart and thence around the pulmonary circulation. Hering's 

 experiments showed that in a horse the time required for the pulmonary 

 circulation plus the circulation through the head was twenty or thirty 

 seconds, and Vierordt found the period in the dog to be about seventeen 

 seconds. Stewart estimates that the total circulation-time in man is 

 about one minute or a little more. These figures suggest vividly how 

 active is the circulation and how completely unified by its means are the 

 semi-fluid tissue-protoplasm and the circulating-liquid. 



