284 



THE CIRCULATION 



FIG. 150 



The Pulse-wave is a rapid impulse sent through the arteries by the 

 ventricular contraction. It must be carefully distinguished from the 



current of blood whose velocity has just 

 been mentioned. They have little in 

 common, and yet they are sometimes at 

 first confused. The wave is accompanied 

 by a progressive distention of the arterial 

 wall, and travels at a rate of from 7 to 9 m. 

 per second. It is greater in the upper 

 extremities than in the lower because the 

 arteries there are more elastic than in the 

 legs (Zermak). The pulse-wave is at 

 least fifty times more rapid than the 

 blood-current through the same blood- 

 vessel. It is not to be found under ordi- 

 nary conditions in the capillaries or in 

 the veins because the extreme smallness 

 of the former tubes prevents its passage 

 through into the latter. The length of 

 the pulse-wave is about 5 m. or would be 

 were the arteries long enough to allow 

 both crests of the wave to be distinguished 

 at one time. (See the sphygmograms in 

 the Appendix.) 



Blood-pressure has of late received much 

 study from surgeons as well as from phy- 

 siologists because of its practical impor- 

 tance, especially in relation to surgical 

 shock. Blood-pressure is the very varying 

 amount of force exerted laterally by the 

 blood against the walls of the heart, 

 arteries, capillaries, and veins enclosing 

 it. Highest within the left ventricle at 

 systole, the blood-pressure falls at first 

 slowly through the arteries until the arte- 

 rioles are reached, when it falls rapidly. 

 Before entering the capillaries, 80 per 

 cent, of the pressure has been taken up 

 in friction and four-fifths of the total 

 fall between the ventricle and the auricle 

 again has occurred. Within the 0.5 mm. 

 or so of the capillaries 10 per cent, more 

 is lost and the blood enters the veins under 

 a pressure of not more than 10 or 20 mm. 



of mercury. From here to the auricle the fall is more nearly uniform 

 than elsewhere in the circulation. At some place within the veins (per- 

 haps in the beginning of the vena cava) the blood loses all its positive 



Ludwig's kymograph-manometer: 

 a, a', artery whose pulse and blood- 

 pressure are being recorded on the 

 smoked drum; the U-tube is filled 

 with mercury, the tube between it 

 and the artery with, say magnesium 

 sulphate solution; / is a float, and S, 

 the writing arm; P is a light pendu- 

 lum resting against the arm to keep 

 it in contact with the drum. 



