BLOOD-PRESSURE AND BLOOD-VELOCITY. 467 



muscles must also influence the blood-flow. The thickening of the 

 fibers in contraction squeezes upon the capillaries and small vessels 

 and tends to empty them. On account of the valves in the veins 

 the blood is forced mainly toward the venous side of the heart; 

 so that rhythmical contractions of the muscles may accelerate the 

 circulation. The conti actions of the smooth muscles, especially 

 in the stomach and intestines during digestion, have a similar effect. 

 The musculature of the spleen also is supposed to aid the circulation 

 through that organ by its rhythmical contractions. 



The Conditions of Pressure and Velocity in the Pulmonary 

 Circulation. The general plan of the smaller circulation from 

 right ventricle to left auricle is the same as in the major or systemic 

 circulation, and the same general principles hold. The right 

 ventricle pumps its blood into the pulmonary artery, and, on ac- 

 count of the peripheral resistance in the lung capillaries, the side 

 pressure in the artery is higher than in the capillaries, and higher in 

 these than in the pulmonary veins. The velocity of movement is 

 least, on the other hand, in the extensive capillary area and greatest 

 in the pulmonary artery and veins, on account of the variations in 

 width of the bed. So also in the pulmonary artery the pressure and 

 velocity must fluctuate between a systolic and diastolic level at each 

 heart beat, while in the pulmonary veins they are more or less uni- 

 form. An interesting difference between the two circulations 

 consists in the fact that the peripheral resistance is evidently much 

 less in the pulmonary circuit, and consequently the pressure in the 

 pulmonary arteries is much less than in the aortic system. The 

 velocity of the flow, as already stated (p. 441), is also greater in 

 the lung capillaries than in the systemic capillaries. Exact deter- 

 minations of the pressure in the pulmonary artery are made with 

 difficulty on account of the position of the vessel.* The results 

 obtained by various observers give such values as the following: 



Mean Pressure. Extreme Variations. 

 Mms. Hg. Mms. Hg. 



Dog 20 10 to 33 



Cat 18 7.5 " 24.7 



Rabbit 12 6 "35 



It will be seen, therefore, that the mean pressure is not more 

 than one-seventh to one-eighth of that prevailing in the aorta. 

 The thinner walls and smaller muscular power of the right ventricle 

 as compared with the left are an indication of the fact that less force 

 is necessary to keep up the circulation through the pulmonary 

 circuit. 



* For a discussion of the special physiology of the pulmonary circulation 

 and for references to literature see Tigerstedt, "Ergebnisse der Physiologie," 

 vol. ii, part n, p. 528, 1903. 



