518 CIRCULATION OF BLOOD AND LYMPH. 



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 cir- 

 culation. This pumping effect of our muscular movements is 

 probably quite an important factor in returning the blood from 

 the lower extremities. In this portion of the body the venous 

 flow to the heart has to overcome the hydrostatic pressure of the 

 column of blood, and it has been shown that when one is standing 

 quite still, the venous pressure alone may be insufficient to overcome 

 this resistance, so that the blood-flow from the feet may be much 

 retarded. Under these circumstances movements of the legs, as 

 in walking, aided by the valves in the veins, probably help to 

 " milk " the blood into the pelvic veins. The contractions 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. 489), 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 



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

 and for references to literature, see Tigerstedt, "Ergebnisse der Physiologic/ 

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



