BLOOD-PRESSURE AND BLOOD-VELOCITY. 481 



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. 454), 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. 



The Variations in Pressure in the Pulmonary Circuit. 

 Experimental results indicate that the pressures in the pulmonary 

 circuit do not undergo as marked changes as in the systemic circu- 

 lation; the flow is characterized by a greater steadiness. With a 

 systemic pressure, as taken in the carotid, varying from 144 to 222 

 mms., that in the pulmonary artery changes correspondingly only 

 from 20 to 26 mms., and extreme variations of pressure in the 

 pulmonary artery probably do not exceed, as a rule, 15 to 20 mms. 

 The regulations of the pressure and flow of blood in the small 

 circulation do not seem to be so direct or complex as in the aortic 

 system. The part taken by the vasomotor nerves is referred to 

 in the chapter upon the innervation of the blood-vessels, and 

 attention may be called here only to the mechanical factors, which, 

 indeed, for this circulation are probably the most important. The 

 output from the right ventricle, and therefore the amount of flow 

 and the pressure in the pulmonary artery, depends mainly on the 

 amount of blood received through the venae cavae by the right auricle. 



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

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

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

 31 



