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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



The concept of critical opening and closure has 

 been invoked to account for certain puzzling responses 

 of the pulmonary circulation (429). These include the 

 exceedingly gradual increase in pulmonary arterial 

 pressure during graded exercise (267), the relative 

 stability of the pulmonary arterial blood pressure 

 during hemorrhage (160), and the pressure gradient 

 between the pulmonary artery and left atrium as the 

 left atrium falls below 7 mm Hg (36, 366). 



However, it is more difficult to prove the operation 

 of critical opening and closure of small vessels in the 

 pulmonary, than in the systemic, circulation. The 

 difficulties are of several different kinds: a) mechanical 

 influences, e.g., local changes in transmural pressure, 

 may open and close vessels independent of vasomotor 

 activity; b) the effects of anomalous viscosity are apt 

 to be more pronounced and to simulate changes in 

 vascular calibers in a low-pressure circulation; c) the 

 pulmonary arterioles are thin-walled, wide-lumened 

 and, in general, poorly constructed to spring shut; 

 d) there are generally alternate, and equally con- 

 vincing mechanisms to account for pulmonary vascu- 

 lar behavior (272); and e) experiments specifically 

 designed to look for signs of critical closure have not 

 always been able to find them (273, 434). 



At present, the experimental evidence for critical 

 opening and closure of small pulmonary vessels — a 

 vasomotor phenomenon — is inconclusive. If the phe- 

 nomenon does occur, it seems to do so when the 

 pressure gradient between the pulmonary artery and 

 left atrium is exceedingly low, i.e., of the order of 1 

 to 2 mm Hg (42); moreover, it does not seem to affect 

 equally all small vessels of comparable dimensions 

 (273, 366). In general, transmural pressures are more 

 apt to be involved in the closure and opening of small 

 pulmonary vessels than is vasomotor activity. It 

 would be of interest to examine such closed pul- 

 monary vessels to see if their lumens are slits (mechani- 

 cal collapse) or circles (concentric obliteration by 

 vasoconstriction ) . 



Potential and Kinetic Energy 



Mechanical energy is imparted by the right ven- 

 tricle to the blood perfusing the pulmonary circulation 

 in two forms, kinetic and potential energy. At rest, 

 the kinetic energy factor in the pulmonary circulation 

 is of the order of 10 per cent or less of the total; on 

 the other hand, both in normal subjects during exer- 

 cise and in patients with pulmonic stenosis or left- 

 to-right shunts, the kinetic energy factor may increase 

 to over 50 per cent of the total (320, 338). 



The usual calculation of resistance deals only with 

 the drop in potential energy (pressure) across the 

 system. It does not take into account the fact that 

 as blood courses down the pulmonary vascular tree, 

 part of the kinetic energy is reconverted to pressure 

 energy as the area of the bed increases; a small frac- 

 tion is dissipated as heat arising from the friction of 

 blood flow (20). In experimental pulmonic valvular 

 insufficiency, the unusually rapid blood flow and 

 turbulence in the pulmonary artery may produce a 

 drop in pressure across the pulmonic valve (123). 



These considerations suggest that at rest, when the 

 kinetic energy factor is small and of the same order of 

 magnitude in the pulmonary arteries and veins, the 

 drop in potential energy (pressure) between the 

 pulmonary arteries and veins provides a rough meas- 

 ure of the decrease in mechanical energy across the 

 pulmonary vascular bed; on the other hand, in 

 normal subjects during exercise, and in patients with 

 cardiac abnormalities characterized by large stroke 

 volumes and rapid rates of pulmonary blood flow, 

 the pressure gradient across the pulmonary circula- 

 tion does not provide an adequate measure of the 

 mechanical energy delivered to the system. 



PULMONARY CAPILLARY CIRCULATION 



Pulmonary Capillary Pressure (P c ) 



Since a direct method for measuring P c pressures 

 is not available, the level of the P c pressure is generally 

 estimated from the pulmonary arterial diastolic 

 pressure on the one hand, and the mean left atrial 

 pressure, on the other. In the normal subject these 

 limits set the mean P c pressure at approximately 10 

 mm Hg. 



Rate of Pulmonary Capillary Blood Flow (Q, c ) 



In the normal animal or man the rate of pulmonary 

 capillary blood flow is virtually identical with the 

 right ventricular output; in left-to-right shunts or 

 extensive collateral arterial circulations, Q c exceeds 

 the right ventricular output. An earlier chapter has 

 analyzed the methods used to measure the cardiac 

 output. Of special interest to the present section is 

 the use of inert soluble gases not only to measure the 

 rate of pulmonary capillary blood flow in man but 

 also to explore the nature of the pulmonary capillary 

 flow. Throughout this section it will be assumed that 

 physiological measurements of pulmonary capillary 



