DYNAMICS OF PULMONARY CIRCULATION 



I707 



exercise. However, it is conceivable that, under 

 certain pathological conditions, such as those which 

 involve a huge pulmonary blood flow through a 

 curtailed pulmonary vascular bed, the contact time 

 may be too brief. Indeed, an inadequate contact 

 time has been invoked to account for peripheral 

 arterial hypoxemia in resting patients with multiple 

 pulmonary emboli and in exercising patients with 

 "alveolar-capillary block" (359)- However, such 

 explanations are not entirely convincing since, in 

 most of these pathologic states, other equally con- 

 vincing explanations for peripheral arterial hy- 

 poxemia, e.g., opening of pulmonary arteriovenous 

 shunts, also exist. 



Pulmonary Capillary Hematocrit 



In the 1930's, Fahraeus and Lindquist pointed out 

 that blood flowing in capillary tubing has a greater 

 ratio of plasma to red cells than does blood in wider 

 streams (18). Since then, it has been repeatedly shown 

 that the hematocrit of blood in many organs is less 

 than in the large vessels which enter and leave them. 

 The lung appears to be no exception: measurements 

 of the hematocrit of blood obtained from whole organ 

 homogenates (161) as well as comparisons of transit 

 time of tagged red cells and plasma (332) indicate 

 that the small vessel hematocrit is regularly lower 

 than that of the large vessels, ranging from 1 7 per 

 cent less at rest to 13 per cent less during exercise. 

 This difference may affect not only the hemodynamic 

 behavior of the pulmonary capillary circulation but 

 also measurements of alveolar-capillary gas exchange, 

 such as the pulmonary diffusing capacity for carbon 

 monoxide, and derivative values, such as the time 

 spent by blood in the pulmonary capillary and the 

 pulmonary capillary blood volume (143, 318). 



Transcapillary Exchange 



Until recently, considerations of the transcapillary 

 movements of water and electrolytes emphasized 

 their bulk transfer and dealt largely with the balances 

 between hydrostatic and oncotic pressures (Starling's 

 law of capillary exchange). Recognizing that the 

 pulmonary capillaries were unique in being "bathed 

 in air rather than in water," such considerations of 

 bulk transfer were sufficient to account for the normal 

 "dry" lungs as well as the "wet" lungs of clinical 

 pulmonary edema. Within the last few years, trans- 

 capillary exchange by diffusion has also been taken 

 into serious account (76). Still incomplete is the 



definition of the role of the pulmonary lymphatics 

 with respect to the water which escapes into the 

 pulmonary interstitium and alveoli. 



Certain aspects of the transcapillary exchange of 

 water seem well established. For example, the osmotic 

 pressure of the plasma colloids (expressed figuratively 

 as an "oncotic pressure") of approximately 25 mm Hg 

 normally suffices to prevent bulk loss of fluid from 

 the pulmonary capillaries, even in the hydrostatically 

 dependent portions of the lung. Also, while trans- 

 capillary molecular exchange rates by diffusion may 

 be of the order of the cardiac output, no net fluid 

 transfer occurs. Ethanol and injected carbon dioxide 

 behave like isotopic water. As expected, the indi- 

 cator-dilution curves for T-1824 (which does not 

 leave the capillaries) and for water (which undergoes 

 rapid to-and-fro transcapillary exchange) are quite 

 different (76). Indeed, labeled water resembles the 

 inert gases in behaving as though the barrier did not 

 exist. 



Quite unexpected is the similarity between the 

 T-1824 indicator-dilution curve and the correspond- 

 ing curves for urea and for the highly diffusible 

 phosphate, potassium, sodium, and chloride ions 

 (16, 77, 360). Virtual identity of these curves has 

 been interpreted to mean that: a) in contrast to the 

 enormous pulmonary volume of distribution of 

 sodium and chloride at equilibrium (134), the volume 

 of dilution available to urea and to the diffusible 

 ions during a single circulation is confined either to 

 the strict pulmonary vascular volume or to the pul- 

 monary vascular volume plus an additional, circum- 

 scribed perivascular volume into which these sub- 

 stances diffuse and then promptly return, and b) 

 because of the ready permeability of the barrier 

 (probably the basement membrane) to water as well 

 as its relative impermeability to ions and urea, the 

 barrier is aqueous rather than lipid in nature. 



MISCELLANEOUS HEMODYNAMIC PHENOMENA 



Pulmonary Arterial Pulse-Wave Velocity 



The pulse-wave velocity is related to relative, 

 rather than to absolute, distensibility (see Chapter 

 24). Estimates of the speed at which the pulse wave 

 travels along the length of the pulmonary arterial 

 tree vary considerably. The discrepancies are at- 

 tributable to three causes: inadequate methodology; 

 the experimental difficulty of controlling the hemo- 

 dynamic influences which modify the speed of the 



