9 66 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



congestion, but also after dextrose infusions, e.g., 

 to 37 mm Hg, and during ureteral occlusion, e.g., 

 to 40 mm Hg, with relatively close parallelism be- 

 tween intratubular, interstitial, and peritubular 

 capillary pressures. Evidently, the hydrostatic pressure 

 difference across the walls of renal peritubular 

 capillaries is far less than across the walls of peripheral 

 capillaries generally. This implies that the full 

 osmotic force of the plasma proteins, unopposed by 

 hydrostatic pressure differences, may be available 

 for withdrawal of tubular reabsorbate from renal 

 interstitial fluid to blood. 



Pulmonary capillary pressure presents an exception 

 in the opposite direction. Though direct measurements 

 arc not available as yet, an indirect "wedging" 

 method (146, 147) has made it clear that in the lung 

 capillary pressure is normally between 5 and 1 5 

 mm Hg in dog and man and is, therefore, well below 

 the osmotic pressure of the plasma proteins. Ab- 

 sorption is favored (55) and ensures a minimum of 

 interstitial fluid in the alveolar walls, which is an 

 important consideration in a tissue the prime function 

 of which is to permit rapid exchange of gases. In 

 normal subjects these "wedge pressures" are quite 

 constant; exercise produces elevations of not more 

 than 3 or 4 mm Hg. Greater elevations than this 

 during exercise have been found to be helpful in 

 detecting early left ventricular failure or slight mitral 

 stenosis not yet severe enough to produce clinical 

 symptoms or signs (291). 



Retinal capillary pressure has not been measured 

 directly, but must be considerably higher than that 

 in muscle or skin in order to maintain blood flow 

 despite an intraocular pressure of about 20 mm Hg. 

 Nor are any reliable figures available for capillary 

 pressures in other special regions, e.g., brain, pleural 

 and peritoneal surfaces, joints, etc. In view of the 

 differences between capillary pressures in skin, 

 kidney, and lung, generalizations are obviously 

 unjustified and direct measurements are needed tor 

 each tissue. 



C. Variability of Capillary Blood Pressures 

 I 'nder Control Conditions 



The average figures so far given would, by them- 

 selves, present an erroneous idea of the potential 

 role of capillary pressure in the filtration and ab- 

 sorption of fluid. In any one tissue capillary pressure, 

 like the more easily observed capillary blood flow, 

 varies from moment to moment and from capillary 

 to capillar) even when they arise from the same 



arteriole. This is to be expected from the responsive- 

 ness of the terminal arterioles and arteriocapillary 

 sphincters to nerve impulses, both constrictor and 

 dilator, to local metabolic products and also to 

 mild injury such as that produced by manipula- 

 tion, exposure to air, and cannulation itself (198, 

 203, 205). In the skin of frog (205) and man (203) 

 the mere introduction of a minute pipette sometimes 

 produces a brief rise of capillary pressure accom- 

 panying the transient vasodilatation of a "'triple 

 response" to injury. 



It must also be emphasized that capillary pressure 

 has been measured directly in relatively few tissues. 

 In man, determinations have been limited to the 

 capillary loops in the nailfold where arteriovenous 

 anastomoses are also present and may influence 

 pressure measurements. As shown in table 2.1, 

 in one series pressure in the arteriolar loops averaged 

 32 mm Hg. However, the single readings ranged from 

 21 to 48 mm Hg; in the venous loops the correspond- 

 ing figures were 12 and from 6 to 18 (203). In an- 

 other series of control measurements Eichna & 

 Bordley (89) found even larger variations, much 

 more overlapping of values, and a smaller average 

 gradient, viz. 31 to 22 mm Hg rather than 32 to 12 

 (table 2.1). These differences may possibly be re- 

 lated to room temperature because the larger gradient 

 and lower pressures in the venous limbs of the capil- 

 laries were found at room temperatures of 18 to qo C 

 (203). The smaller gradient and higher venous capil- 

 lary pressures were observed in a warm room where 

 temperatures were 23 to 28 C (89). It seems likely 

 that capillary pressures in human digital skin, par- 

 ticularly in the venous limbs next to the subpapillary 

 venous plexus, can be influenced by the state of the 

 arteriovenous anastomoses. At higher room tempera- 

 tures opening of these large channels, and increased 

 blood flow direct from larger arterioles to larger 

 venules, may well increase pressure locally in the 

 subpapillary plexus into which the true capillaries 

 also discharge their blood. 



Chambers & Zweifach (37) have suggested, in 

 addition, a division of function in the minute vessels, 

 viz. that higher pressures, and hence filtration, may 

 occur chiefly in the direct, arteriovenous channels, 

 with lower pressures and absorption located in the 

 true capillaries. For such specialization, however, 

 no supporting evidence in the form of pressure 

 measurements in direct channels is available. More- 

 over, very high pressures and filtration rates were 

 frequently found in true capillaries (200). Zweifach 

 (387) also suggested that "the arrangement whereby 



