EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



967 



the true capillaries come off at right angles to the 

 A-V vessels favors the development of "suction forces,' 

 especially where a rapid, continuous flow courses 

 only through the A-V channels." No suction forces 

 were encountered in any of the many direct measure- 

 ments of capillary pressure. The reason for this 

 becomes clear in considering actual rather than 

 apparent velocities of flow in the minute vessels. 

 Under the microscope, which magnifies linear velocity 

 as well as size, flows that seem very rapid indeed are 

 really between 1 and 2 mm per sec. Calculation of the 

 magnitude of the corresponding velocity effect by 

 the Bernoulli equation shows the insignificance of 

 any possible suction force, viz. for a linear velocity of 

 2 mm per sec a pressure difference of only .000015 

 mm Hg. 



Far more important is the conspicuous variability 

 of pressures found throughout the entire minute 

 vessel system as shown in figure 2.4 for the exposed 

 frog's mesentery where measurements could include 

 larger vessels as well as capillaries. In general, high 

 capillary pressures were associated with very rapid 

 flows (if venous outflow was normally free) and with 

 increased pulse pressure in the capillary network. 

 Lower pressures, approaching venous pressure, 

 were associated with slower flow or absence of flow 

 (ig8). Hence, as arteriolar diameter or tone of 

 precapillary sphincters changes from moment to 

 moment, even under resting or control conditions, 

 the average balance between highly variable capillary 

 pressures and the much less variable osmotic pressure 

 of the plasma proteins often includes temporary 

 imbalances in single capillaries and corresponding 

 shifts toward periods of filtration or absorption. 

 McMaster (235) has suggested that such shifts ex- 



plain, in part, the intermittent entry of Locke's 

 solution at atmospheric pressure into the skin through 

 a fine needle introduced carefully to avoid both 

 blood vessels and lymphatics. 



Position of a capillary bed, relative to the heart, 

 affects capillary blood pressure in general accordance 

 with changes of hydrostatic pressure (203). In the 

 finger tip of man at heart level average pressure in 

 the arteriolar portion of the capillary loop was 32 

 mm Hg and in the venous portion 1 2 mm Hg, with 

 large individual variations in single capillaries around 

 these averages. When the hand was 30 era above 

 heart level these average pressures became 23 and 

 10 mm Hg, respecti\ely, further drop being ar- 

 rested presumably because of collapse of the thin- 

 walled veins in the arm. Conversely, lowering the 

 forearm to 40 cm below heart level increased average 

 arteriolar and venous capillary pressure to 45 and 

 33 mm Hg, respectively. 



The relation between capillary blood pressure and 

 the osmotic pressure of the plasma proteins is there- 

 fore extremely labile, both as to time and the area 

 of capillary wall involved. Absorption may be favored 

 in a large segment of the capillary bed for consider- 

 able periods, e.g., during vasoconstriction or eleva- 

 tion of an extremity and filtration favored for other 

 periods, e.g., during vasodilatation or dependency. 

 Nevertheless, a net equilibrium is maintained and 

 favors constancy of plasma volume and interstitial 

 fluid volume. Under exceptional conditions, e.g., 

 muscular activity, prolonged dependency of an 

 extremity, high temperature, injury, and inflamma- 

 tion, excessive capillary filtrate must be returned to 

 the blood stream by the lymphatic vessels. These 

 ancillary vessels, as described in the following section, 



n _□ 



40 



u 24 



-Arterioles 



, 1 rr 



-Capillaries 

 arteriolar 

 venous 



Artery Artery -first 



bifurcation 





L E NGTH - MM 

 J I I I L. 



35 



30 



25 



20 



15 



10 



fig. 2.4. Chart showing vari- 

 ability of capillary blood pressure 

 and of pressure gradient in the 

 blood vessels of the frog's mes- 

 entery. The higher capillary 

 pressures and increased capillary 

 pulse pressure are characteristic 

 of vasodilatation. The lower 

 capillary pressures and absence 

 of measurable pulse pressure 

 are characteristic of vasocon- 

 striction. [From Landis (198).] 



10 



14 



16 



18 



20 



