282 RESPIRATION 



contracted and impermeable to blood, so that neither blood cor- 

 puscles nor even the finest particles of Indian ink can pass 

 through them. Nor is the full arterial blood pressure capable of 

 forcing them open. Whenever the tissue is stimulated to activity, 

 however, these capillaries open wide, so that blood can pass 

 through them freely. He found, for instance, that in muscle of the 

 guinea pig about twenty times as many capillaries were open 

 during activity of the muscle as during rest. The active contrac- 

 tility of capillaries had been directly observed by Roy and Gra- 

 ham Brown in 1880, but the real significance of this observation 

 had not been realized. 



Krogh's observations have thrown a flood of new light on the 

 exchange of gases and other material between the blood and the 

 living tissues : for the opening out of new capillary paths when- 

 ever a greater exchange of material is taking place must facili- 

 tate enormously the exchange, and thus furnish a means of keep- 

 ing the gas pressures in the tissues approximately normal in spite 

 of great variations in metabolism. During muscular work, for 

 instance, the immense increase of capillary paths will greatly 

 facilitate the exchange of oxygen and carbonic acid between the 

 blood and the muscle fibers. There must be a great tendency to 

 fall in the oxygen pressure of the blood passing through the 

 muscle capillaries during muscular work. Unless this fall were 

 approximately compensated for by the opening out of new capil- 

 laries, it is difficult to see how a sufficient oxygen supply could be 

 maintained, as in all probability the oxygen consumption in a 

 muscle during very hard work is twenty or thirty times as great 

 as during rest. We can also now understand much better how it 

 comes about, for instance, that when the skin circulation is cut 

 down to the utmost by vasoconstriction in the prevention of un- 

 necessary loss of heat from the body, the skin, though more or 

 less blue from greatly diminished blood flow, may be still full of 

 blood, as shown by the full blue color. 



Probably it is the stimulus of the presence in excess of certain 

 metabolic products, particularly carbonic acid, and the deficiency 

 of others, particularly oxygen, that determines the relaxation of 

 the capillary walls. There can also be little doubt that the same 

 stimuli, acting reflexly, determine the activity of local vasomotor 

 nerves. Temperature stimuli, or irritation stimuli, appear to act 

 in a similar manner. Stimuli may also act centrally, however, as 

 in the general regulation of body temperature by variations in the 

 skin circulation, or in emotional vasomotor changes. 



