112 RESPIRATORY MECHANISMS 



not combine with CO. The dissociation curve for 2 is fairly 

 steep (t u about 8 mm, at 1 9°C) and C0 2 has no effect. 



The oxygen capacity of the haemerythrin in the coelomic 

 fluid of Sipunculus is quite low, 1.6 vols. % on an average, but 

 the quantity of fluid is very large, and it would appear that 

 the pigment acts mainly as a store of oxygen to be utilized in 

 adverse conditions in the marine mud in which the animal 

 lives. 



The adaptation of the circulation to respiratory needs. The rate 

 of blood flow is evidently of no less importance for the trans- 

 port of the gases than the transport capacity, but unfortunately 

 very little is known about it, except in the case of man and 

 a few mammals. For these it has been shown that the adapta- 

 tion of the circulation to the varying respiratory demands is 

 of the same order of importance and perfection as the adapta- 

 tion of the lung ventilation. The following points are note- 

 worthy and probably applicable to a very large number of 

 forms besides the few for which they have been definitely 

 ascertained. 



Increased activity of an organ demands and secures an 

 increase in the blood-flow through that particular organ. 

 This increase, which is brought about mainly by relaxation of 

 arteries and especially of arterioles, is always correlated with a 

 dilatation of capillaries and in many cases, especially in 

 muscles during work, with an opening up of a large number of 

 capillaries previously closed as described in the writer's 

 monograph on The Anatomy and Physiology of Capillaries (Re- 

 vised ed. 1 929) . The number of open capillaries may increase 

 more than tenfold, and the utilization of the oxygen supplied 

 through the arterial blood may increase from the usual 33% 

 to about 90%. An increase in flow to a single organ is to 

 some extent compensated by a restriction of the flow to the 

 others, but it is the general rule that it involves an increase in 

 the total circulation through the lungs as measured by the 

 "minute volume" of the right heart. In man this minute 

 volume is of the order of 4-5 1 during rest in a reclining posi- 



