THE RESPIRATORY SYSTEM. 279 



tissues and least in the muscles ; this tension is so low that, as the 

 blood traverses the capillaries, its oxyhsemoglobin undergoes dissocia- 

 tion to a considerable extent. The oxygen thus set free passes into 

 solution in the blood plasma and diffuses into the tissues. Further, the 

 rate at which oxyhsemoglobin dissociates at this tension is increased by 

 the addition to the blood of carbonic acid, and often of lactic acid, as it 

 passes through the capillaries. 



In addition to these two factors, the amount of oxygen available 

 for the use of the tissues depends upon the rate at which the blood is 

 flowing through the capillaries. 



Carbonic acid also passes by diffusion from the tissues, in which its 

 tension is high, into the plasma, in which its tension is much lower. 

 The tension of carbonic acid in the tissues is ascertained indirectly by 

 measuring its tension in bile or urine, and is about 8 to 9 per cent, of an 

 atmosphere, whereas its tension in blood is only 5 to 6 per cent, of an 

 atmosphere. 



The Consumption of Oxygen by the Tissues. The tissues are 

 constantly taking up oxygen, and the amount used by any tissue, for 

 example the kidney, can be determined by ascertaining, first, the 

 difference in the quantity of oxygen present in 1 c.c. of the blood 

 entering and leaving the organ respectively ; secondly, the amount of 

 blood flowing through the organ in a given time ; and, thirdly, the weight 

 of the organ. The degree to which the blood is saturated with oxygen 

 is measured by Barcroft's blood-gas manometer. The rate of blood 

 flow through the organ is ascertained by observing directly the quantity 

 escaping from it in a given time. To take an example, if 1 c.c. of 

 arterial blood contains O18 c.c. oxygen, and 1 c.c. of blood from the 

 renal vein contains 0*13 c.c. of oxygen, each c.c. of blood passing 

 through the kidney loses 0'05 c.c. oxygen. Supposing the rate of 

 blood flow through the kidney to be 50 c.c. per minute, the amount of 

 oxygen taken up by the kidney cells is 2*5 c.c. ; and if the weight of 

 the kidney is 30 grams, 1 gram of kidney uses 0'08 c.c. oxygen 

 per minute. Experiments made in this way show that the amount of 

 oxygen consumed by the different tissues of the body varies greatly ; 

 the heart and kidney use more oxygen than any other organ. 



2 CONSUMED BY 1 GRAM OF THE TISSUE PER MINUTE. 



Skeletal muscle (resting) .... 0'003 c.c. 



(active) .... 0'03 c.c. 



Heart muscle . . . . . 0'05 c.c. 



Kidney 0'03-0'06 c.c. 



Submaxillary gland (resting) . . . s 0*02 3 c.c. 



