CH. XXVII.] TISSUE RESPIRATION 395 



to these pressure differences, and as the amount varies greatly at 

 different times, it is obvious that the pressure differences vary 

 greatly also. When the muscle is at rest, the oxygen pressure in 

 the capillaries is very near to that in the muscle fibre ; when the 

 muscle is active and using large quantities of oxygen, the intra- 

 capillary oxygen pressure is much greater than the intra-muscular 

 oxygen pressure. Such a change might be brought about by a rise 

 in the intra-capillary oxygen pressure, or a fall in the intra-muscular 

 oxygen pressure, or by both taking place simultaneously. Let us 

 therefore enquire what is known about these quantities. 



The tension of oxygen in muscle has recently been calculated 

 as being at most equal to 19 mm. of mercury; from this it may 

 vary down to zero. Within these limits the conditions for diffusion 

 can be increased by a drop in the intra-muscular oxygen pressure. 



There is, in addition, a mechanism for raising the intra-capillary 

 oxygen pressure. This is the increased quantity of acid (carbonic 

 and sarco-lactic acids) which is thrown into the blood as the result 

 of muscular metabolism. The following diagram (fig. 307) shows 

 the extent, both in degree and time, of this pouring of acid into the 

 blood as the result of a tetanic contraction of a muscle lasting only 

 a few seconds. 



10 30 50 70 90 110 130 J50 170 1913 210 230 250 270 290 



Time In seconds. 



FIG. 307. The black area represents the lactic acid thrown into the blood during the time following 

 a tetanus that lasted 84 seconds; the work done by the muscle was 70 gramme-centimetres; the 

 total quantity of lactic acid formed was O'OOS grammes. The figures on the vertical line represent 

 fractions of a gramme of lactic acid per second. 



In glandular structures the oxygen pressure is higher than 

 in muscle; probably owing to the relatively more copious blood- 

 supply of glands, equilibrium is more readily established between 

 the blood and the gland cells, the oxygen pressure in the cells being 

 almost that present in venous blood. 



The quantity of oxygen used by different tissues varies not only 

 with the degree of their activity, but also with the nature of the 

 tissues. On the whole it may be said that, weight for weight, 

 glandular tissue uses most oxygen ; next in order come the 

 muscular tissues, and last of all, the connective tissues. There are 

 some important tissues, notably the nervous system, about which 

 little is known in this connection. The amount of oxygen used by 



