RESPIRATION 547 



is at a tension of a little over 40 mm. Hg in venous blood. 

 Hence there is a constant passage of carbon dioxide from the 

 tissues to the blood. The amount of C0„ which the blood 

 carries from the tissues to the lungs to be eliminated is a very 

 small proportion of the total amount carried — only between 

 8 and 9 per cent, of the whole. Possibly this amount may be 

 carried by the haemoglobin (p. 496). 



C. INTERNAL RESPIRATION. 



This has been already considered under muscle (p. 254 

 et seq.). 



The rate of internal respiration depends upon the activity 

 of the tissues, and not upon the amount of oxygen in the blood. 

 It has been shown that, when a tissue is stimulated, the 

 increased activity precedes the increased taking up of oxygen 

 and giving off of carbon dioxide, thus confirming the view that 



Oct ' ~ 



the evolution of energy is not due to a direct oxidation 

 (p. 249 et seq.). 



When the oxidation processes in the tissues are decreased 

 as in poisoning with cyanides, the oxygen tension in the 

 tissues is not lowered and COo is not evolved. Under 

 these conditions the sarcolactic acid liberated passes into 

 the blood and increases the 0^. But without the low 

 tension in the tissues the partial pressure of the oxygen in the 

 blood plasma remains so high that dissociation of HbOg does 

 not take place and bright red blood passes on to the veins. 



D. EXTENT OF RESPIRATORY EXCHANGE. 



This has been studied under metabolism of muscle (p. 254 

 et seq.). 



The extent of the respiratory interchange in the lungs is 

 governed by the extent of the internal respiratory changes, 

 i.e. by the activity of the tissues and chiefly of muscle. 

 Merely increasing the number or depth of the respirations has 

 only a transient influence on the amount of the respiratory 

 interchanges. Every factor which increases the activity of the 

 metabolic changes in the tissues increases the intake of oxygen 

 and the output of carbon dioxide by the lungs. 



