394 RELATION OF RESPIRATION TO OTHER PROCESSES [CH. XXVII. 



crease of weight in bottles 4 and 5 weighed together gives the amount 

 of carbonic acid produced by the animal in the same time. 



Eanke gives the following numbers from experiments made on a 

 man, who was taking a mixed diet consisting of 100 grammes of 

 protein, 100 of fat, and 250 of carbohydrate in the twenty-four hours. 

 The amount of oxygen absorbed in the same time was 666 grammes ; 

 of which 560 passed off as carbonic acid, 9 in urea, 19 as water 

 formed from the hydrogen of the protein, and 78 from that of the fat. 



Vierordt from a number of experiments on human beings gives the 

 following average numbers : the amount of oxygen absorbed in the 

 twenty-four hours is 744 grammes ; this leads to the formation of 900 

 grammes of carbonic acid (this contains about half a pound of carbon) 

 and 360 grammes of water. 



The respiratory interchange is lessened during sleep. It is especi- 

 ally small in the winter sleep of hibernating animals. During hiber- 

 nation the respiratory quotient sinks to 0'5, so that the animals 

 actually gain weight from retention of oxygen. This aspect of respira- 

 tion is essentially so much a part of " metabolism " that it will be dealt 

 with more in detail in the chapters which deal with that subject. 

 (See especially the respiration calorimeter in Chap. XLTI.) 



Tissue Respiration. As has been already stated, respiration 

 may be divided into internal or tissue respiration and external or 

 pulmonary respiration. External respiration is much the less 

 obscure, and we have treated of it at considerable length, not only 

 on this account, but also on account of the very frequent impair- 

 ments of the pulmonary mechanism which are met with in disease. 

 It must be borne in mind, however, that pulmonary respiration is 

 but the means, and tissue respiration is the end. 



Tissue respiration consists in the passage of oxygen from the 

 blood of the capillaries to the cells of the tissues, and the passage 

 of carbonic acid in the reverse direction. This gaseous interchange 

 is no doubt brought about by a simple process of diffusion. The 



oxygen passes out of the plasma 

 of the blood through the capillary 

 wall, and then through the lymph 

 , , until it reaches the cell in which 



(iJIfliM it is going to be used, which 



we will suppose is a muscle fibre 

 FIG. BOG. (fig. 306). In order that a con- 



stant stream of oxygen may pass 



from the blood to the fibre, there must be a difference of oxygen 

 pressure between the oxygen dissolved in the plasma, and that 

 dissolved in the lymph, and the latter^must be at a greater pressure 

 than that dissolved in the muscle fibre. The amount of oxygen 

 which passes will, other things being equal, be directly proportional 



