INTERNAL OR TISSUE RESPIRATION 273 



assumed that it is also variable in the tissues. There is no evidence 

 and little probability that the variations are always parallel. Quan- 

 titative and even qualitative differences in the substances which can 

 bind carbon dioxide are known to exist between the blood and the 

 tissues, and it is uncertain how much interchange of such compounds 

 carrying with them combined carbon dioxide, which may after- 

 wards become dissociated, takes place between the blood and the 

 tissue lymph or the cells. However probable, then, it may be that 

 the transportation of carbon dioxide from the cells to the lymph, 

 and from the lymph to the blood is managed in the same way as 

 that of the carbon dioxide from the blood to the alveolar air, namely, 

 by diffusion of the gas in solution, it cannot be said that at present 

 clear proof of this has been obtained. Results are, indeed, on 

 record, which purport to show that the partial pressure of carbon 

 dioxide in various tissues or in physiological liquids which have 

 been in contact with them is higher than that of the arterial or even 

 than that of the venous blood. But these results are of unequal 

 and some of them of doubtful value for the solution of the problem 

 under discussion. 



Lymph, bile, urine, and the serous fluids contain, so much carbon 

 dioxide that the pressure of that gas in all of them is greater than in 

 arterial blood, while in lymph alone (taken from the large thoracic 

 duct) has it been found less than that of venous blood. And it is pro- 

 bable that lymph gathered nearer the primary seats of its production 

 (the spaces of areolar tissue) would show a higher proportion of carbon 

 dioxide. Strassburg found that with a pressure of carbon dioxide in 

 the arterial blood of 21 mm. of mercury, the pressure in bile was 50 mm. , 

 in peritoneal fluid 58 mm., in urine 68 mm., in the surface of the empty 

 intestine 58 mm. Saliva, pancreatic juice, and milk, also contain much 

 carbon dioxide. From muscle as much as 15 volumes per too of carbon 

 dioxide can be pumped out, some of which is free that is, is given 

 up to the vacuum alone while some of it is fixed, and only comes off 

 after the addition of an acid. 



Before leaving the subject of the gaseous exchange between 

 tissues and blood and between blood and air some important con- 

 sequences of the form of the dissociation curve of oxyhsemoglobin, 

 and of the modifications produced in it by change of temperature, 

 by salts and by acids, must be alluded to. They have been developed 

 in masterly fashion by Barcroft. The flattening of the hyperbolic 

 dissociation curve of dialyzed oxyhaemoglobin when the tempera- 

 ture is raised (Fig. 119) makes it obvious that for any percentage 

 saturation which can exist in blood as it leaves the systemic capil- 

 laries, the corresponding partial pressure of oxygen must be much 

 greater at the body temperature (38 C.) than at 16 C. For haemo- 

 globin which is 50 per cent, saturated it is twenty-five times as 

 great. This favours the giving up of oxygen to the tissues by blood 

 in which the percentage saturation is considerably reduced. On 



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