136 ENERGY TRANSFORMATIONS IN LIVING MATTER 



dioxide is greater in the secretion than in the venous blood flowing 

 from the organ. Here, just as truly as in the alveoli of the lung, 

 volume energy is increased in the process of secretion. In a similar 

 manner hypertonic salt solutions are taken up from the serous 

 cavities by the blood and lymph capillaries lining their walls against 

 the gradient of osmotic pressure. The absorption of iso tonic 

 effusions, whether normal or pathological, is a process which also 

 demonstrates that the cells lining the vessels are not inert membranes 

 comparable in action to non-living membranes, whether permeable 

 or semi-permeable; but are living cells, capable of acting as active 

 absorbent channels, by behaving as machines possessing the im- 

 portant function of the conversion of chemical into volume energy. 

 But it is in the case of the typical secretory and excretory cells of 

 the body that this function of the living cell of acting as an energy 

 transformer between volume energy and chemical energy is seen 

 developed to its highest degree. In these cases, it is observed not 

 only that the amount of volume energy developed is larger, but 

 that the action is eminently selective according to the nature of the 

 dissolved substance. 



This subject will be gone into in greater detail in the chapter 

 on secretion and excretion; we need not therefore enter more fully 

 upon the matter here than to indicate that, in the kidney, for 

 example, urea solution is concentrated from less than -04 per cent, 

 in the circulating blood up to 2 per cent, in the urine, and that 

 in this process, no matter what may be the intermediate stages, 

 the kidney cells develop volume energy, against the usual laws 

 applicable to inert semipermeable membranes, just as much as a 

 mechanical engine attached to a piston and cylinder would do 

 in compressing a gas from a pressure of about 130 mm. of mercury 

 and a volume of 75,000 c.c. to a pressure of about 6500 mm. of 

 mercury and a volume of 1500 c.c., these being the volumes of the 

 blood and urine and pressures of the -04 and 2 per cent, solutions 

 respectively. 



In the face of this experimental evidence, surely it is time to 

 cease regarding kidney cells as semi-permeable membranes to 

 which the laws of osmosis apply. The case is exactly the reverse 

 of the semi-permeable membrane, in which the solvent passes 

 through, tending to equalise pressures and reduce volume energy ; 

 for in the kidney cell the dissolved substance passes through at a 

 greater rate than the solvent, increasing the difference in pressure 

 on the two sides and developing volume energy. 



