12 ENERGY TRANSFORMATIONS 



but living cells displaying biotic energy, taking up energy from 

 the plasma in chemical form and using that energy by con- 

 verting it into volume or osmotic energy, and effecting thereby 

 separation of substances in solution in greater concentration 

 that is to say, such cells act as energy-transformers, the ultimate 

 conversion being from chemical into volume energy. 



Such a change is seen in the secretion of gases both in the 

 swimming bladder of the fish, and, according to Bohr and other 

 observers, in the mammalian lung, where the partial pressure of 

 the carbon-dioxide separated in the alveolar air is higher than that 

 in the blood of the pulmonary capillaries. Here the cells of the 

 lung are acting against pressure, and, no matter what the inter- 

 mediate steps may be, volume energy is increased in the process, 

 and must be obtained from the chemical energy of the cell, the 

 cell-protoplasm acting as the energy machine or transformer. 



Similar instances of gaseous secretion are seen (with the differ- 

 ence in these instances that the gas is retained in solution, and 

 the increased pressure is osmotic) in the case of the secretion of 

 saliva and of bile, where in both cases the pressure of dissolved 

 carbon-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 hyper-tonic 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 isotonic 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 important 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 



