SECRETION 



WE have seen in the preceding chapters how important is the function of enzymes 

 in the regulation of the chemical changes of living organisms. Now there is a 

 liquid, one amongst others of the same class, and known as the pancreatic juice, 

 which is formed by the cells of a certain organ and poured into the cavity of the 

 intestine. Its chief properties are due to the variety of enzymes which it contains, 

 although there are other substances present. It may be considered as a typical 

 case of secretion. The cells of the pancreas produce substances which are not 

 present in the blood bathing them and, at the same time, they separate water 

 from the blood in order to carry off these substances in solution. Along with the 

 water we find, as a rule, some other constituents of the blood transferred to the 

 secretion, especially diffusible salts, such as sodium chloride. 



There are, however, included under the general name of secretion, the activities 

 of such an organ as the kidney, whose chief function is to separate from the blood 

 certain products of metabolism, such as urea, which would be injurious to the 

 organism unless removed. There are, moreover, the so-called "internal secretions," 

 where substances having special actions on other parts of the organism are formed, 

 but, instead of leaving the cells in which they are produced by a surface in 

 connection with a special channel, the duct of the gland, they are sent in the 

 other direction into the blood current. In such cases, materials supplied by the 

 blood are converted by the organ in question into " chemical messengers " or 

 " hormones," and returned to the blood in this altered form. 



It will thus be seen that, xinder certain aspects, the process of secretion is a part of the 

 general cell metabolism, especially in the case of the internal secretions The manner in 

 which the passage of water is effected in the case of the typical external secretions is a 

 question of much interest, together with the way in which it is regulated. The influences 

 at work causing the production of the specific contents of the secretion will also require 

 our consideration. 



In the present state of knowledge, it is impossible to treat the subject from a really general 

 point of view. Perhaps we may look upon the transfer of water from the blood to the secretion 

 as a property common to the majority of cases, so that this phenomenon will be discussed in 

 the first place. It will afterwards be necessary to take special instances, and, as far as possible, 

 our chief attention will be given to those points of most general application. 



SECRETION OF WATER 



The most obvious hypothesis to make is that the layer of cells forming the 

 membrane intervening between the blood vessels and the lumen of the duct has 

 the properties of a semi-permeable membrane, so that, supposing the pressure in 

 the blood vessels to be higher than the osmotic pressure of the blood, pure water 

 will be forced through. But the osmotic pressure of the blood, as we have seen 

 (page 165), is as high as 6'5 atmospheres, or 5,000 mm. of mercury, whereas 

 200 mm. of mercury is a high value for the blood pressure. Such a hypothesis is 

 clearly an impossible one. But it is very rarely, if ever, that a secretion consists 

 of pure water, so that the difference of osmotic pressures is not so great as that 

 given. If the membrane, or one of the membranes, intervening is permeable only 

 to the colloids of the blood, such as a gelatine membrane, a very much lower 

 arterial pressure will suffice to filter off a solution containing all the crystalloid 

 components of the blood, in the same concentration as in it. We shall presently 

 see reason to believe that this is the case with the "glomerulus" of the kidney, 



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