166 PRINCIPLES OF GENERAL PHYSIOLOGY 



(1896 and 1912) has insisted on the importance of osmotic processes in addition. 

 It is, in fact, clear that a rise in the osmotic pressure of the lymph, however this 

 rise is produced, will result in the passage of water from the blood to the lymph, 

 and an increase in the volume present. We see then why the amount of lymph 

 flow from an organ is increased by activity of that organ. The energy required 

 for activity is afforded by chemical processes which result in the production of a 

 larger number of small molecules from larger ones, with a consequent increase in 

 the molar concentration and osmotic pressure of the contents of the cells. These 

 metabolic products diffuse from the cells into the lymph surrounding them, thus 

 raising its osmotic pressure above that of the blood, with the result that water 

 passes from the latter to the lymph and causes an increase in its volume. 



Under other conditions, fluid is absorbed from the tissue spaces into the blood. 

 After loss of blood by haemorrhage, for example, water is taken by the blood from 

 the tissue spaces. According to Starling (1896, p. 321) the process depends on 

 the osmotic pressure of the colloids of the blood. Although lymph contains a 

 certain amount of protein, this amount is normally small as compared with that 

 in the blood plasma, so that the osmotic pressure of the latter is higher than that 

 of the lymph. Under normal conditions, this would result in absorption of water 

 by the blood, were it not that the difference of osmotic pressure is balanced by the 

 difference of mechanical pressure in favour of the contents of the blood vessels. 

 If this latter pressure rises above the osmotic pressure of the colloids of the blood, 

 water will be driven into the tissue spaces and the blood will become more 

 concentrated. If it falls, as after loss of blood, water will pass in the other 

 direction, and the volume of the blood will be increased at the expense of the tissue 

 fluids. It is assumed that the walls of the blood vessels are permeable to all the 

 solutes of blood and lymph, with the exception of those in the colloidal state. 



USE OF THE EXPRESSION "OSMOTIC PRESSURE" 



It is, perhaps, well to make a few remarks with respect to the view held by some that 

 osmotic pressure only exists in the presence of a semi -permeable membrane. If this is so, 

 we are incorrect in speaking of the osmotic pressure of a solution under any circumstances 

 except those in which it is separated from pure solvent by means of a membrane impermeable 

 to solutes. When, therefore, that property of a solution which would cause it to show 

 osmotic pressure under these special circumstances is determined by some other method, 

 such as freezing point, another name must be used. 



It is clear that such a practice, although perhaps in agreement with the original meaning 

 of osmosis as used by Dutrochet, would give rise to much inconvenience, and even confusion. 

 We need a word to express the total concentration of a solution in such elements as act as 

 molecules in the sense of Avogadro's law, since the molar concentration does not afford the 

 information in the case of electrolytes and colloids. It seems to me that we are quite 

 justified, even in theory, in speaking of the osmotic pressure of the blood, for example, 

 without any reference, even in thought, to a semi-permeaDle membrane. We mean to express 

 those properties conferred by the kinetic energy of the molecules, or elements equivalent 

 to them, of the solutes. In the presence of a semi-permeable membrane it would be shown 

 as a definite pressure, capable of measurement by a manometer ; but the phenomenon which 

 causes this pressure is always there and leads to diffusion, amongst other things. 



This denying of the existence of osmotic pressure except in relation to a membrane leads 

 to the denial of its existence altogether, since we know of no perfect semi-permeable membrane. 



No objection is made to the statement that the air in a vessel open to the atmosphere has 

 a pressure of 760 mm. of mercury, although it is not to be detected unless the vessel is closed 

 and provided with a manometer while the outer atmosphere is removed. 



In the present book I intend to continue to make use of the words "osmotic pressure," 

 meaning thereby that property of solutions conferred upon them by the kinetic energy of the 

 solutes. 



The name " tonicity " is sometimes used, especially in reference to blood corpuscles and 

 living cells in general, but it is not necessarily the same as osmotic pressure, unless we admit 

 that the latter may vary according to the membrane used. For example, we say that a 

 solution of sodium chloride is "isotonic" with mammalian blood corpuscles, because it 

 produces no change in their volume. But we might add an equivalent amount of urea to this 

 solution without making it less " isotonic" with the blood corpuscles, because their membrane 

 is permeable to urea. On the other hand, its osmotic pressure is really doubled, as shown by 

 vapour pressure measurements. The word "isotonic" can only be used when the nature of 

 the particular membrane is specified and refers only to those constituents of the solution to 

 which the membrane is impermeable ; osmotic pressure refers to the total concentration, 

 assuming that the membrane is impermeable to all the solutes, permeable to the solvent. 



