pressure of blood serum is

•56 × 17,024
1.87

=

5,000 mm. of mercury approxi

mately, or a pressure of nearly 7 atmospheres.

The osmotic pressure of solutions may also be compared by observing their effect on red corpuscles, or on vegetable cells such as those in Tradescantia. If the solution is hypertonic, i.e. has a greater osmotic pressure than the cell contents, the protoplasm shrinks and loses water, or, if red corpuscles are used, they become crenated. If the solution is hypotonic, e.g. has a smaller osmotic pressure than the material within the cell-wall, no shrinking of the protoplasm in the vegetable cell occurs, and if red corpuscles are used they swell and liberate their pigment. Isotonic solutions produce neither of these effects, because they have the same molecular concentration and osmotic pressure as the material within the cell-wall.

Physiological Applications.—It will at once be seen how important all these considerations are from the physiological standpoint. In the body we have aqueous solutions of various substances separated from one another by membranes. Thus we have the endothelial walls of the capillaries separating the blood from the lymph; we have the epithelial walls of the kidney tubules separating the blood and lymph from the urine; we have similar epithelium in all secreting glands; and we have the wall of the alimentary canal separating the digested food from the blood-vessels and lacteals. In such important problems, then, as lymph-formation, the formation of urine and other excretions and secretions, and absorption of food, we have to take into account the laws which regulate the movements both of water and of substances which are held in solution by the water. In the body osmosis is not the only force at work, but we have also to consider filtration: that is, the forcible passage of materials through membranes, due to differences of mechanical pressure. Further complicating these two processes we have to take into account another force: namely, the secretory or selective activity of the living cells of which the membranes in question are composed. This is sometimes called by the name vital action, which is an unsatisfactory and unscientific expression. The laws which regulate filtration, imbibition, and osmosis are fairly well known and can be experimentally verified. But we have undoubtedly some other force, or some other manifestation of force, in the case of living membranes. It probably is some physical or chemical property of living matter which has not yet been brought into line with the known chemical and physical forces which operate in the inorganic world. We cannot deny its existence, for it sometimes operates so as to neutralise the known forces of osmosis and filtration.

The more one studies the question of lymph-formation, the more convinced one becomes that mere osmosis and filtration will not explain it entirely. The basis of the action is no doubt physical, but the living cells do not behave like the dead membranes of a dialyser; they have a selective action, picking out some substances and passing them through to the lymph, while they reject others.

The question of gaseous interchanges in the lungs has been another battlefield of a similar kind. Some maintain that all can be explained by the laws of diffusion of gases; others assert that the action is wholly

vital. Probably those are most correct who admit a certain amount of truth in both views; the main facts are explicable on a physical basis, but there are also some puzzling data which show that the pulmonary epithelium is able to exercise some other force as well, which interferes to some extent with the known physical process. Take again the case of absorption. The object of digestion is to render the food soluble and diffusible; it can hardly be supposed that this is useless; the readily diffusible substances will pass more easily through into the blood and lymph: but still, as Waymouth Reid has shown, if the living epithelium of the intestine is removed, absorption comes very nearly to a standstill, although from the purely physical standpoint removal of the thick columnar epithelium would increase the facilities for osmosis and filtration.

The osmotic pressure exerted by crystalloids is very considerable, but their ready diffusibility limits their influence on the flow of water in the body. Thus, if a strong solution of salt is injected into the blood, the first effect will be the setting up of an osmotic stream from the tissues to the blood. The salt, however, would soon diffuse out into the tissues, and would now exert osmotic pressure in the opposite direction. Moreover, both effects will be but temporary, because excess of salt is soon got rid of by the excretions.

Osmotic Pressure of Proteins.-It has been generally assumed that proteins, the most abundant and important constituents of the blood, exert little or no osmotic pressure. Starling, however, has claimed that they have a small osmotic pressure; if this is so, it is of importance, for proteins, unlike salt, do not diffuse readily, and their effect therefore remains as an almost permanent factor in the blood. Starling gives the osmotic pressure of the proteins of the blood-plasma as equal to 30 mm. of mercury. By others this is attributed to the inorganic salts with which proteins are always closely associated. Moore, for instance, finds that the purer a protein is, the less is its osmotic pressure; the same is true for other colloidal substances. It really does not matter much, if the osmotic force exists, whether it is due to the protein itself, or to the saline constituents which are almost an integral part of a protein. It is merely interesting from the theoretical point of view. We should from the theoretical standpoint find it difficult to imagine that a pure protein can exert more than a minimal osmotic pressure. It is made up of such huge molecules that, even when the proteins are present to the extent of 7 or 8 per cent., as they are in blood-plasma, there are comparatively few protein molecules in solution, and probably none in true solution. Still, by means of this weak but constant pressure it is possible to explain the fact that an isotonic or even a hypertonic solution of a diffusible crystalloid may be completely absorbed from the peritoneal cavity into the blood.

The functional activity of the tissue elements is accompanied by the breaking down of their protein constituents into such simple materials as urea (and its precursors), sulphates, and phosphates. These materials pass into the lymph, and increase its molecular concentration and its osmotic pressure; thus water is attracted (to use the older way of putting it) from the blood to the lymph, and so the volume of the lymph rises and its flow increases. On the other hand, as these substances accumulate in the lymph they will in

time attain there a greater concentration than in the blood, and so they will diffuse towards the blood, by which they are carried to the organs of excretion.

But, again, we have a difficulty with the proteins; they are most important for the nutrition of the tissues, but they are practically indiffusible. We must provisionally assume that their presence in the lymph is due to filtration from the blood. The plasma in the capillaries is under a somewhat higher pressure than the lymph in the tissues, and this tends to squeeze the constituents of the blood, including the proteins, through the capillary walls. I have, however, already indicated that the question of lymph-formation is one of the many physiological problems which await solution by the physiologists of the future.

INDEX

In cases where several references are made to any subject, the figure in heavy type indicates
where the principal matter in relation to the subject is to be found.