Physics in Medicine

Acidity then is due to the preponderance of hydrogen ions over hydroxyl ions, and alkalinity is due to the converse. Neutrality is an equilibrium of hydrogen and hydroxyl ions. Certain sub stances—called amphoteric electrolytes—produce both hydrogen ions and hydroxyl ions on dissociation, and may therefore combine as acids with bases, or as bases with acids. Proteins are of this class, and so of course, is water.

An alteration in acidity is the causative factor in the regula tion of respiration, the activity of muscle, and the excitability of nerve and plays an important part in regulating excretion and secretion.

The powerful effects of variation in hydrogen ion concentration on physiological processes require the existence of mechanism for the prevention of considerable changes of this kind. One of the main functions of blood is to provide this. It can itself with stand the addition of relatively considerable amounts of free acid or free alkali without much change in its reaction, largely owing to the presence of carbonates and phosphates. The electrolytic dissociation theory provides the only satisfactory explanation of this.

Colloids and Membranes.

The colloidal state is recognised as a permanent suspension of solid particles or liquid globules in a continuous medium of a different kind. In contrast to true solutions, colloids are heterogeneous systems in which the sus pended particles are large enough to possess the special properties peculiar to surfaces. It is easily seen that, since molecules at the surface of separation of two phases are attracted more power fully in the direction of the phase to which they belong than in the opposite direction, the interface will be in a state of tension. Consequently any change in the area of a surface film will result in a loss or gain of surface energy. Again, particles of colloid solutions usually possess an electrical charge, which is localized on its surface. Such particles are to a large extent prevented from aggregating together and falling as precipitates by the mutual repulsion of their electrical charges.

The deposition of charged ions on a colloidal particle or any surface would confer on the surface a charge of corresponding sign. This process is called adsorption and it may or may not be

accompanied by chemical action between the constituents of the surface and the adsorbed material. The neutralization of elec trical charges by adsorption of oppositely charged ions from added neutral salts results in precipitation. A similar effect may be produced by oppositely charged colloids.

An important case of adsorption is that occurring at the semi permeable membrane which covers cells. Physical principles show that any material which lowers surface energy will be ac cumulated in the surface. Certain substances, such as proteins, may form solid deposits in the surface films owing to their con centration brought about in this way. These deposits are capable in some cases of being redissolved if carried into the interior of the cell. Hence the cell membrane must have a variable struc ture in equilibrium with its contents.

Osmotic Pressure.

Semi-permeable membranes, i.e., mem branes which allow water to pass freely, but hold back dissolved substances exhibit another important property. Water will tend to diffuse more rapidly through the membrane towards that side on which the concentration of dissolved substance is greater, than in the reverse direction. This would cause a difference in hydro static pressure on the two sides if an increase in volume of the more concentrated solution is prevented by a non-yielding en velope. This pressure is called osmotic pressure and the uni lateral passage of fluid osmosis (q.v.). Solutions which exert the same osmotic pressure are said to be isotonic.

It is easy to see that the solutions which bathe a cell not en closed in a rigid envelope must, for equilibrium, possess the same osmotic pressure as the cell contents. Otherwise the cell will contract through loss or expand through gain of water, and will be destroyed unless the osmotic pressures can be equalized in this way. In any case if any change of concentration occurs, the functional activity of the cell will be affected. The mem brane covering individual cells and blood corpuscles is imperme able to salts, glucose, etc. ; that forming the walls of the capillary blood vessels is permeable to these but not to colloids.