ROBERT C. PLUMB
chemical principles exemplified
Worcester Polytechnic lnrliluls Worces1.r. M a % m h w t l a01609
Stomach Upset Caused by Aspirin Illustrating principles of acid-base equilibrium and solubility Conrnbut~onh\ l'rf,\~.swrI!'illiom I ) Hohr), Il'owucwcr /+I/\rechn~clnrfirute, adopred from orticl~ah\ Ilr,rocr I\:Uownporr' The reader of this exemplum must adopt a mature, non-alarmist perspective. The exemplum is concerned with a side effect of one of the most effective and safe pain relievers known to man. The effect takes place in all people, hut for most individuals it is entirely inconsequential. When two aspirin tablets are swallowed, the tissue of the stomach wall suffers a small amount of damage; for an average person the damage will cause from 0.5 to 2.0 ml of blood to drain into the stomach cavity. This is an entirely trivial amount of damage and the body soon heals itself. However, some susceptible individuals may lose hundreds of milliliters of blood as a reaction to aspirin, and some habitual users of aspirin may even develop anemia hecause of the high rate of bleeding of the stomach wall. The mechanism by which aspirin damages the stomach wall provides an interesting illustration of some principles of acid-base equilihrium and solubility. The digestive secretions (principally hydrochloric acid and pepsinogen) are generated in the stomach lining (the mucosa). These substances, separately, reach the stomach cavity through tubules which pass through a harrier layer on the inner surface of the stomach lining. The barrier layer is made up of columnar epithelial cells tightly fused together and coated with an ordered array of lipid and protein. When the hydrochloric acid and the pepsinogen meet in the stomach cavity, they react to form pepsin, an enzyme that cleaves protein chains. The fatty lipids are not attacked by the pepsin, and the harrier layer serves to protect the mucosa from the digestive juices. Although lipids do not dissolve in the digestive juices, some contents of the stomach can dissolve in the lipids and here lies the crux of the problem. Highly polar and ionic substances dissolve in water, whose molecules are polar, while nonpolar molecules may dissolve in lipids. The hydrogen ion from the hydrochloric acid normally remains in aqueous solution in the stomach cavity since the nonpolar lipid layer will not dissolve ionic substances.
'Davenport, Horace W., The New England Journal of Medicine, 276, 1307 (1967); Scientific American, 226 (No. 11, 86 (January 1972). Both articles are quite readable, but the second presents more of the physiological background. 2A series of numerical problems illustrating this point can be easily generated by the teacher of equilibria. =This statement is not to be taken as an endorsement of anv pmdurt. There
