This form of hypertrophy implies that, as a result of some defect in the organism, some function has been called into unusual exercise. As a result of the continued excessive exercise, the tissue is increased. The necessity for this increased exercise may arise in one of two ways; there may be from atrophy or destruction an actual loss of tissue, and the remaining tissue enlarges to bring it up to the normal amount; or the circumstances may be such as to call for the exercise of a particular function in excess of what is usual, so that, in relation to the increased need, the existing tissue is defective in amount. In this case the new-formed tissue constitutes by so much an absolute excess over the average normal.
The most striking instance of compensatory hypertrophy from loss of tissue is that afforded by the enlargement of one kidney as a result of destruction or disease of the other. The hypertrophied kidney sometimes attains to the bulk of the two normal ones, especially if the lesion has occurred in a young person. A similar hypertrophy occurs in the lung in cases of congenital non-inflation of one lung; also in the testicle, where one is wanting or defective in its development, and in the liver, where, from destruction of a large portion of the right lobe, the left may attain to the size which the right normally presents. (See under the affections of these various organs).
It is necessary to distinguish Pseudo-hypertrophy from the proper compensatory hypertrophy illustrated above. Atrophy of a tissue may be accompanied by an excessive growth of tissue different from that which has been lost, and the new tissue may not only make up the normal bulk but exceed it, so as to give an appearance of hypertrophy. Atrophy of muscle is frequently accompanied by development of adipose tissue between the ultimate fibres, and in pseudo-hyper-trophic paralysis a deceptive appearance of hypertrophy of the muscles is produced. A similar excessive production of adipose tissue sometimes occurs around effete and disused glands. (See under Fatty infiltration).
The other form of compensatory hypertrophy, that characterized by absolute excess of tissue, is exhibited chiefly in muscular organs. In the case of canals with muscular walls, constrictions of the canals or orifices and defects in the valves frequently occur, and these may necessitate increased muscular effort to compensate for the defects. Thus the walls of the heart frequently hypertrophy from disease at the orifices, in the valves, or in the general vascular system. The urinary bladder shows hypertrophy of its muscular coat in consequence of obstruction at its neck (enlarged prostate) or in the urethra. The muscular coat of the stomach frequently hypertrophies from obstruction of the pylorus, and that of the intestine from obstruction of its calibre. (See under the organs named).