Essentially two conditions are produced.

In one the thickened, unyielding valves project across the openings they should guard, and thus by constricting the opening interfere with the passage of blood either through the heart or from it.

In the other the valves are so damaged that they cannot properly close the orifices they guard, and on or after the contraction of the cavities there is back flow or regurgitation of the blood.

If, for instance, the orifice of the heart into the aorta is narrowed, then the left ventricle can only accomplish its work of projecting into the aorta a given amount of blood in a given time by contracting with greater force and giving a greater rapidity to the stream passing through the narrow orifice.
This the heart can do because, like all other organs of the body, it has a large reserve force which enables it, even suddenly, to meet demands double the usual, and like all other muscles of the body it becomes larger and stronger by increased work.

The condition here is much simpler than when the same valve is incapable of perfect closure, or when both obstruction and imperfect closure, are combined as they not infrequently are.