In such cases the ventricle must do more than in the first case.

It must force through the orifice, which may be narrowed, the amount of blood which is necessary to keep up the pressure within the aorta and give to the circulation the necessary rapidity of flow, and also the amount which flows back into the heart through the imperfectly acting valve.

This it can do by contracting with greater force upon a larger amount of blood, the cavity becoming enlarged to receive this.

Not only may such damage to the valves be produced, but the muscular tissue of the heart may suffer from defective nutrition or from the effect of poisons, whether these are formed in the body as the effect of disease or introduced from without; or in consequence of disease in the lungs the flow of blood through them may be impeded, or disease elsewhere in the body, as in the kidneys may, by increasing the pressure of the blood within the arteries, throw more than the usual amount of work upon the heart.
The power of the heart in meeting these conditions, however various they are and however variously they act, seems little short of marvellous, and it goes on throwing three and one-third ounces of blood seventy or eighty times a minute into a tube against nine feet of water pressure, working often perfectly under conditions which would be fatal to a machine.

As long as this goes on the injury is said to be compensated for; the increased work which the heart is able to accomplish by the exercise of its reserve force and by becoming larger and stronger enables it to cope with the adverse conditions.
With increased demand for work there is a gradual diminution of the reserve force.