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. An individual may be able to carry easily forty pounds
up a hill and by exerting all his force may carry eighty pounds, but
if he habitually carries the eighty pounds, even though the muscles
become stronger by exercise the load cannot be again doubled. The
dilatation of the heart which is so important in compensation is
fraught with danger, because any weakening of the muscle increases the
dilatation, until a point is reached when, owing to the dilatation of
the orifices between auricles and ventricles, the valves become
incompetent to close them.

When the heart is not able to accomplish its work, the effect of the
condition becomes apparent by the accumulation of blood within the
veins and a less active circulation. This affects the nutrition and
the capacity for work of all the organs of the body, and the imperfect
function of the organs may in a variety of ways make still greater
demands upon an already overloaded heart. Other conditions supervene.
The increased pressure within the veins and capillaries due to the
impossibility of the blood in the usual amount passing through or from
the heart increases the amount of fluid in the tissues. There is
always an interchange between the blood within the vessels and the
fluid outside of them; the passage of fluid from the vessels is
facilitated by the increased pressure within them, just as pressure
upon a filtering fluid increases the rapidity of filtration, and the
increase of pressure within veins and capillaries impedes passage of
tissue fluid into them. The fluid accumulates within the tissues
leading to dropsy, or the accumulation may take place in some of the