After severe muscular exercise the amount of blood returned by the veins to the heart increases. Something similar occurs when there is anemia, lack of oxygen, overactivity of the thyroid gland or fever.
Increase in materials carried in the blood occurs also in the toxemia of pregnancy, in dropsy, or with disturbances in the way in which the body uses salt and water.
As the inflow increases, the pressure in the blood vessels into which blood is pumped lessens. The heart beats more rapidly and strongly. As long as the heart can handle the change in its load of work, the patient may not be too much disturbed. When the load gets too big and the reserve power of the heart is damaged, emptying of the heart will fail to keep up with the inflow of blood. This may cause beginning failure of the heart.
If the flow of blood into the heart is inadequate, the tissues of the body will not get the blood and oxygen that they need. If this occurs suddenly, as it may do from a variety of causes, the brain does not get the blood and oxygen that it needs and the person faints or collapses. If the onset is gradual the blood vessels may accommodate themselves by contracting in less vital areas.
A sudden, severe lessening of inflow may occur after a large hemorrhage, by loss of fluid into the tissues as occurs in shock and bums, or as a result of insufficient water intake, or large loss of water as takes place in diarrhea. Pouring of much blood into the legs with large, dilated veins will lessen heart inflow, as will also some disturbances of the nervous system. Some drugs that greatly dilate veins or permit pouring of fluid into the tissues have a similar effect.