This is the amount of blood returning to the heart after it has been circulated and deoxygenated by the end-organ tissues. Preload is important to cardiac output because if there is a reduction in blood returning to the heart, the downstream effect would be a reduction in cardiac output.
Afterload
This is the pressure against which the left ventricle must contract to push out the cardiac output required by the body at a given time. If the afterload is elevated for a period of time long enough for the heart muscle to fatigue, left ventricular clearing is reduced, which can have negative effects once the blood begins to collect in the pulmonary vasculature. When the heart muscle itself becomes fatigued and left ventricular performance is reduced, as is the cardiac output secondary to the loss of contractility.
