Preload refers to the amount of blood that is returned to the right atria. When preload is increased, it triggers a mechanism called the Frank-Starling effect that stretches the heart. Much like a rubber band, the greater the stretch, the greater the contraction. This results in an increase in cardiac output.
Afterload refers to the amount of pressure that the left ventricle must overcome in order to eject blood into the aorta. Decreasing the afterload will result in an increase in preload; an increase will have the opposite effect.
