Preload is also called as the left ventricular end diastolic pressure is the amount of stretch of the ventricle at the end of diastole. Preload is the volume of blood left at the end of diastole and it stretches the LV to its maximum under normal physiological conditions. You can relate it to a balloon when you blow air into the balloon, it stretches, same as heart while we blow blood in during diastole.
Afterload is also called as systemic vascular resistance and it is the pressure overcome by the heart to eject the blood during systole. Alternatively we can say, it is the vascular resistance, overcome by the heart to open the aortic valve, to pump blood into the systemic circulation.
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