Final answer:
Preload is the volume of blood in the ventricles at end of diastole, while afterload is the peripheral resistance against which the left ventricle must pump. Diastole is the relaxation of the myocardium, and systole is the contraction of the myocardium.
Step-by-step explanation:
Preload: Preload is another way of expressing EDV. Therefore, the greater the EDV is, the greater the preload is. One of the primary factors to consider is filling time, or the duration of ventricular diastole during which filling occurs. The more rapidly the heart contracts, the shorter the filling time becomes, and the lower the EDV and preload are.
Afterload: Afterload refers to the tension that the ventricles must develop to pump blood effectively against the resistance in the vascular system. Any condition that increases resistance requires a greater afterload to force open the semilunar valves and pump the blood. Damage to the valves, such as stenosis, which makes them harder to open will also increase afterload. Any decrease in resistance decreases the afterload.
Diastole: Diastole is the relaxation of the myocardium. In the cardiac cycle, diastole corresponds to the phase when the heart muscles are relaxed and blood flows into the heart.
Systole: Systole is the contraction of the myocardium. In the cardiac cycle, systole corresponds to the phase when the ventricles of the heart contract, pushing blood out of the heart.