Afterload and preload ï»¿ Preload is what comes to the heart before contraction. The heart muscle is much like a rubber band. The more you stretch it, the better it will contract. This stretch is accomplished in the heart through filling of the ventricles with blood. Therefore, preload is related to the amount of blood in the ventricle before contraction. If a person is overhydrated, preload will increase. If a person is dehydrated, preload will decrease. Afterload is what comes after ventricular contraction or the resistance against which the heart must pump blood. Afterload is determined by two conditions; the blood volume ejected from the ventricle and the compliance of the vascular space into which the blood is ejected. Think of afterload as a hose nozzle. If the hose nozzle is wide open, afterload is decreased due to decreased compliance. If the hose nozzle is almost closed, afterload will increase because the water has so much resistance to push against. Now, if you increase or decrease the amount of water that comes from the nozzle, there will be a further effect on the afterload. Think of afterload as blood pressure. Increased blood pressure is increased afterload, while decreased blood pressure is decreased afterload.