The driving force of blood circulation is the venous return, driven by the difference between mean systemic filling and right atrial pressure. The heart is a complex pump that acts as a put-through system and at the same time increases the pressure of the blood in the arterial system. The relatively high arterial pressure is necessary to meet the flow demand of the tissues in times of need. The output of the heart (stroke volume) multiplied by the heart rate determines the cardiac output (L/min). The performance of the heart is determined by preload, afterload, and contractility. In young children, afterload seems to be the most important factor for the performance of the left ventricle. With growing age, this shifts more to the preload. Since blood pressure is the product of systemic vascular resistance and cardiac output, a low blood pressure can be the result of a low cardiac output, a low systemic vascular resistance, or both.