Cardiac output is the amount of blood pumped by the heart into the aorta in one minute. Heart beats 70-72 times per minute and pumps out about 70 ml blood during each beat. Mechanism of regulating the heart beat : The normal rate of heart beat is maintained by the regulating mechanisms.

1. Nervous control: The SA node (pace maker) is the immediate regulator of heart rate. But the action of the SA node is controlled by the cardiovascular centre located in the medulla oblongata. Two sets of nerves lead from Cardiovascular centre in the brain. These are sympathetic that stimulates the SA node and accelerate the heart beat and parasympathetic or vagus nerve that inhibit the SA node and retards the heart beat.

2. Hormonal control: Adrenalin and thyroxin influence the heart rate independent of the nervous system. Adrenalin increases heart beat by directly influencing SA node. Thyroxin increases oxidative metabolism of body cells. This requires more oxygen and thus indirectly increases heart rate.