The cardiac output is the volume of blood pumped from the heart every minute. It is obtained by the volume pumped with each beat (stroke volume) multiplied by the heart rate. The cardiac output increases during exercise and falls in the common type of heart failure. The stroke volume depends in part on the amount of blood in the ventricule at the end of diastole. This determines the degree of stretch of the myocardial fibre. The greater the stretch (within limits) the stronger the force of contraction and the greater the stroke volume (Starlings law of the heart).
Control of the heart rate depends on the ratio of the sympathetic (increases) and parasympathetic (decreases) impulses to the sino atrial node which is the cardiac pacemaker. Receptors sensitive to pressure are located in the carotid sinus and the wall of the arch of the aorta.When the carotid sinus detects a sudden rise in arterial pressure, impulses pass from the carotid baroreceptors to the cardiac control centre in the medulla and impulses sent through the parasympathetic system reduce the heart rate. Similar effects result from the aortic baroreceptors as well. If the blood pressure drops the heart rate is reflexly increased. Various other reflexes from anger, fear, pain, temperature, etc. also increase the heart rate.