Cardiac output is a function of heart rate and stroke volume. Itis equal to the heartbeat multiplied by the stroke volume, and is measuring the volume of blood leaving the heart per unit of time. Heart is determined by the rate of depolarization in auto-rhythmic cells which can be decreased by parasympathetic (ach) innervations or increased by sympathetic innervation (epi from adrenal medulla).
Stroke volume is determined by the force of contraction in ventricular myocardium and is influenced by contractility and length-tension relationship of muscle fibers. The length-tension relationship of muscle fibers caries with venous return which is aided by skeletal muscle pup and respiratory pump.