Cardiac output is the volume of blood the heart pumps each minute. It is calculated by multiplying the stroke volume by the heart rate: CO = SV x HR. Stroke volume is the volume of blood pumped with each beat (cardiac cycle). Heart rate is the number of times the heart beats in one minute.
Heart rate is controlled by parasympathetic and sympathetic innervation and circulating hormones. Parasympathetic innervation is by the vagus nerve, which slows heart rate by decreasing the firing rate of the SA (sinoatrial) node. Sympathetic innervation and circulating catecholamines (epinephrine) increase heart rate and the strength of the cardiac contraction. Thus, parasympathetic innervation (“rest-and-digest”) decreases cardiac output and sympathetic activation (“fight-or-flight”) increases cardiac output.
Making a Difference in the World 