Cardiac output (CO) is defined as the product of the Stroke Volume (amount of blood pumped per heart beat) times the Heart Rate (Number of times the heart beats per minute).
Cardiac output (CO) describes the volume or amount of blood that is pumped by the heart and distributed to the body in one minute. Cardiac output can be increased by either increasing the stroke volume (SV), increasing heart rate (HR) or by increasing both variables.
Stroke volume, the amount of blood pumped per heart beat, can be increased by two mechanisms. The first mechanism is to increase the contractility of the cardiac muscle fibers. Contractility is the strength or force that is generated by the muscle cells during a contraction. Increasing the contractility of the cardiac muscles increases the force or strength of the contraction of the heart muscles. With a stronger, more forceful cardiac contraction, more blood will be pumped from the heart ventricles. The Sympathetic nervous system innervates the heart myocardial or contractile cells, regulating myocardial cell contractility. With increased sympathetic nervous stimulation, the strength or contractility of the myocardial cells increases. The second way to increase stroke volume is to increase the End Diastolic Volume (EDV). The End Diastolic Volume is the volume of blood in the heart ventricles at the end of the diastolic phase, just before the ventricles contract. During work activities, more blood volume fills the ventricles, so when the ventricle muscles contract; more blood volume also leaves the heart. Both mechanisms regulate the amount of blood pumped out of the heart ventricles with each heartbeat.
The Cardiac output can also be influenced by the heart rate. Heart rate is also increased with increased sympathetic nervous system stimulation. The sympathetic nervous activity causes the pacemaker cells of the heart to fire more rapidly therefore causing an increase in heart rate. With an increased heart rate more blood will be pumped from the heart per minute, thus increasing the flow of blood to the active working skeletal muscles. Cardiac output will increase during work activities in direct proportion to the metabolic rate required to perform the exercise or endurance activity.
The second physiological response of the cardiovascular system to meet the oxygen demands of working skeletal muscle during endurance exercise is to redistribute the flow of blood from inactive organs, such as the stomach and gastrointestinal tract to the working muscles. At rest, approximately 15-20% of total cardiac output is distributed to skeletal muscle, while during maximal exercise, 80 -85% of the total cardiac output is redistributed to the contracting muscles. The vascular system accomplishes this redistribution by vasodilating, or relaxing the blood vessels in active muscles, thus allowing increased flow through these vessels and increasing the number of capillaries to these active muscles. The vascular system also vasoconstricts the arteries, or shunts the blood flow leading to internal organs. As a result, the amount of blood flow to the abdominal organs is reduced during work activity as compared to resting conditions.