Regulation of Cardiac Output

The main changes in cardiac output are normally determined by the metabolic requirements of the body, and as such, the heart plays a “permissive” role and does not regulate its own output. Thus, in the human circulation the heart will eject only the minimum volume demanded of it.

The output of the heart represents a balance between the venous return and the ability of the heart to meet the demands of venous return. The healthy normal heart is well able to meet a large range of demands, however when its capabilities are exceeded signs and symptoms of cardiac failure appear.

Factors Determining Cardiac Output

The essential function of the beating heart can be viewed as transporting blood from the low-pressure high-compliance venous system to the high-pressure low compliance arterial system. As such, venous return has a significant influence on the amount of blood ejected from the heart. Under circumstances in which no other controls are operative, the venous return to the right atrium may be determined as the balance between the filling pressure (the force returning blood to the right atrium) and the right arterial pressure (the opposing force). The resulting venous return at different levels of filling pressure and arterial pressure is represented in the below venous return curve.

As illustrated in the classic Starling curve, after an increase in ventricular end-diastolic volume, the succeeding contraction will result in an increased intraventricular pressure. This in turn will result in an increased stroke volume and cardiac output. Thus, cardiac output can be related to right atrial pressure.

Starling curve