13.6: Cardiac Output I:Effect of Heart Rate on Cardiac Output

Cardiac Output

Cardiac output (CO) refers to the total amount of blood ejected by one of the ventricles in liters per minute (L/min). In a resting adult, CO ranges from 5 to 6 L/min, adjusting according to the body's metabolic requirements.

Effect of Heart Rate on Cardiac Output

Cardiac output adapts to metabolic demands during stress, physical activity, or illness. The autonomic nervous system regulates heart rate via the sinoatrial node. The parasympathetic nervous system decreases heart rate through the vagus nerve. In contrast, the sympathetic nervous system increases heart rate by stimulating beta-1 receptors in the sinoatrial node and releasing catecholamines, such as adrenaline. Thyroid hormones also elevate heart rate by mimicking catecholamine effects.

Baroreceptors, specialized nerve cells located in the aortic arch and the internal carotid arteries, play a crucial role in sensing fluctuations in blood pressure, which in turn influences heart rate.

In hypertension, they increase their discharge rate, sending signals to the cerebral medulla.

This triggers parasympathetic activity and suppresses the sympathetic response, reducing heart rate and blood pressure.

Conversely, during hypotension, reduced baroreceptor activity enhances sympathetic responses, raising heart rate and inducing vasoconstriction to elevate blood pressure.

These mechanisms help maintain stable blood pressure.

Cardiac Cycle and Output Calculation

The cardiac cycle includes systole (ventricular contraction) and diastole (ventricular relaxation). During systole, the ventricles eject blood; in diastole, the ventricles fill with blood.

Cardiac output is derived by multiplying stroke volume (SV), the amount of blood ejected per heartbeat, by the heart rate (HR).