ABSTRACT
Study objectives Describe the factors that determine cardiac output Distinguish between cardiac output, cardiac reserve, and cardiac index Discuss the factors that control heart rate Distinguish between the terms chronotropic and inotropic Discuss the factors that control stroke volume Distinguish between preload and afterload Describe the Frank-Starling Law of the Heart Understand how the cardiac function curve is generated Explain the mechanism of action of diuretics in congestive heart failure and hypertension Dene ejection fraction Describe how cardiac output varies in a sedentary individual versus an endurance-trained athlete
. Introduction The primary function of the heart is to deliver a sufcient volume of blood (oxygen and nutrients, and so forth) to the tissues so that they may carry out their functions effectively. As the metabolic activity of a tissue varies, so will its need for blood. An important factor involved in meeting this demand is cardiac output (CO) or the volume of blood pumped into the aorta per minute. Cardiac output is determined by heart rate multiplied by stroke volume:
Cardiac output (CO) = Heart rate (HR) × Stroke volume (SV)
An average adult at rest may have a heart rate of 70 beats per minute and a stroke volume of 70 milliliters per beat. In this case, the cardiac output would be
CO = 70 beats/min × 70 ml/beat
= 4900 ml/min
≈ 5 L/min
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This is approximately equal to the total volume of blood in the body. Therefore, the entire blood volume is pumped by the heart each minute.
