Introduction

The primary function of the heart is to deliver a sufficient volume of blood (oxygen and nutrients, etc.) 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) x stroke volume (SV)

An average adult at rest may have a heart rate of 70 beats per minute and a stroke volume of 70 ml per beat. In this case, the cardiac output would be:

CO = 70 beats/min x 70 ml/beat = 4900 ml/min = 5 l/min

Table 14.1 Factors Affecting Cardiac Output

Heart rate

Autonomic nervous system

Catecholamines (epinephrine and norepinephrine) Body temperature

Stroke volume

Length of diastole Venous return (preload) Contractility of the myocardium Afterload Heart rate

Miscellaneous

Activity level Body size Age

This is approximately equal to the total blood volume in the body. Therefore, the entire blood volume is pumped by the heart each minute.

Many factors are involved in determining cardiac output in a given individual (see Table 14.1):

• Activity level

A primary determinant is the level of activity of the body. During intense exercise in an average sedentary person, cardiac output may increase to 18 to 20 l/min. In a trained athlete, the increase in cardiac output is even greater and may be as much as 30 to 35 l/min. Cardiac reserve is the difference between the cardiac output at rest and the maximum volume of blood that the heart is capable of pumping per minute. The effect of endurance training is to increase cardiac reserve significantly so that a greater volume of blood can be pumped to the working muscles. In this way, exercise performance is maximized and muscle fatigue is delayed. On the other hand, patients with heart conditions such as congestive heart failure or mitral valve stenosis are not able to increase their cardiac output as much, if at all, during exercise. Therefore, these patients are forced to limit their level of exertion as the disease process progresses.

The size of the body is another factor that determines cardiac output. Healthy young men have a cardiac output of about 5.5 to 6.0 l/min; the cardiac output in women averages 4.5 to 5.0 l/min. This difference does not involve gender per se, but rather the mass of body tissue that must be perfused with blood. Cardiac index normalizes cardiac output for body size and is calculated by the cardiac output per square meter of body surface area. An average person weighing 70 kg has a body surface area of approximately 1.7 square meters. Therefore:

Cardiac output also varies with age. Expressed as cardiac index, it rapidly rises to a peak of more than 4 l/min/m2 at age 10 and then steadily declines to about 2.4 l/min/m2 at the age of 80. This decrease in cardiac output is a function of overall metabolic activity and therefore indicative of declining activity with age.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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