Essentials of Anatomy and Physiology

This exercise cardiac output is twice the resting
cardiac output we first calculated, which should not be
considered unusual. The cardiac output of a healthy
young person may increase up to four times the rest-
ing level during strenuous exercise. This difference is
the cardiac reserve, the extra volume the heart can
pump when necessary. If resting cardiac output is 5
liters and exercise cardiac output is 20 liters, the car-
diac reserve is 15 liters. The marathon runner’s cardiac
output may increase six times or more compared to
the resting level, and cardiac reserve is even greater
than for the average young person; this is the result of
the marathoner’s extremely efficient heart. Because of
Starling’s law, it is almost impossible to overwork a
healthy heart. No matter how much the volume of
venous return increases, the ventricles simply pump
more forcefully and increase the stroke volume and
cardiac output.
Also related to cardiac output, and another measure
of the health of the heart, is the ejection fraction.
This is the percent of the blood in a ventricle that is
pumped during systole. A ventricle does not empty
completely when it contracts, but should pump out
60% to 70% of the blood within it. A lower percent-
age would indicate that the ventricle is weakening.
These aspects of physiology are summarized in Table
12–2.

REGULATION OF HEART RATE

Although the heart generates and maintains its own
beat, the rate of contraction can be changed to adapt

to different situations. The nervous system can and

does bring about necessary changes in heart rate as

well as in force of contraction.

The medullaof the brain contains the two cardiac

centers, the accelerator centerand the inhibitory

center. These centers send impulses to the heart

along autonomic nerves. Recall from Chapter 8 that

the autonomic nervous system has two divisions: sym-

pathetic and parasympathetic. Sympathetic impulses

from the accelerator center along sympathetic nerves

increase heart rate and force of contraction during

exercise and stressful situations. Parasympathetic

impulses from the inhibitory center along the vagus

nerves decrease the heart rate. At rest these impulses

slow down the depolarization of the SA node to what

we consider a normal resting rate, and they also slow

the heart after exercise is over.

Our next question might be: What information is

received by the medulla to initiate changes? Because

the heart pumps blood, it is essential to maintain nor-

mal blood pressure. Blood contains oxygen, which all

tissues must receive continuously. Therefore, changes

in blood pressure and oxygen level of the blood are

stimuli for changes in heart rate.

You may also recall from Chapter 9 that presso-

receptors and chemoreceptors are located in the

carotid arteries and aortic arch. Pressoreceptorsin

the carotid sinuses and aortic sinus detect changes in

blood pressure. Chemoreceptors in the carotid

bodies and aortic body detect changes in the oxygen

content of the blood. The sensory nerves for the

carotid receptors are the glossopharyngeal (9th cra-

nial) nerves; the sensory nerves for the aortic arch

284 The Heart

Table 12–2 PHYSIOLOGY OF THE HEART

Aspect and

Normal Range Description

Heart rate (pulse):

60–80 bpm

Stroke volume:

60–80 mL/beat

Cardiac output:

5–6 L/min

Ejection fraction:

60%–70%

Cardiac reserve:

15 liters or more

Generated by the SA node, propagated through the conduction pathway; parasympathetic

impulses (vagus nerves) decrease the rate; sympathetic impulses increase the rate

The amount of blood pumped by a ventricle in one beat

The volume of blood pumped by a ventricle in 1 minute; stroke volume x pulse

The percentage of blood within a ventricle that is pumped out per beat

The difference between resting cardiac output and maximum cardiac output during exercise