fective for pumping blood (see Box 12–4: Arrhyth-
mias).
HEART RATE
A healthy adult has a resting heart rate (pulse) of 60 to
80 beats per minute, which is the rate of depolariza-
tion of the SA node. (The SA node actually has a
slightly faster rate, closer to 100 beats per minute, but
is slowed by parasympathetic nerve impulses to what
we consider a normal resting rate.) A rate less than 60
(except for athletes) is called bradycardia; a prolonged
or consistent rate greater than 100 beats per minute is
called tachycardia.
A child’s normal heart rate may be as high as 100
beats per minute, that of an infant as high as 120, and
that of a near-term fetus as high as 140 beats per
minute. These higher rates are not related to age, but
rather to size: the smaller the individual, the higher the
metabolic rate and the faster the heart rate. Parallels
may be found among animals of different sizes; the
heart rate of a mouse is about 200 beats per minute
and that of an elephant about 30 beats per minute.
Let us return to the adult heart rate and consider
the person who is in excellent physical condition. As
you may know, well-conditioned athletes have low
resting pulse rates. Those of basketball players are
often around 50 beats per minute, and the pulse of a
marathon runner often ranges from 35 to 40 beats per
minute. To understand why this is so, remember that
the heart is a muscle. When our skeletal muscles are
exercised, they become stronger and more efficient.
The same is true for the heart; consistent exercise
makes it a more efficient pump, as you will see in the
next section.
CARDIAC OUTPUT
Cardiac outputis the amount of blood pumped by a
ventricle in 1 minute. A certain level of cardiac output
is needed at all times to transport oxygen to tissues and
to remove waste products. During exercise, cardiac
output must increase to meet the body’s need for more
oxygen. We will return to exercise after first consider-
ing resting cardiac output.
To calculate cardiac output, we must know the
pulse rate and how much blood is pumped per beat.
Stroke volumeis the term for the amount of blood
pumped by a ventricle per beat; an average resting
stroke volume is 60 to 80 mL per beat. A simple for-
mula then enables us to determine cardiac output:
Cardiac output stroke volume pulse (heart rate)
Let us put into this formula an average resting
stroke volume, 70 mL, and an average resting pulse,
70 beats per minute (bpm):
Cardiac output 70 mL 70 bpm
Cardiac output 4900 mL per minute
(approximately 5 liters)
Naturally, cardiac output varies with the size of the
person, but the average resting cardiac output is 5 to 6
liters per minute. Notice that this amount is just about
the same as a person’s average volume of blood. At
rest, the heart pumps all of the blood in the body
within about a minute. Changes are possible, depend-
ing on circumstances and extent of physical activity.
If we now reconsider the athlete, you will be able to
see precisely why the athlete has a low resting pulse. In
our formula, we will use an average resting cardiac
output (5 liters) and an athlete’s pulse rate (50):
Cardiac output stroke volume pulse
5000 mL stroke volume 50 bpm
5000/50 stroke volume
100 mL stroke volume
Notice that the athlete’s resting stroke volume is
significantly higher than the average. The athlete’s
more efficient heart pumps more blood with each beat
and so can maintain a normal resting cardiac output
with fewer beats.
Now let us see how the heart responds to exercise.
Heart rate (pulse) increases during exercise, and so
does stroke volume. The increase in stroke volume is
the result of Starling’s law of the heart, which states
that the more the cardiac muscle fibers are stretched,
the more forcefully they contract. During exercise,
more blood returns to the heart; this is called venous
return. Increased venous return stretches the myocar-
dium of the ventricles, which contract more forcefully
and pump more blood, thereby increasing stroke vol-
ume. Therefore, during exercise, our formula might
be the following:Cardiac output stroke volume pulseCardiac output 100 mL 100 bpmCardiac output 10,000 mL (10 liters)The Heart 283