Cardiac Output, Blood Flow, and Blood Pressure 45114.1 Cardiac Output
The pumping ability of the heart is a function of the beats
per minute (cardiac rate) and the volume of blood ejected
per beat (stroke volume). The cardiac rate and stroke vol-
ume are regulated by autonomic nerves and by mecha-
nisms intrinsic to the cardiovascular system.
ventricle. In order for this to be true, the pulmonary circulation
must have a low resistance, low pressure, and high blood flow
compared to the systemic circulation. For comparison, the mean
arterial pressure (discussed in section 14.3) of the pulmonary
circulation is 10–20 mmHg compared to 70–105 mmHg in the
systemic circulation.
The total blood volume also averages about 5.5 L. This
means that each ventricle pumps the equivalent of the total blood
volume each minute under resting conditions. Put another way, it
takes about a minute for a drop of blood to complete the systemic
and pulmonary circuits. An increase in cardiac output, as occurs
during exercise, must thus be accompanied by an increased rate
of blood flow through the circulation. This is accomplished by
factors that regulate the cardiac rate and stroke volume.Regulation of Cardiac Rate
In the complete absence of neural influences, the heart will
continue to beat as long as the myocardial cells are alive.
This automatic rhythm is a result of a spontaneous, diastolic
depolarization of the pacemaker cells in the SA node. This
pacemaker potential is produced by mechanisms discussed
in chapter 13, section 13.5, and serves as the depolarization
stimulus for the production of action potentials ( fig. 14.1 ).Mark complained of abdominal pain and swelling in his
legs, and his physician said that he had edema and hypo-
proteinemia. After a colonoscopy with biopsies, he was
diagnosed with Crohn’s disease, an autoimmune intesti-
nal disorder that caused plasma proteins to leak through
his intestinal lining. He later began an intense exercise
program, lifting heavy weights and training to run mara-
thons. The day after a long run on a hot day, he noticed
that he got very dizzy when he stood up and was told that
he should drink more and switch to sports drinks during
such training. A couple of years later, Mark was diag-
nosed with essential hypertension and was prescribed an
ACE inhibitor. The physician advised him not to hold his
breath when he lifted heavy weights.
Some of the new terms and concepts you will
encounter include:- Oncotic pressure and interstitial fluid formation
- Cardiac output, blood volume, and peripheral
resistance. - Baroreceptor reflex and Valsalva maneuver
Clinical Investigation
LEARNING OUTCOMES
After studying this section, you should be able to:- Describe the extrinsic regulation of cardiac rate and
contractility. - Explain the relationship between stroke volume and
venous return. - Explain the Frank-Starling law of the heart.
The cardiac output is the volume of blood pumped per minute
by each ventricle. The average resting cardiac rate in an adult
is 70 beats per minute; the average stroke volume (volume of
blood pumped per beat by each ventricle) is 70 to 80 ml per
beat. The product of these two variables gives an average car-
diac output of 5,500 ml (5.5 L) per minute:
Cardiac output 5 Stroke volume 3 cardiac rate
(ml ∕ min) (ml ∕ beat) (beats ∕min)Because the cardiac output of the right ventricle is normally
the same as that of the left ventricle, the lungs receive the entire
cardiac output while other organs share the output of the left
Figure 14.1 The effect of autonomic nerves on the
pacemaker potentials in the SA node. The heart’s rhythm
is set by the rate of spontaneous depolarization in the SA node.
This spontaneous depolarization is known as the pacemaker
potential, and its rate is increased by sympathetic nerve stimula-
tion and decreased by parasympathetic nerve inhibition.ThresholdControlSympathetic nerve effectParasympathetic nerve effect- 50mV
- 50mV
- 50mV
Time (msec)= Pacemaker potential250 500 750 1000