Download Unit One: Introduction to Physiology: The Cell and General Physiology

Chapter 20: Cardiac Output, Venous Return,
And Their Regulation
Guyton and Hall, Textbook of Medical Physiology, 12 edition
Normal Values for CO at Rest and During Activity
• Factors Directly Affecting Cardiac Output (CO)
a. The basic level of body metabolism
b. Whether the person in exercising
c. Age
d. Body size
Normal Values for CO at Rest and During Activity
• Effect of Age on CO—cardiac index is the CO per
square meter of body surface area
Fig. 20.1
Control of CO by Venous Return
• Frank-Starling Law of the Heart
a. When increased quantities of blood flow
into the heart, the increased blood
stretches the walls of the chambers. As a
result the heart contracts with increased
force, and this empties the extra blood
into the systemic circulation.
b. Stretching of the heart causes the heart to
pump faster at an increased rate—
Bainbridge Reflex
Control of CO by Venous Return
• CO Regulation-sum of blood flow regulation in all
of the local tissues of the body
Fig. 20.2
Control of CO by Venous Return
• Effect of Total Peripheral Resistance on LongTerm CO
Fig. 20.3
Control of CO (cont.)
• Effect of Total Peripheral Resistance on LongTerm CO
Control of CO
• Limits for CO
Fig. 20.4 CO curves for the normal heart and for
hypoefffective and hypereffective hearts
Control of CO
• Factors That Cause a Hypereffective Heart
a. Nervous excitation: sympathetic stimulation or
parasympathetic inhibition
b. Increased pumping effectiveness caused by
hypertrophy of the cardiac muscle (but not
to the point of damaging the heart
Control of CO
• Factors That Cause a Hypoeffective Heart
a. Increased arterial pressure against which the heart
must pump (i.e. hypertension)
b. Inhibition of nervous excitation
c. Pathological factors causing abnormal rhythm or
rate of heart beat
d. Coronary artery blockage
e. Valvular heart disease
f. Congenital heart disease
g. Myocarditis
h. Cardica hypoxia
Control of CO
• Role of the Nervous System
Fig. 20.5 Experiment in a dog to demonstrate the importance
of nervous maintenance of the arterial pressure as a
prerequisite for cardiac output control.
Control of CO
• Low Cardiac Output Caused by Cardiac Factors
a. Severe coronary vessel blockage and myocardial
infarction
b. Severe valvular heart disease
c. Myocarditis
d. Cardiac tamponade
e. Cardiac metabolic derangements
Fig. 20.6 CO in different pathological conditions. The number in parentheses indicate
the number of patients studied in each condition.
Control of CO (cont.)
•
Decrease Caused by Non-cardiac Peripheral Factors
a. Decreased blood volume
b. Acute venous dilation
c. Obstruction of large veins
d. Decreased tissue mass, especially skeletal muscle
e. Decreased metabolic rate of the tissuesa
Quantitative Analysis of CO Regulation
•
Fig. 20.7
Effect of External Pressure Outside the Heart on
CO curves
Quantitative Analysis of CO Regulation
•
Factors That Can Shift the CO Curve
a. Cyclical changes in intrapleural pressure during
respiration
b. Breathing against a negative pressure
c. Positive pressure breathing
d. Opening the thoracic cage
e. Cardiac tamponade (fluid accumulation in the
pericardial cavity
•
Simultaneous Changes in External Cardiac
Pressure and the Effectiveness of the Heart
as a Pump
Fig. 20.8
Venous Return Curves
• Three Principal Factors Affecting Venous Return
to the Heart
a. Right atrial pressure
b. Mean systemic filling pressure
c. Resistance to blood flow
Venous Return Curves
• Normal Venous Return Curve
Fig. 20.9 Normal venous return curve
Venous Return Curves
• Venous Return Curves
a. Plateau at negative atrial pressure caused by collapse
of large veins
b. Effect of blood volume on mean circulatory filling
pressure (psf )
c. Effect of sympathetic stimulation of circulation on
mean circulatory filling pressure (psf)
The greater the distance between the psf and right atrial
pressure, the greater the venous return
Fig. 20.11
Fig. 20.11
Resistance to Venous Return
VR = Venous return
Psf = Mean systemic filling pressure
PRA = Right atrial pressure
RVR = Resistance to venous return
Resistance to Venous Return
• Effect of Resistance to Venous Return on the Venous
Return Curve
Fig. 20.12
Resistance to Venous Return
• Combinations of Venous Return Patterns
Fig. 20.13
Resistance to Venous Return
• Effect of Increased Blood Volume on Cardiac Output
Fig. 20.14 The two solid curves demonstrate an analysis of cardiac output and right atrial pressure when the CO (red line)
and venous return (blue line) lines are normal. Transfusion of blood equal to 20 per cent of the blood volume
causes the venous return curve to become the dashed line; as a result, the cardiac output and right atrial pressure
shift from point A to point B
Resistance to Venous Return
• Further Compensatory Effects
a. Increased CO increases capillary pressure
b. Reduces mean systemic pressure (stress-relaxation)
c. Increases the resistance to venous return
Resistance to Venous Return
• Effects of Sympathetic Stimulation on CO
Fig. 20.15
Resistance to Venous Return
• Effects of Sympathetic Inhibition on CO
Fig. 20.16