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Chapter 42
Circulation and Gas
Exchange
The Mammalian Heart: A Closer Look
• A closer look at the mammalian heart provides a
better understanding of double circulation
LE 42-6
Pulmonary artery
Aorta
Anterior
vena cava
Pulmonary
artery
Right
atrium
Left
atrium
Pulmonary
veins
Pulmonary
veins
Semilunar
valve
Semilunar
valve
Atrioventricular
valve
Atrioventricular
valve
Posterior
vena cava
Right
ventricle
Left
ventricle
• The heart contracts and relaxes in a rhythmic cycle
called the cardiac cycle
• The contraction, or pumping, phase is called systole
• The relaxation, or filling, phase is called diastole
LE 42-7
Atrial systole;
ventricular
diastole
Semilunar
valves
closed
0.1 sec
AV valves
open
0.3 sec
Semilunar
valves
open
0.4 sec
Atrial and
ventricular
diastole
AV valves
closed
Ventricular systole;
atrial diastole
• The heart rate, also called the pulse, is the number
of beats per minute
• The cardiac output is the volume of blood pumped
into the systemic circulation per minute
Maintaining the Heart’s Rhythmic Beat
• Some cardiac muscle cells are self-excitable, meaning
they contract without any signal from the nervous
system
• The sinoatrial (SA) node, or pacemaker, sets the rate
and timing at which cardiac muscle cells contract
• Impulses from the SA node travel to the
atrioventricular (AV) node
• At the AV node, the impulses are delayed and then
travel to the bundle of His then to the Purkinje
fibers that make the ventricles contract
• Impulses that travel during the cardiac cycle can be
recorded as an electrocardiogram (ECG or EKG)
Pacemaker
generates wave of
signals to contract.
SA node
(pacemaker)
Signals pass
to heart apex.
Signals are delayed
at AV node.
AV
node
Bundle
branches
ECG
Signals spread
throughout
ventricles.
Heart
apex
Purkinje
fibers
• The pacemaker is influenced by nerves, hormones,
body temperature, and exercise
Concept 42.3: Physical principles govern
blood circulation
• The physical principles that govern movement of
water in plumbing systems also influence the
functioning of animal circulatory systems
Blood Vessel Structure and Function
• The “infrastructure” of the circulatory system is its
network of blood vessels
• All blood vessels are built of similar tissues and have
three similar layers
LE 42-9
Vein
Artery
100 µm
Endothelium
Valve
Basement
membrane
Endothelium
Endothelium
Smooth
muscle
Capillary
Connective
tissue
Smooth
muscle
Connective
tissue
Vein
Artery
Arteriole
Venule
• Structural differences in arteries, veins, and
capillaries correlate with functions
• Arteries have thicker walls that accommodate the
high pressure of blood pumped from the heart
• In the thinner-walled veins, blood flows back to the
heart mainly as a result of muscle action
LE 42-10
Direction of blood flow
in vein (toward heart)
Valve (open)
Skeletal muscle
Valve (closed)
Blood Flow Velocity
• Physical laws governing movement of fluids through
pipes affect blood flow and blood pressure
• Velocity of blood flow is slowest in the capillary
beds, as a result of the high resistance and large
total cross-sectional area
Venae cavae
Veins
Venules
Capillaries
Arterioles
Arteries
120
100
80
60
40
20
0
Aorta
Pressure (mm Hg)
Velocity (cm/sec)
Area (cm2)
LE 42-11
5,000
4,000
3,000
2,000
1,000
0
50
40
30
20
10
0
Systolic
pressure
Diastolic
pressure
Blood Pressure
• Blood pressure is the hydrostatic pressure that blood
exerts against the wall of a vessel
• Systolic pressure is the pressure in the arteries
during ventricular systole; it is the highest pressure
in the arteries
• Diastolic pressure is the pressure in the arteries
during diastole; it is lower than systolic pressure
• Blood pressure is determined by cardiac output and
peripheral resistance due to constriction of arterioles
LE 42-12_4
Blood pressure
reading: 120/70
Pressure
in cuff
above 120
120
Rubber cuff
inflated
with air
Artery
Pressure
in cuff
below 120
120
Pressure
in cuff
below 70
70
Artery
closed
Sounds
audible in
stethoscope
Sounds
stop
Capillary Function
• Capillaries in major organs are usually filled to
capacity
• Blood supply varies in many other sites
• Two mechanisms regulate distribution of blood in
capillary beds:
– Contraction of the smooth muscle layer in the wall of an
arteriole constricts the vessel
– Precapillary sphincters control flow of blood between
arterioles and venules
LE 42-13ab
Precapillary sphincters
Arteriole
Thoroughfare
channel
Venule
Capillaries
Sphincters relaxed
Arteriole
Sphincters contracted
Venule
LE 42-13c
Capillaries and larger vessels (SEM)
20 µm
• The critical exchange of substances between the
blood and interstitial fluid takes place across the
thin endothelial walls of the capillaries
• The difference between blood pressure and osmotic
pressure drives fluids out of capillaries at the
arteriole end and into capillaries at the venule end
LE 42-14
Tissue cell
Capillary
Red
blood
cell
Net fluid
movement out
Net fluid
movement in
15 µm
Direction of
blood flow
Blood pressure
Osmotic pressure
Inward flow
Pressure
Capillary
INTERSTITIAL FLUID
Outward flow
Arterial end of capillary
Venous end
Fluid Return by the Lymphatic System
• The lymphatic system returns fluid to the body from
the capillary beds
• This system aids in body defense
• Fluid reenters the circulation directly at the venous
end of the capillary bed and indirectly through the
lymphatic system