49
Circulatory Systems: Pumps, Vessels, and Blood
• A circulatory system is composed of a pump
(heart), fluid (blood), and conduits (blood vessels).
• This is also called a cardiovascular system.
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Overview: Trading Places
• Every organism must exchange materials with
its environment
• Exchanges ultimately occur at the cellular level
• In unicellular organisms, these exchanges occur
directly with the environment
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Concept 42.1: Circulatory systems link exchange
surfaces with cells throughout the body
• In small and/or thin animals, cells can exchange
materials directly with the surrounding medium
• In most animals, transport systems connect the
organs of exchange with the body cells
• Most complex animals have internal transport
systems that circulate fluid
49
Circulatory Systems: Pumps, Vessels, and Blood
• Some simple aquatic animals do not have
circulatory systems.
• In small animals, nutrients, gases, and wastes can
diffuse between the cells and the environment and
a circulatory system is not needed.
• Some aquatic animals have a flat body shape or a
highly-branched gastrovascular cavity (cnidaria)
to provide maximum surface area for exchange.
• Larger animals with many cell layers must rely on
extracellular tissue fluids to carry materials to and
from cells.
Figure 49.1 Open Circulatory Systems
Not very efficient
-as hemolymph
under low
pressure
-not able to be
directed to tissues
that may need
more oxygen
Figure 49.2 A Closed Circulatory System
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Circulatory Systems: Pumps, Vessels, and Blood
• Closed systems have advantages over open
systems:
 Blood flow, nutrient delivery and waste
removal are more rapid.
 Closed systems can direct the blood to
specific tissues.
 Cellular elements and large molecules that aid
in transport can be kept within the vessels.
• Closed systems generally support higher levels of
metabolic activities.
• Insects are an exception to this rule, but they do
not rely on their circulatory systems for gas
exchange.
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• Vertebrate circulatory systems
AMPHIBIANS
REPTILES (EXCEPT BIRDS)
MAMMALS AND BIRDS
Lung and skin capillaries
Lung capillaries
Lung capillaries
FISHES
Gill capillaries
Artery
Pulmocutaneous
circuit
Gill
circulation
Heart:
ventricle (V)
A
Atrium (A)
Systemic
circulation
Vein
Systemic capillaries
Right
systemic
aorta
Pulmonary
circuit
A
A
V
Right
V
Left
Right
Systemic
circuit
Systemic capillaries
Figure 42.4
Pulmonary
circuit
Left
Systemic
V aorta
Left
A
Systemic capillaries
A
V
Right
A
V
Left
Systemic
circuit
Systemic capillaries
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Vertebrate Circulatory Systems
• The four-chambered hearts of birds and mammals
completely separate the pulmonary and systemic
circuits.
• The advantages of separate circuits are:
 Oxygenated and deoxygenated blood cannot
mix.
 Gas exchange is maximized because the
lungs receive only blood with low O2 and high
CO2 content.
 The separate circuits can operate at different
pressures.
Figure 49.3 The Human Heart and Circulation (Part 2)
49
• The mammalian cardiovascular system
7
Capillaries of
head and
forelimbs
Anterior
vena cava
Pulmonary
artery
Aorta
Pulmonary
artery
9
6
Capillaries
of right lung
Capillaries
of left lung
2
4
3
Pulmonary
vein
5
1
Right atrium
3
11
Left atrium
Pulmonary
vein
10
Left ventricle
Right ventricle
Aorta
Posterior
vena cava
8
Figure 42.5
Capillaries of
abdominal organs
and hind limbs
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• The heart contracts and relaxes
 In a rhythmic cycle called the cardiac cycle
• The contraction, or pumping, phase of the cycle
 Is called systole
• The relaxation, or filling, phase of the cycle
 Is called diastole
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• The cardiac cycle
2 Atrial systole;
ventricular
diastole
Semilunar
valves
closed
0.1 sec
Semilunar
valves
open
0.3 sec
0.4 sec
AV valves
open
1 Atrial and
ventricular
diastole
Figure 42.7
AV valves
closed
3 Ventricular systole;
atrial diastole
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• The heart rate, also called the pulse, is the
number of beats per minute
• The stroke volume is the amount of blood
pumped in a single contraction
• The cardiac output is the volume of blood
pumped into the systemic circulation per minute
and depends on both the heart rate and stroke
volume
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Maintaining the Heart’s Rhythmic Beat
• Some cardiac muscle cells are self-excitable
 Meaning they contract without any signal from the nervous system. The
electrical impulse is then rapidly conducted due to the branching nature of
cardiac muscle and the presence of intercalated discs (which have gap
junctions to allow rapid flow of charge)
• The impulses that travel during the cardiac cycle
 Can be recorded as an electrocardiogram (ECG or EKG)
• The pacemaker is influenced by
 Nerves, hormones, body temperature, and exercise
• The autonomic nervous system controls heart rate by influencing the rate at
which pacemaker cells gradually depolarize.
• Acetylcholine and norepinephrine from the parasympathetic and sympathetic
nerve endings slow and increase the rate, respectively
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• The control of heart rhythm
1 Pacemaker generates
wave of signals
to contract.
SA node
(pacemaker)
2 Signals are delayed
3 Signals pass
at AV node.
AV node
to heart apex.
4 Signals spread
Throughout
ventricles.
Bundle
branches
Heart
apex
ECG
Figure 42.8
Purkinje
fibers
Fig. 42-10
Artery
Vein
SEM
Valve
100 µm
Basal lamina
Endothelium
Smooth
muscle
Connective
tissue
Endothelium
Capillary
Smooth
muscle
Connective
tissue
Artery
Vein
Capillary
15 µm
Red blood cell
Venule
LM
Arteriole
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• Capillaries have thin walls, the endothelium plus its
basement membrane, to facilitate the exchange of
materials
• Arteries and veins have an endothelium, smooth
muscle, and connective tissue
Structural differences in arteries, veins, and
capillaries correlate with their different functions
Arteries have thicker walls than veins to
accommodate the high pressure of blood pumped
from the heart
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• All blood vessels
 Are built of similar tissues
 Have three similar layers
49
Figure 49.14 Atherosclerotic Plaque (Part 1)
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Figure 49.14 Atherosclerotic Plaque (Part 2)
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The Vascular System:
Arteries, Capillaries, and Veins
• If the coronary arteries are affected, blood supply
to the heart decreases.
• A thrombus here (coronary thrombosis) can
block an artery, causing a heart attack
(myocardial infarction, or MI).
• If part of the thrombus breaks away (an
embolism), lodges in the brain, and blocks blood
flow, stroke may occur.
• The best approach to reducing heart disease is
prevention.
• Risk factors include high-fat/high-cholesterol
diets, smoking, a sedentary life style, and obesity.
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• Blood tends to accumulate in veins. Veins are
called capacitance vessels because of their
high capacity to store blood.
Direction of blood flow
in vein (toward heart)
Valve (open)
• Pressure in veins is very low,
Contraction of skeletal muscles
pushes blood toward the heart
because one-way valves in veins
prevent backflow.
• If veins become stretched, the
valves no longer do their job and
varicose veins develop.
Skeletal muscle
Valve (closed)
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The Vascular System:
Arteries, Capillaries, and Veins
• During walking or running, the legs act as
auxiliary vascular pumps and return blood to the
heart from the veins of the lower body.
• A greater volume of blood is returned to the heart,
which stretches the cardiac muscle cells, and the
heart contracts more forcefully. This is known as
the Frank-Starling law.
• Breathing also helps return venous blood by
creating negative pressure (suction), which pulls
blood and lymph toward the chest area.
• In large veins near the heart, smooth muscles
also contract with exercise, increasing venous
return and cardiac output.
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• Physical laws
governing the
movement of fluids
through pipes
 Influence blood
flow and blood
pressure
• The velocity of
blood flow varies in
the circulatory
system
 And is slowest in
the capillary beds
as a result of the
high resistance
and large total
cross-sectional
area
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• 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
 Is the highest pressure in the arteries
• Diastolic pressure
 Is the pressure in the arteries during diastole
 Is lower than systolic pressure
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• Two mechanisms
 Regulate the distribution of blood in capillary beds
• In one mechanism
 Contraction of the smooth muscle layer in the wall of an
arteriole constricts the vessel
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• In a second mechanism
 Precapillary sphincters control the flow of
blood between arterioles and venules
Figure 49.12 Starling’s Forces (Part 1)
Figure 49.12 Starling’s Forces (Part 2)
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The Vascular System:
Arteries, Capillaries, and Veins
• The medical phenomenon called edema (tissue
swelling) in certain diseases, or the inflammation
accompanying injury or an allergic reaction,
supports this model.
• However, edema does not occur during strenuous
exercise, for example, or in birds, which have
higher arteriole pressure and lower osmotic
pressure than mammals.
49
The Vascular System:
Arteries, Capillaries, and Veins
• CO2 and bicarbonate ions (HCO3–) are major
factors that pull water back into the capillaries.
• As blood flows through the capillary, CO2 diffuses
into the plasma and is converted into HCO3–.
• The increasing bicarbonate concentration causes
osmotic pressure to be higher at the venous end,
especially during exercise.
• In fact, CO2 and HCO3– are the major factors that
pull water back into the capillaries, not colloidal
osmotic pressure.
Figure 49.15 The Composition of Blood
Figure 49.16 Blood Clotting (Part 1)
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Blood pressure is determined
• partly by cardiac output
• And partly by peripheral resistance due to
variable constriction of the arterioles
Figure 49.18 Control of Blood Pressure through Vascular Resistance
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Control and Regulation of Circulation
• When atria are receiving too much venous return
they release a hormone called atrial natriuretic
factor, which stimulates the kidney to excrete
sodium and water, resulting in a reduced blood
volume.
Figure 49.19 Regulating Blood Pressure
49
Control and Regulation of Circulation
• Several adaptations permit the air-breathing seal to
remain under water for long periods of time.
• Seals have greater blood volume, greater blood O2
carrying capacity, and more myoglobin in their
muscles than do humans.
• The diving reflex is the most important adaptation.
• During a dive, the heart rate slows, and all major
blood vessels are constricted except those critical
to survival under water.
• Metabolic rate slows, and tissues switch to
glycolytic (anaerobic) metabolism.
Figure 49.20 The Diving Reflex