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42
Circulation
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Concept 42.1: Circulatory systems link
exchange surfaces with cells throughout
the body
 In small or thin animals, cells can exchange materials
directly with the surrounding medium by diffusion but
diffusion is only efficient over small distances.
 In most animals, cells exchange materials with the
environment via a fluid-filled circulatory system
 A circulatory system has
 A circulatory fluid
 A set of interconnecting vessels
 A muscular pump, the heart
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Open and Closed Circulatory Systems
 The circulatory system connects the fluid that surrounds
cells with the organs that exchange gases, absorb
nutrients, and dispose of wastes
 Circulatory systems can be open or closed and vary in the
number of circuits in the body
 In insects, other arthropods, and some molluscs,
circulatory fluid called hemolymph bathes the organs
directly in an open circulatory system
 In a closed circulatory system, blood is confined to
vessels and is distinct from the interstitial fluid. Annelids,
cephalopods, and vertebrates have
closed circulatory systems
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Figure 42.3
(a) An open circulatory system
(b) A closed circulatory system
Heart
Heart
Interstitial fluid
Blood
Hemolymph in sinuses
Small
branch vessels
in each organ
Pores
Dorsal
vessel
(main
heart)
Tubular heart
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Auxiliary
hearts
Ventral vessels
Organization of Vertebrate Circulatory Systems
 Humans and other vertebrates have a closed circulatory
system called the cardiovascular system
 The three main types of blood vessels are arteries, veins,
and capillaries. Blood flow is one way in these vessels
 Arteries and veins are distinguished by the direction of blood
flow, not by O2 content
 Arteries branch into arterioles and carry blood away from
the heart to capillaries
 Networks of capillaries called capillary beds are the sites of
chemical exchange between the blood and interstitial fluid
 Venules converge into veins and return blood from
capillaries to the heart
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Figure 42.4a
Bony fishes,
rays, and
sharks have
single circulation
with a
two-chambered
heart
In single
circulation,
blood leaving
the heart passes
through two
capillary beds
before
returning
(a) Single circulation:
fish
Gill
capillaries
Artery
Heart:
Atrium (A)
Ventricle (V)
Vein
Body
capillaries
Key
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Oxygen-rich blood
Oxygen-poor blood
Double Circulation
 Amphibians, reptiles, and mammals have double
circulation
 Oxygen-poor and oxygen-rich blood are pumped
separately from the right and left sides of the heart
 In reptiles and mammals, oxygen-poor blood flows through
the pulmonary circuit to pick up oxygen through the lungs
 In amphibians, oxygen-poor blood flows through a
pulmocutaneous circuit to pick up oxygen through the
lungs and skin
 Oxygen-rich blood delivers oxygen through the systemic
circuit
 Double circulation maintains higher blood pressure in the
organs than does single circulation
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 Frogs and other amphibians have a three-chambered heart:
two atria and one ventricle
 The ventricle pumps blood into a forked artery that splits
the ventricle’s output into the pulmocutaneous circuit and
the systemic circuit
 When underwater, blood flow to the lungs is nearly shut off
 Turtles, snakes, and lizards have a three-chambered heart:
two atria and one ventricle, partially divided by an incomplete
septum
 In alligators, caimans, and other crocodilians a septum
divides the ventricles but pulmonary and systemic circuits
connect where arteries exit
the heart
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Figure 42.4b
(b) Double circulation:
amphibian
Pulmocutaneous circuit
Lung
and skin
capillaries
Atrium
(A)
Atrium
(A)
Right
Left
Ventricle (V)
Systemic
capillaries
Key
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Systemic circuit
Oxygen-rich blood
Oxygen-poor blood
 Mammals and birds have a four-chambered heart
with two atria and two ventricles
 The left side of the heart pumps and receives only
oxygen-rich blood, while the right side receives
and pumps only oxygen-poor blood
 Mammals and birds are endotherms and require
more O2 than ectotherms
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Figure 42.4c
(c) Double circulation:
mammal
Pulmonary circuit
Lung
capillaries
A
A
V
V
Right
Left
Systemic
capillaries
Systemic circuit
Key
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Oxygen-rich blood
Oxygen-poor blood
Coordinated cycles of heart contraction drive
double circulation in mammals
 Blood begins its flow with the right ventricle pumping blood to the
lungs via the pulmonary arteries
 In the lungs, the blood loads O2 and unloads CO2
 Oxygen-rich blood from the lungs enters the heart at the left atrium
via the pulmonary veins
 It is pumped through the aorta to the body tissues by the left
ventricle
 The aorta provides blood to the heart through the coronary
arteries
 Blood returns to the heart through the superior vena cava (blood
from head, neck, and forelimbs) and inferior vena cava (blood
from trunk and hind limbs)
 The superior vena cava and inferior vena cava flow into the right
atrium
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Figure 42.5
Superior
vena cava
Capillaries of
head and
forelimbs
7
Pulmonary
artery
Capillaries
of right lung
Pulmonary
artery
Aorta
9
Capillaries
of left lung
6
2
3
3
4
11
Pulmonary
vein
Right atrium
Right ventricle
1
Pulmonary
vein
Left atrium
Left ventricle
5
10
Aorta
Inferior
vena cava
8
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Capillaries of
abdominal organs
and hind limbs
The Mammalian Heart: A Closer Look
 A closer look at the mammalian heart provides
a better understanding of double circulation
 The two atria have relatively thin walls and serve
as collection chambers for blood returning to
the heart
 The ventricles have thicker walls and contract
much more forcefully
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Figure 42.6
Aorta
Pulmonary artery
Pulmonary
artery
Right
atrium
Left
atrium
Semilunar
valve
Semilunar
valve
Atrioventricular
(AV) valve
Atrioventricular
(AV) valve
Right
ventricle
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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
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Figure 42.7-3
2 Atrial systole and
ventricular diastole
1 Atrial and
ventricular diastole
0.1
sec
0.4
sec
0.3 sec
3 Ventricular systole
and 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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 Four valves prevent backflow of blood in the heart
 The atrioventricular (AV) valves separate each
atrium and ventricle
 The semilunar valves control blood flow to the
aorta and the pulmonary artery
 The “lub-dup” sound of a heart beat is caused by
the recoil of blood against the AV valves (lub) then
against the semilunar (dup) valves
 Backflow of blood through a defective valve
causes a heart murmur
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Maintaining the Heart’s Rhythmic Beat
 Some cardiac muscle cells are autorhythmic,
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 that travel during the cardiac cycle
can be recorded as an electrocardiogram
(ECG or EKG)
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Figure 42.8-4
1 Signals (yellow)
from SA node
spread through
atria.
SA node
(pacemaker)
ECG
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2 Signals are
delayed at AV
node.
AV
node
3 Bundle branches
pass signals to
heart apex.
Bundle
branches
Heart
apex
4 Signals
spread
throughout
ventricles.
Purkinje
fibers
 Impulses from the SA node travel to the atrioventricular
(AV) node
 At the AV node, the impulses are delayed and
then travel to the Purkinje fibers that make the ventricles
contract
 The pacemaker is regulated by two portions of
the nervous system: the sympathetic and parasympathetic
divisions
 The sympathetic division speeds up the pacemaker
 The parasympathetic division slows down the pacemaker
 The pacemaker is also regulated by hormones
and temperature
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Blood Vessel Structure and Function
 A vessel’s cavity is called the central lumen
 The epithelial layer that lines blood vessels is called the
endothelium
 The endothelium is smooth and minimizes resistance
 Capillaries are only slightly wider than a red
blood cell
 Capillaries have thin walls, the endothelium plus its basal
lamina, to facilitate the exchange of materials
 Arteries and veins have an endothelium, smooth muscle, and
connective tissue
 Arteries have thicker walls than veins to 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
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Figure 42.9a
Valve
Basal lamina
Endothelium
Smooth
muscle
Connective
Capillary
tissue
Endothelium
Smooth
muscle
Connective
tissue
Artery
Vein
Arteriole
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Venule
Figure 42.9b
Vein
LM
Artery
Capillary
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LM
Red blood cell
15 µm
Red blood cells 100 µm
Figure 42.10
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Systolic
pressure
Venae
cavae
Veins
Venules
Capillaries
Diastolic
pressure
Arterioles
120
100
80
60
40
20
0
Arteries
50
40
30
20
10
0
Aorta
Velocity
(cm/sec)
 Blood flow in
capillaries is
necessarily slow
for exchange of
materials
5,000
4,000
3,000
2,000
1,000
0
Pressure
(mm Hg)
 Velocity of blood
flow is slowest in
the capillary
beds, as a result
of the high
resistance and
large total crosssectional area
Area (cm2)
Blood Flow Velocity
Blood Pressure
 Blood pressure is the pressure that blood exerts
in all directions, including against the walls of
blood vessels. Blood flows from areas of higher pressure to
areas of lower pressure
 The recoil of elastic arterial walls plays a role in maintaining
blood pressure
 The resistance to blood flow in the narrow diameters of tiny
capillaries and arterioles dissipates much of the pressure
 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
 A pulse is the rhythmic bulging of artery walls with each
heartbeat
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Regulation of Blood Pressure
 Homeostatic mechanisms regulate arterial blood pressure
by altering the diameter of arterioles
 Vasoconstriction is the contraction of smooth muscle in
arteriole walls; it increases blood pressure
 Vasodilation is the relaxation of smooth muscles in the
arterioles; it causes blood pressure to fall
 Nitric oxide is a major inducer of vasodilation
 The peptide endothelin is a strong inducer of
vasoconstriction
 Vasoconstriction and vasodilation are often coupled to
changes in cardiac output that affect blood pressure
 Fainting is caused by inadequate blood flow to
the head
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Figure 42.11
 Blood pressure is generally measured for an artery in the
arm at the same height as the heart
Pressure in cuff
greater than
120 mm Hg
Cuff
inflated
with air
Pressure in cuff
drops below
120 mm Hg
120
Pressure in
cuff below
70 mm Hg
120
70
Artery
closed
1
2
Sounds
audible in
stethoscope
Sounds
stop
3
 Blood pressure for a healthy 20-year-old human at rest is
about 120 mm Hg at systole and 70 mm Hg at diastole
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Figure 42.12
 Blood is moved
through veins by
smooth muscle
contraction, skeletal
muscle contraction,
and expansion of the
vena cava with
inhalation
 One-way valves in
veins prevent
backflow of blood
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Direction of blood flow
in vein (toward heart)
Valve (open)
Skeletal muscle
Valve (closed)
Capillary Function
 Blood flows through only 5–10% of the body’s capillaries at
any given time. While 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
 Constriction or dilation of arterioles that supply
capillary beds
 Precapillary sphincters that control flow of blood
between arterioles and venules
 Blood flow is regulated by nerve impulses, hormones, and
other chemicals
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Figure 42.13
Precapillary sphincters
Arteriole
Thoroughfare
channel
Capillaries
Venule
(a) Sphincters relaxed
Arteriole
(b) Sphincters contracted
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Venule
 The 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
 Most blood proteins and all blood cells are too
large to pass through the endothelium
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Figure 42.14
INTERSTITIAL
FLUID
Net fluid movement out
Body cell
Blood
pressure
Osmotic
pressure
Arterial end
of capillary
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Direction of blood flow
Venous end
of capillary
Fluid Return by the Lymphatic System
 The lymphatic system returns fluid that leaks
out from the capillary beds
 Fluid lost by capillaries is called lymph
 The lymphatic system drains into veins in the neck
 Valves in lymph vessels prevent the backflow
of fluid
 Edema is swelling caused by disruptions in the flow of
lymph
 Lymph nodes are organs that filter lymph and
play an important role in the body’s defense
 When the body is fighting an infection, lymph nodes
become swollen and tender
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Concept 42.4: Blood components function in
exchange, transport, and defense
 The closed circulatory systems of vertebrates contain a more
highly specialized fluid called blood
 Blood in vertebrates is a connective tissue consisting of
several kinds of cells suspended in a liquid matrix called
plasma
 The cellular elements occupy about 45% of the volume of
blood
 Suspended in blood plasma are two types of cells
 Red blood cells (erythrocytes) transport O2
 White blood cells (leukocytes) function in defense
 Platelets are fragments of cells that are involved in clotting
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Figure 42.16
Plasma 55%
Constituent
Major functions
Water
Solvent
Ions (blood
electrolytes)
Sodium
Potassium
Calcium
Magnesium
Chloride
Bicarbonate
Osmotic balance,
pH buffering,
and regulation
of membrane
permeability
Plasma proteins
Albumin
Cellular elements 45%
Separated
blood
elements
Osmotic balance,
pH buffering
Fibrinogen
Clotting
Functions
Defense and
immunity
Lymphocytes
Eosinophils
Immunoglobulins Defense
(antibodies)
Lipid transport
5,000–10,000
Leukocytes
(white blood cells)
Basophils
Apolipoproteins
Number per
µL (mm3) of blood
Cell type
Neutrophils
Platelets
Monocytes
250,000–400,000
Blood clotting
Substances transported by blood
Nutrients (such as glucose,
fatty acids, vitamins)
Waste products of metabolism
Respiratory gases (O2 and CO2)
Hormones
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Erythrocytes
(red blood cells)
5,000,000–6,000,000 Transport of O2
and some CO2
Plasma
 Plasma contains inorganic salts as dissolved
ions, sometimes called electrolytes
 Plasma proteins influence blood pH and help
maintain osmotic balance between blood and
interstitial fluid
 Particular plasma proteins function in lipid
transport, immunity, and blood clotting
 Plasma is similar in composition to interstitial
fluid, but plasma has a much higher protein
concentration
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Erythrocytes
 Red blood cells, or erythrocytes, are the most
numerous blood cells
 They contain hemoglobin, the iron-containing
protein that transports O2
 Each molecule of hemoglobin binds up to four
molecules of O2
 In mammals, mature erythrocytes lack nuclei
and mitochondria
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Leukocytes
 There are five major types of white blood cells, or
leukocytes: monocytes, neutrophils, basophils,
eosinophils, and lymphocytes
 They function in defense either by phagocytizing bacteria
and debris or by mounting immune responses against
foreign substances
 They are found both in and outside of the circulatory
system
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Stem Cells and the Replacement of Cellular
Elements
 Erythrocytes, leukocytes, and platelets all develop
from a common source of stem cells in the red
marrow of bones, especially ribs, vertebrae,
sternum, and pelvis
 The hormone erythropoietin (EPO) stimulates
erythrocyte production when O2 delivery is low
 Physicians can use recombinant EPO to treat
people with conditions such as anemia
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Figure 42.17
Stem cells
(in bone marrow)
Myeloid
stem cells
Lymphoid
stem cells
B cells T cells
Erythrocytes
Neutrophils
Basophils
Lymphocytes
Monocytes
Platelets
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Eosinophils
Blood Clotting
 Coagulation is the
formation of a solid
clot from liquid blood
 A cascade of
complex reactions
converts inactive
fibrinogen to fibrin,
forming a clot
 A blood clot formed
within a blood vessel
is called a thrombus
and can block blood
flow
Red blood cells caught
in threads of fibrin
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5 µm
Figure 42.18a
1
2
3
Platelet
plug
Collagen fibers
Platelet
Clotting factors from:
Platelets
Damaged cells
Plasma (factors include calcium, vitamin K)
Enzymatic cascade
Prothrombin
Thrombin
Fibrinogen
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Fibrin
Fibrin clot
Fibrin clot
formation
Cardiovascular Disease
 Cardiovascular diseases are disorders of the heart and the
blood vessels
 These diseases range in seriousness from minor
disturbances of vein or heart function to life-threatening
disruptions of blood flow to the heart
or brain
 One type of cardiovascular disease, atherosclerosis, is
caused by the buildup of
Endothelium
fatty deposits (plaque) within
arteries
Lumen
 Cholesterol is a key player
Thrombus
in the development of
Plaque
atherosclerosis
 Inflammation plays a role in
atherosclerosis and thrombus formation
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Coronary circulation
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 A heart attack, or myocardial infarction, is the damage or
death of cardiac muscle tissue resulting from blockage of
one or more coronary arteries
 A stroke is the death of nervous tissue in the brain, usually
resulting from rupture or blockage
of arteries in the head
 Aspirin inhibits inflammation and reduces the risk of heart
attacks and stroke
 Angina pectoris is chest pain caused by partial blockage
of the coronary arteries
 Hypertension, or high blood pressure, also contributes to
heart attack and stroke, as well as other health problems.
Hypertension can be controlled by dietary changes,
exercise, and/or medication
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