Circulatory System
Do Now
Why is it important for your heart to
continue beating even when you’re
sleeping?
 What does your body need?
 What are some wastes?
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Circulation and Respiration
Each breath brings oxygen rich air into
your body
 Your cells need that oxygen
 Your heart delivers oxygen to your cells
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Working together, your circulatory and
respiratory systems supply cells
throughout the body with the nutrients
and oxygen that they need to stay alive!
Multicellular Needs
Unicellular organisms don’t need a circulatory
system, because the cell is in direct contact with
the environment and oxygen, nutrients and
wastes can easily diffuse across the cell
membrane by diffusion.
 Multicellular organisms need a circulatory system
to transport substances made in one part of the
body to sites where they are needed in another
part of the body.
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Function
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The circulatory system transports
substances including oxygen, nutrients
and wastes to and from cells responding
to changing demands by diffusion (from
high to low concentration along
concentration gradient).
Structure
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Humans have a closed circulatory system.
– Blood is pumped through a system of vessels
(In an open system, blood flows in vessels and
sinuses/gills)
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Sometimes the circulatory system is also called
the “cardiovascular system” because:
– Cardio = heart
– Vascular = vessels
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The human circulatory system consists of:
– The heart
– A series of blood vessels
– Blood that flows through them
The Heart
Located near the center of your chest
 A hollow organ about the size of your fist composed of
cardiac muscle.
 Enclosed in a protective sac of tissue called the
pericardium
 Inside there are two thin layers of epithelial and
connective tissue
 Contractions of the myocardium, a thick cardiac muscle,
pump blood through the circulatory system
 The heart contracts about 72 times a minute
 Each contraction pumps about 70 mL of blood
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Heart
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Septum, or wall,
separates the right side
form the left side
preventing mixing of
oxygen-rich blood and
oxygen-poor blood
Flaps of connective tissue
called valves divide each
side into 2 chambers:
totaling 4 chambers
– Upper chambers receive
blood = atrium
– Lower chambers pump
blood out of heart =
ventricle
Types of Circulation
Pulmonary circulation = from right side of
the heart to lungs where carbon dioxide
leaves the blood and oxygen is absorbed
 Systemic circulation = from left side of
the heart to organs

– Coronary circulation = through heart tissue
Pulmonary Circulation
The right side of the heart pumps blood from
the heart to the lungs
 In the lungs, carbon dioxide leaves the blood
while oxygen is absorbed.
 The oxygen-rich blood goes into the left side of
the heart
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Systemic Circulation
The oxygen-rich blood from
the left side of the heart is
pumped to the rest of the
body
 Oxygen-poor blood returns
to the right side of the heart
 This blood is oxygen-poor
because the cells absorbed
the oxygen and released
carbon dioxide into the
blood
 The oxygen-poor blood is
ready for another trip to the
lungs to get oxygen again

Figure 37-2 The Circulatory
System
Section 37-1
Capillaries of
head and arms
Superior
vena cava
Pulmonary
vein
Capillaries of
right lung
Aorta
Pulmonary
artery
Capillaries
of left lung
Inferior
vena cava
Capillaries of
abdominal organs
and legs
Coronary Circulation
Remember: the heart
is an organ and needs
nutrients, oxygen and
creates wastes.
 Blood flows to the
tissues of the heart
too!

Blood Flow through the heart

Blood leaves the heart in arteries, and blood returns to heart in veins.

Oxygenated blood returns from the lungs through the pulmonary veins to the left
atrium.
Oxygenated blood is pumped from the left atrium through the mitral valve to the left
ventricle.
Oxygenated blood leaves the left ventricle through the aortic valve to the aorta,
which is the largest artery of your body.
The aorta branches into various arteries pumping blood through your body.
Deoxygenated blood returns from the top of your body through the superior vena
cava and from the bottom of your body through the inferior vena cava to the right
atrium.
Deoxygenated blood is pumped from the right atrium through the tricuspid valve to
the right ventricle.
Deoxygenated blood leaves the right ventricle through the pulmonary valve to the
pulmonary arteries.
The pulmonary arteries pump blood to the lungs to absorb oxygen and release
carbon dioxide.
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Heart circulation animation:
http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_pumping.html
The Path of Blood
Valves
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Blood enters into the atria of the heart, separated from the
ventricles by valves, preventing back-flow of blood keeping the
blood flowing in one direction
When the atria contract, the valves open and blood flows into the
ventricles
When the ventricles contract, the valves close preventing blood
from flowing back into the atria and blood flows out of the heart
At the exits of the ventricles, there are valves that prevent blood
from flowing back into the heart
The “lub-dup” sound of your heart is caused by the closing of the
heart’s valves. The “lub” is when the ventricles contract and blood
being forced against the artioventricular or A-V (tricuspid or mitral)
valves. The “dup” is the blood being forced against the semilunar
(aortic or pulmonary) valves.
Figure 37-3 The Structures of the Heart
Section 37-1
Superior Vena Cava
Large vein that brings oxygen-poor blood from the
upper part of the body to the right atrium
Aorta
Brings oxygen-rich blood from the left
ventricle to the rest of the body
Pulmonary Arteries
Bring oxygen-poor blood
to the lungs
Pulmonary Veins
Bring oxygen-rich blood from each
of the lungs to the left atrium
Left Atrium
Pulmonary Valve
Prevents blood from flowing
back into the right ventricle
after it has entered the
pulmonary artery
Right Atrium
Tricuspid Valve
Prevents blood from flowing
back into the right atrium after it
has entered the right ventricle
Aortic Valve
Prevents blood from flowing
back into the left ventricle
after it has entered the aorta
Mitral Valve
Prevents blood from flowing back
into the left atrium after it has
entered the left ventricle
Left Ventricle
Inferior Vena Cava
Vein that brings oxygen-poor
blood from the lower part of
the body to the right atrium
Septum
Right Ventricle
Heartbeat

There are two muscle contractions in the heart:
– The atria
– The ventricles
Each contraction begins in a small group of cardiac
muscle cells in the right atrium that stimulate the rest of
the muscle cells = sinoatrial node (SA node)
 Since the sinoatrial node sets the pace for the heart it is
also called “the pacemaker”
 The impulse spreads from the pacemaker through fibers
in the atria to the atrioventricular node (AV node) and
through fibers in the ventricles
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When the atria contract, blood flows into the ventricles
When the ventricle contract, blood flows out of the heart
The Sinoatrial Node
Section 37-1
Contraction of Atria
Contraction of Ventricles
Sinoatrial
(SA) node
Conducting
fibers
Atrioventricular
(AV) node
Changing Heartbeat
Your heart can beat faster or slower, depending
on your body’s need for oxygen-rich blood
 When you exercise, your heart rate can increase
to 200 beats per minute
 The autonomic nervous system influences heart
rate

– Neurotransmitters released by neurons in the
sympathetic nervous system can increase heart rate,
and those released by the parasympathetic nervous
system can decrease heart rate
Blood vessels
Blood circulates in one direction and it is
moved by the pumping of the heart
 As blood flows through the circulatory
system, it moves through three types of
blood vessels:
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– Arteries
– Capillaries
– Veins
Arteries
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Large vessels that carry blood
away from the heart to tissues of
the body
Except for the pulmonary
arteries, all arteries carry
oxygen-rich blood.
Arteries have thick walls of
elastic connective tissue,
contractible smooth muscle, and
epithelial cells that help them
withstand the powerful pressure
produced when the heart
contracts and pushes blood into
the arteries.
Capillaries
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The smallest of the blood
vessels connecting
arteries and veins
Walls are one cell thick
allowing for easier
diffusion of nutrients and
oxygen from capillaries to
body cells and wastes
and carbon dioxide from
body cells to capillaries
Veins
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Return blood to the heart
Veins have walls of connective
tissue and smooth muscle
Large veins contain valves that
keep blood flowing towards the
heart
Many veins are located near
skeletal muscles, so when the
muscles contract, they help force
blood through the veins, even
against gravity
Exercise helps prevent
accumulation of blood in limbs
and stretching veins out of shape
Figure 37-5 The Three Types
of Blood Vessels
Section 37-1
Vein
Artery
Endothelium
Arteriole
Capillary
Venule
Connective
tissue
Connective
tissue
Smooth
muscle
Endothelium
Smooth
muscle
Endothelium
Valve
Blood Pressure
The heart produces pressure when it contracts.
 The force of blood on the arteries’ walls = blood
pressure
 Blood pressure decreases when the heart relaxes, but
there must always be some pressure to keep the blood
flowing
 Doctors measure blood pressure with a
sphygmomanometer recording two numbers
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– Systolic pressure = force felt in arteries when ventricles contract
– Diastolic pressure = force of blood felt in arteries when
ventricles relax
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Average adult’s blood pressure = 120/80
Regulating Blood Pressure
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With the nervous system:
– Sensory neurons at several places in the body detect blood
pressure and send impulses to brain stem (medulla oblongata)
– When too high, the autonomic nervous system releases
neurotransmitters that cause the smooth muscles around blood
vessels to relax, lowering blood pressure.
– When too low, neurotransmitters are released that cause the
smooth muscles to contract, elevating blood pressure.
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With the endocrine/excretory system:
– Hormones produced by the heart and other organs cause
kidneys to remove more water from the blood when blood
pressure is too high, reducing blood volume and lowering blood
pressure
Disorders
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Disorders of the circulatory
system are very common:
– High Blood Pressure
– Heart Attack
– Stroke
Most stem from atherosclerosis
= fatty deposits (plaque) builds
up on walls of arteries,
obstructing blood flow,
increasing blood pressure and
risk of blood clots
High Blood Pressure
Also known as Hypertension
 Forces heart to work harder, which may weaken
or damage the heart muscle and vessels
 More likely to develop heart disease and
increased risk of heart attack and stroke
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Heart Attack
A medical emergency
 Coronary arteries (supplying heart blood)
bring oxygen and nutrients to the heart
muscle itself
 Blockage of coronary artery may damage
or kill part of heart muscle (myocardium)
due to lack of oxygen = heart attack
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– Symptoms include: chest pain/pressure,
feeling of heartburn/indigestion, sudden
dizziness, or brief loss of consciousness
Stroke
Blood clots may break free from vessels and get stuck in
a blood vessel leading to a part of the brain = stroke
 Brain cells relying on that vessel may begin to die from
lack of oxygen and brain function in that region may be
lost
 Strokes can also occur when a weakened artery in the
brain burst, flooding the area with blood
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Prevention
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Cardiovascular diseases are
easy to prevent:
– Exercise – increases
respiratory system’s
efficiency
– Weight control – reduces
body fat and stress
– Sensible diet – low in
saturated fat reduces risk of
heart disease
– Not smoking – reduces risk
of heart disease