The Circulatory System
Chapter 42
Why have a circulatory system?
Diffusion is too slow a process to transport
chemicals through the body of an animal.
 The presence of a circulatory system
reduces the distance a substance must
diffuse to enter or leave a cell.
 The circulatory system does more than
move gases, it is a critical component to
maintaining homeostasis of the body.
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Components of the cardiovascular
system
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Heart
– Four chambers in humans
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Arteries
– Carry blood away from heart
– Branch into smaller arterioles
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Veins
– Carry blood toward the heart
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Capillaries
– The site of chemical exchange between blood and
interstitial fluid
– Connect arteries to veins
Blood flow pattern in humans
‘Double-loop’ circulation
 body  vena cava  right atrium  right
ventricle  pulmonary artery  lung 
pulmonary vein  left atrium  left ventricle 
aorta  arteries  arterioles  capillaries 
venules  veins  vena cava
 The blood that is low in oxygen is completely
separated from the blood that is rich in oxygen
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The heart
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Located beneath the sternum
Cone-shaped and about the size of a clenched
fist
Surrounded by a double layered sac
Comprised mostly of cardiac muscle tissue
The 2 atria are thin-walled and collect blood
The ventricles are thick-walled and pump blood
There are 4 valves which prevent backflow of
blood during contraction
The heartbeat
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A complete sequence of contraction and
relaxation is the cardiac cycle
Systole – the heart muscle contracts and the
chambers pump blood
Diastole – the heart muscle relaxes and the
chambers fill with blood
The tempo of contraction is controlled by the
sinoatrial (SA) node, sometimes called the
pacemaker
The SA node can be influenced by hormones,
body temperature, exercise
Blood pressure
The force that blood exerts against a vessel wall
Pressure is greatest in arteries during ventricular
systole – this is the main force propelling blood
from the heart through the vessels
 Stress may trigger responses that cause the
smooth muscles of vessel walls to contract,
increasing blood flow and pressure
 Blood in the veins is under very little pressure –
it is aided in returning to the heart by skeletal
muscles
 Veins have valves that prevent backflow of blood
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Blood flow to different body parts is
regulated
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All tissues and organs receive enough blood
even though only 5% - 10% of the capillaries
are carrying blood at any given time
– The brain, heart, kidneys, and liver usually carry a full
load of blood
Blood flow to capillaries in other areas is
controlled by smooth muscles in the artery walls
 After a meal  increased flow to digestive tract
 Exercise  increased flow to skeletal muscles
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Exchange of materials
Occurs across the thin walls of the
capillaries
 The capillary wall is a single ‘leaky’ layer
of endothelial cells
 About 85% of the fluid that leaves the
blood at the arteriole end of the capillary
re-enters from the interstitial fluid at the
venous end
 The remaining 15% of the fluid is lymph
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The lymphatic system
Lymph is similar in composition to
interstitial fluid
 It enters the lymphatic system by diffusing
into lymph capillaries
 It then drains back into the circulatory
system at two locations near the shoulders
 Lymph nodes are specialized swellings
along the system that filter the lymph and
attack viruses and bacteria
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Blood
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Connective tissue – cells suspended in liquid plasma
– Average person has 4-6 L of whole blood
Plasma
– 90% water
– Also contains electrolytes and plasma proteins
Erythrocytes (RBCs = red blood cells)
– Transport oxygen
– No nuclei
– Contain hemoglobin
Leukocytes (white blood cells)
– Function in defense and immunity
– Number will increase during times of infection
Platelets
– Function in blood clotting
Blood clotting
Platelets clump together and form a
temporary plug
 Clotting factors are released that convert
fibrinogen to fibrin
 Fibrin forms threads to form the clot
 Hemophilia – caused by a defect in the
genes that control this process
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Cardiovascular disease
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Accounts for more than 50% of all U.S. deaths
Heart attack – blockage of coronary arteries
which leads to cardiac muscle damage
Stroke – blockage of arteries in the brain
Atherosclerosis – artery walls develop plaques
which narrow the space and increase chance of
clot formation and heart attack
Hypertension – chronic high blood pressure
which can promote atherosclerosis