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CHAPTER 50
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The Circulatory System
Blood
• Type of connective tissue composed
– Fluid matrix called plasma
– Formed elements
• Functions of circulating blood
1. Transportation
2. Regulation
3. Protection
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Blood plasma
• 92% water
• Contains the following solutes
– Nutrients, wastes, and hormones
– Ions
– Proteins
• Albumin
• Fibrinogen
– If removed, plasma is called serum
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Formed elements
• Red blood cells (erythrocytes)
– About 5 million per microliter of blood
– Hematocrit is the fraction of the total blood
volume occupied by red blood cells
– Mature mammalian erythrocytes lack nuclei
– RBCs of vertebrates contain hemoglobin
• Pigment that binds and transports oxygen
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Formed elements
• White blood cells (leukocytes)
– Less than 1% of blood cells
– Larger than erythrocytes and have nuclei
– Can migrate out of capillaries into tissue fluid
– Types
• Granular leukocytes
– Neutrophils, eosinophils, and basophils
• Agranular leukocytes
– Monocytes and lymphocytes
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Formed elements
• Platelets
• Cell fragments that pinch off from larger cells
in the bone marrow
• Function in the formation of blood clots
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Formed elements
• All develop from pluripotent stem cells
• Occurs in the bone marrow
• Erythropoietin stimulates the production of
erythrocytes (erythropoiesis)
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Invertebrate Circulatory
Systems
• Nature of the circulatory system in multicellular
invertebrates is directly related to the size,
complexity, and lifestyle of the organism
• No circulatory system
– Sponges and most cnidarians utilize water from the
environment as a circulatory fluid
• Gastrovascular cavity
– Nematodes
• Use the fluids of the body cavity for circulation
• Small or long and thin
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Invertebrate Circulatory
Systems
• Larger animals require a separate circulatory
system for nutrient and waste transport
• Open circulatory system
– No distinction between circulating and extracellular
fluid
– Fluid called hemolymph
• Closed circulatory system
– Distinct circulatory fluid enclosed in blood vessels and
transported away from and back to the heart
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Vertebrate Circulatory Systems
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Vertebrate Circulatory Systems
• Amphibian heart
– 3-chambered heart
• 2 atria and 1 ventricle
– Separation of the pulmonary and systemic
circulations is incomplete
– Amphibians living in water obtain additional
oxygen by diffusion through their skin
– Reptiles have a septum that partially
subdivides the ventricle, thereby further
reducing the mixing of blood in the heart
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Vertebrate Circulatory Systems
• Mammals, birds, and crocodilians
– 4-chambered heart
– 2 separate atria and 2 separate ventricles
– Right atrium receives deoxygenated blood
from the body and delivers it to the right
ventricle, which pumps it to the lungs
– Left atrium receives oxygenated blood from
the lungs and delivers it to the left ventricle,
which pumps it to rest of the body
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The Cardiac Cycle
• Heart has two pairs of valves
– Atrioventricular (AV) valves
• Maintain unidirectional blood flow between atria
and ventricles
• Tricuspid valve = On the right
• Bicuspid, or mitral, valve = On the left
– Semilunar valves
• Ensure one-way flow out of the ventricles to the
arterial systems
• Pulmonary valve located at the exit of the right
ventricle
• Aortic valve located at the exit of the left ventricle
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The Cardiac Cycle
• Valves open and close as the heart goes
through the cardiac cycle
• Ventricles relaxed and filling (diastole)
• Ventricles contracted and pumping
(systole)
• “Lub-dub” sounds heard with stethoscope
– Lub – AV valves closing
– Dub – closing of semilunar valves
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Pulmonary
valve
Aortic
valve
Right
atrium
Left
atrium
Left
AV
valves Right ventricle ventricle
2. “Lub”: The ventricles
contract, the
atrioventricular (AV)
valves close, and
pressure in the
ventricles builds
up until the aortic
and pulmonary
valves open.
1. The atria contract.
3. Blood is pumped out
of ventricles and
into the aorta and
pulmonary artery.
Diastole
4. “Dup”: The ventricles
relax, the pressure in
the ventricles falls at
the end of systole, and
since pressure is now
greater in the aorta and
pulmonary artery, the
aortic and pulmonary
valves slam shut.
Systole
5. The ventricles fill
with blood.
Diastole
pressure in
left ventricle
Pressure (mm Hg)
125
130 mL
100
1.
2.
4.
75
5.
3.
“Lub”
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“Dup”
25
65 mL
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (seconds)
0.7
0.8
0.9
1.0
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The Cardiac Cycle
• Heart contains “self-excitable”
autorhythmic fibers
• Most important is the sinoatrial (SA) node
– Located in wall of right atrium
– Acts as pacemaker
– Autonomic nervous system can modulate rate
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The Cardiac Cycle
• Each SA depolarization transmitted
– To left atrium
– To right atrium and atrioventricular (AV) node
• AV node is only pathway for conduction to
ventricles
– Spreads through atrioventricular bundle
– Purkinje fibers
– Directly stimulate the myocardial cells of both
ventricles to contract
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The Cardiac Cycle
• Electrical activity can be recorded on an
electrocardiogram (ECG or EKG)
– First peak (P) is produced by depolarization of
atria (atrial systole)
– Second, larger peak (QRS) is produced by
ventricular depolarization (ventricular systole)
– Last peak (T) is produced by repolarization of
ventricles (ventricular diastole)
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Right atrium
Left atrium
SA node
(pacemaker)
Internodal
pathway
AV
AV node
Interventricular
septum
AV bundle
AV bundle
Purkinje fibers
Purkinje fibers
Left and right
bundle branches
1. The impulse begins at the SA node and travels to the
AV node.
2. The impulse is delayed at the AV node. It
then travels to the AV bundle.
R
Millivotts
+1
P wave
T wave
0
Q
S
1 sec
-1
Seconds
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
AV bundle
Interventricular
septum
Left and right
bundle branches
3. From the AV bundle, the impulse travels
down the interventricular septum.
4. The impulse spreads to branches from the
interventricular septum.
Purkinje fibers
5. Finally reaching the Purkinje fibers, the impulse
is distributed throughout the ventricles.
R
Millivotts
+1
P wave
T wave
0
Q
S
1 sec
-1
Seconds
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The Cardiac Cycle
• Right and left pulmonary arteries deliver
oxygen-depleted blood from the right
ventricle to the right and left lungs
• Pulmonary veins return oxygenated blood
from the lungs to the left atrium of the
heart
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The Cardiac Cycle
• Aorta and all its branches are systemic
arteries, carrying oxygen-rich blood from
the left ventricle to all parts of the body
– Coronary arteries supply oxygenated blood to
the heart muscle
• Blood from the body drains into the right
atrium
– Superior vena cava drains upper body
– Inferior vena cava drains lower body
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The Cardiac Cycle
• Arterial blood pressure can be measured
with a sphygmomanometer
• Systolic pressure is the peak pressure at
which ventricles are contracting
• Diastolic pressure is the minimum
pressure between heartbeats at which the
ventricles are relaxed
• Blood pressure is written as a ratio of
systolic over diastolic pressure
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Characteristics of Blood Vessels
• Blood leaves heart through the arteries
• Arterioles are the finest, microscopic
branches of the arterial tree
• Blood from arterioles enters capillaries
• Blood is collected into venules, which lead
to larger vessels, veins
• Veins carry blood back to heart
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Characteristics of Blood Vessels
• Arteries and veins are composed of four tissue
layers
– Endothelium, elastic fibers, smooth muscle, and
connective tissue
– Walls too thick for exchange of materials across the
wall
• Capillaries are composed of only a single layer
of endothelial cells
– Allow rapid exchange of gases and metabolites
between blood and body cells
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Characteristics of Blood Vessels
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Characteristics of Blood Vessels
• Arteries and arterioles
– Larger arteries contain more elastic fibers in their
walls than other blood vessels
• Recoil each time they receive blood from the heart
– Contraction of the smooth muscle layer of the
arterioles results in vasoconstriction
• Greatly increases resistance and decreases flow
• Chronic vasoconstriction can result in hypertension
– Relaxation of the smooth muscle layer results in
vasodilation
• Decreasing resistance and increasing blood flow to an organ
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Characteristics of Blood Vessels
• Capillaries
– Every cell in the body is within 100
micrometers (μm) of a capillary
– Although each capillary is very narrow, so
many of them exist that the capillaries have
the greatest total cross-sectional area of any
other type of vessel
• Slows blood flow to allow for exchange with
extracellular fluid
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Characteristics of Blood Vessels
• Veins and venules
– Thinner layer of
smooth muscles than
arteries
– Venous pump helps
return blood to heart
• Skeletal muscle
contractions and oneway venous valves
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The Lymphatic System
• Consists of lymphatic capillaries, lymphatic
vessels, lymph nodes, and lymphatic
organs (spleen and thymus)
• Excess fluid in the tissues drains into
blind-ended lymph capillaries
• Lymph passes into progressively larger
vessels with one-way valves
• Eventually drains into subclavian veins
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Cardiovascular Diseases
• Leading cause of death in the United States
• Atherosclerosis
– Accumulation of fatty material within arteries
– Impedes blood flow
• Arteriosclerosis
– Arterial hardening due to calcium deposition
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Cardiovascular Diseases
• Heart attacks (myocardial infarctions)
– Main cause of cardiovascular deaths in U.S.
– Insufficient supply of blood to heart
• Angina pectoris (“chest pain”)
– Warning sign that the blood supply to the
heart is inadequate but is still sufficient to
avoid myocardial cell death
• Stroke
– Interference with blood supply to the brain
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Blood Flow and Blood Pressure
• Cardiac center of the medulla oblongata
modulates heart rate
• Blood pressure increases with blood
volume
• Blood volume is regulated by four
hormones
– Antidiuretic hormone (ADH)
– Aldosterone
– Atrial natriuretic hormone
– Nitric oxide (NO)
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Blood Flow and Blood Pressure
• Blood pressure increases with blood
volume
• Blood volume is regulated by four
hormones
– Antidiuretic hormone (ADH)
– Aldosterone
– Atrial natriuretic hormone
– Nitric oxide (NO)
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