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Chapter 20:
The Heart
Primary sources for figures and content:
Marieb, E. N. Human Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.
Martini, F. H. Fundamentals of Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin
Cummings, 2004.
The cardiovascular system and
heart.
The Cardiovascular System
Figure 20–1
The Cardiovascular System
• Pulmonary circuit:
– Carries blood to and from gas exchange
surfaces of lungs
– Right ventricle  lungs  left atrium
• Systemic circuit:
– Carries blood to and from the body
– Left ventricle  body  right atrium
3 Types of Blood Vessels
• Arteries:
– carry blood away from heart
• Veins:
– carry blood to heart
• Capillaries (exchange vessels):
– networks between arteries and veins
– Exchange materials between blood and tissues
– Dissolved gases, nutrients, wastes
The heart and its general
features.
Anatomy of the Heart
• Located directly behind sternum
Figure 20–2a
The Heart
• Location:
– Left of midline, between 2nd rib and 5th
intercostal space
– posterior to sternum, in pericardial cavity in
mediastinum
• Heart is fist sized, < 1 lb
• Beats 100,000 times/day moving 8,000 Liters
blood/day
• Surrounded by pericardium:
– serous and fibrous layers
• Serous membranes: visceral and parietal
– secrete pericardial fluid, reduce friction
The Heart
1. Pericarditis
– Inflammation of pericardium, usually due
to infection
– Causes friction
2. Cardiac tamponade
– Buildup of fluid in pericardial space
– Restricts heart movement
Internal Anatomy
Figure 20–6a
4 Chambers of the Heart
• 2 for each circuit:
– left and right: 2 ventricles and 2 atria
2 Atria
–
–
–
–
Superior, thin walls
Smooth posterior walls internally
Pectinate muscles (ridges) anteriorly
has expandable flap called an auricle lateral
and superior
– Left and right separated by interatrial septum
4 Chambers of the Heart
2 Ventricles
– Inferior, thick walls, lined with trabeculae
carneae (muscular ridges)
– Left and right separated by
interventricular septum
– Left ventricle 3x thicker, 5x more friction
while pumping, same volume as right
• WHY?
– Left is round, right is crescent
Left and Right Ventricles
• Have significant
structural differences
Figure 20–7
The Left Ventricle
• Systemic circulation:
– blood leaves left ventricle through aortic
valve into ascending aorta
– ascending aorta  aortic arch
4 Chambers of the Heart
• Right atrium:
– collects blood from systemic circuit
• Right ventricle:
– pumps blood to pulmonary circuit
• Left atrium:
– collects blood from pulmonary circuit
• Left ventricle:
– pumps blood to systemic circuit
Relation to Thoracic Cavity
Figure 20–2b
The Heart:
External Divisions
• Great veins and arteries at the base
• Pointed tip is apex
Figure 20–2c
2 Layers of Pericardium
1. Parietal pericardium:
–
outer layer
2. Visceral pericardium:
–
inner layer of pericardium
Superficial Anatomy of the Heart
Figure 20–3
Sulci
• Coronary sulcus:
– divides atria and ventricles
• Anterior and posterior interventricular
sulci:
– separate left and right ventricles
– contain blood vessels of cardiac muscle
The layers
of the heart wall.
The Heart Wall
Figure 20–4
3 Layers of the Heart Wall
1. Epicardium (thin): outer layer
– Visceral pericardium: serous membrane
• Loose CT attached to myocardium
2. Myocardium (thick): middle layer
– Cardiac muscle tissue with CT, vessels, and
nerves
3. Endocardium (thin): inner layer
– Simple squamous epithelium lining with basal
lamina
– Continuous with endothelium blood vessesl
Cardiac Muscle Cells
Figure 20–5
Cardiac Muscle Tissue
• Muscle cells = cardiocytes
• Uses actin and myosin sliding filaments to
contract
• Rich in mitochondria, resists fatigue but
dependent on aerobic respiration
• Cells connected by intercalated discs
• Contraction is all or none
• Longer contractile phase than skeletal muscle
• Fibrous skeleton of the heart (tough CT) acts as
the tendon
Cardiac Muscle Cells
• Intercalated discs:
–
–
–
–
–
interconnect cardiac muscle cells
secured by desmosomes
linked by gap junctions
convey force of contraction
propagate action potentials
The path of blood flow through
the heart, and the major blood
vessels, chambers, and heart
valves.
Internal Anatomy
Figure 20–6a
The Vena Cava
• Delivers systemic circulation to right
atrium
• Superior vena cava:
– receives blood from head, neck, upper
limbs, and chest
• Inferior vena cava:
– receives blood from trunk, and viscera,
lower limbs
The Heart Valves
• One-way valves
prevent backflow
during contraction
Figure 20–8
Atrioventricular (AV) Valves
• Connect atria to ventricles
• Permit blood flow in 1 direction:
– atria to ventricles
• Flaps = cusps
• Pressure closes valve cusps during
ventricular contraction
1. Tricuspid valve: right side, 3 cusps
2. Bicuspid/Mitral valve: left side, 2 cusps
Cusps
• Cusps attached to chordae tendineae
from papillary muscles on ventricle wall
• Contraction of papillary muscles prevent
cusps opening backward during ventricle
contraction
– Prevent back flow
• Cusps hang loose when ventricles not
contraction, allow ventricles to fill with
blood
Semilunar Valves
•
•
•
•
Between ventricles and arteries
3 cusps
No chordae tendineae or muscles
Forced open by blood from ventricular
contraction
• Snap closed to prevent backflow
Heart Disorders
1. Valvular heart disease
- Valve function deteriorates to extent that
heart cannot maintain adequate circulation
- Rheumatic fever: childhood reaction to
streptococcal infection, chronic carditis,
VHD in adult
2. Heart murmur
- Leaky valve
- Mitral valve prolapse: murmur of left AV
valve, cusps don’t close properly, blood
regurgitates back into left atrium
Heart Disorders
3. Congestive heart failure
– Decreased pumping efficiency
• Diseased valves, damaged muscle
– Blood backs up  fluid leaks from vessels
and collects in lungs and tissues
Blood Flow Through the Heart
The Pulmonary Circuit
• Pulmonary trunk divides into left and
right pulmonary arteries
• Blood flows from right ventricle to
pulmonary trunk through pulmonary
semilunar valve
Return from Pulmonary Circuit
• Blood from the lungs gathers into left
and right pulmonary veins
• Pulmonary veins deliver to left atrium
• Blood from left atrium passes to left
ventricle through left atrioventricular
(AV) valve (bicuspid valve or mitral
valve)
Damage to the semilunar valve on
the right side of the heart would
affect blood flow to which vessel?
A.
B.
C.
D.
aorta
superior vena cava
pulmonary artery
coronary artery
What prevents the AV valves from
opening back into the atria?
A.
B.
C.
D.
papillary muscles
valve cusps
moderator band
chordae tendineae
Why is the left ventricle more
muscular than the right ventricle?
A. Because it must pump a larger
volume.
B. Because it must generate more
force.
C. Because it must open a tighter
atrioventricular valve.
D. Both A and B are correct.
Fetal Heart:
Adapted to bypass lungs
Foramen Ovale
• Before birth, is an opening through
interatrial septum in the right atrium
• Connects the 2 atria
• ~25% of blood bypasses directly the left
atrium
• Closes at birth leaving scar called fossa
ovalis
Ductus arteriosus
• Connects pulmonary trunk to aorta
• ~90% of blood bypasses lungs
• Closes at birth leaving the ligamentum
arteriosum
Failure of either to close = poor
oxygenation of blood, cyanosis, “blue
baby syndrome”
KEY CONCEPT
• The heart has 4 chambers:
– 2 for pulmonary circuit:
• right atrium and right ventricle
– 2 for systemic circuit:
• left atrium and left ventricle
• Left ventricle has a greater workload
– more massive than right ventricle, but the two
chambers pump equal amounts of blood
• AV valves prevent backflow from ventricles into
atria
• Semilunar valves prevent backflow from aortic
and pulmonary trunks into ventricles
Blood supply to the heart.
Blood Supply to the Heart
• Coronary circulation
Figure 20–9
Coronary Circulation
•
•
•
•
•
Coronary arteries and cardiac veins
Supplies blood to muscle tissue of heart
Heart = <1% body mass, requires 5% of blood
Too thick for diffusion
Coronary arteries
– originate at base of ascending aorta
– branch to capillary beds for diffusion
• Blood returns via cardiac veins
– Cardiac veins empty into right atrium
Right Coronary Artery
• Supplies blood to:
– right atrium
– portions of both ventricles
– cells of sinoatrial (SA) and atrioventricular
nodes
– marginal arteries
• surface of right ventricle
Left Coronary Artery
• Supplies blood to:
– left ventricle
– left atrium
– interventricular septum
Coronary artery disease
• Partial or complete block of coronary
circulation, results in coronary ischemia
• Can lead to myocardial infraction (heart
attack)
– Heart tissue denied oxygen dies
• Common symptom
– Angina pectoralis:
• plan in the chest, especially during activity, as a
result of ischemia
Coronary bypass surgery
• Use healthy veins (from legs) to create
anatomoses around blockages
• Most people have 4 major coronary
arteries  “quadruple bypass”
The Cardiac Cycle
Figure 20–11
The Heartbeat
• A single contraction of the heart
• 1% myocardial cells autorhythmic
– Depolarize without neural or endocrine
stimulation
• Depolarization transmitted to other
myocardial cells through cardiac
conduction system
1. Sinoatrial (SA) node
2. Atrioventricular (AV) node
3. Conducting cells
Structures of the
Conducting System
1. SA node:
– Right atrium wall near superior vena cava
2. AV node:
– Inferior portion of interatrial septum
above tricuspid valve
Structures of the
Conducting System
3. Conducting cells:
– controls and coordinates heartbeat
– In the atrium:
• Interconnect SA and AV nodes
– Connect nodes and myocardium
– Distribute stimulus through myocardium
• Run down interventricular septum and around
apex
– In the ventricles: AV bundle, bundle
branches and Purkinji fibers
The Cardiac Cycle
• Begins with action potential at SA node
– gradually depolarizes toward threshold
– transmitted through conducting system
– produces action potentials in cardiac muscle
cells (contractile cells)
• SA node Also called pacemaker potential
• SA node depolarizes first, establishing
heart rate
• Cells of nodes cannot maintain resting membrane
potential, drift to depolarization:
– SA node: 80-100 action potentials/min
• “natural pacemaker”
– AV node: 40-60 action potentials/min
• Resting rate (sinus rhythm)
– ~75 bpm set by SA node + parasympathetic stimulation
Figure 20–12
The Conducting System
Figure 20–12
The components and
functions of the conducting
system of the heart.
Impulse Conduction through the Heart
Figure 20–13
The Sinoatrial (SA) Node
• In posterior wall of right atrium
• Contains pacemaker cells
• Connected to AV node by internodal
pathways
• Begins atrial activation (Step 1)
The Atrioventricular (AV) Node
•
•
•
•
In floor of right atrium
Receives impulse from SA node (Step 2)
Delays impulse (Step 3)
Atrial contraction begins
The AV Bundle
• In the septum
• Carries impulse to left and right bundle
branches:
– which conduct to Purkinje fibers (Step 4)
• And to the moderator band:
– which conducts to papillary muscles
4. The Purkinje Fibers
• Distribute impulse through ventricles
(Step 5)
• Atrial contraction is completed
• Ventricular contraction begins
Total time = ~370 ms
Abnormal Pacemaker Function
• Normal average heart rate = ~70-80
bpm
– Max = ~230 bpm, but inefficient above 180
1. Bradycardia: <60 bpm
– abnormally slow heart rate
2. Tachycardia: >100 bpm
– abnormally fast heart rate
If the cells of the SA node failed to
function, how would the heart rate
be affected?
A.
B.
C.
D.
It
It
It
It
would
would
would
would
be faster.
be slower.
be irregular.
not change.
In a person with bradycardia, is
cardiac output likely to be greater
than or less than normal?
A. greater than normal
B. less than normal
Why is it important for impulses from the
atria to be delayed at the AV node before
they pass into the ventricles?
A. to prevent backflow of atrial
blood
B. to allow complete ventricular
contraction
C. to allow complete atrial emptying
D. to create more forceful
ventricular contraction
The electrical events
that are associated with a
normal electrocardiogram
(EKG).
The Electrocardiogram
Figure 20–14b
Electrocardiogram (ECG or EKG)
• A recording of electrical events in the
heart
• Obtained by electrodes at specific
body locations
• Abnormal patterns diagnose damage
Features of an ECG
• P wave: atria depolarize
– Depolarization wave from SA node through
atria ~80ms
• QRS complex: ventricles depolarize
– Atrial repolarization and ventricle
depolarization ~80ms
• T wave:
– Ventricle repolarization ~160ms
Time Intervals & Diagnosis of
Heart Problems
• P–R interval:
– from start of atrial depolarization, to start of
QRS complex
– P-R longer than 200 ms = damage to AV node or
conducting cells
• Total heart block =
– Cause = damage AV node,
– Effect = no impulses transmitted through,
atria and ventricles beat independently
(atria fast, ventricles slow)
Time Intervals & Diagnosis of
Heart Problems
• Q–T interval:
– from ventricular depolarization, to
ventricular repolarization
– Longer than 380 ms = coronary ischemia or
myocardial damage
EKG used to diagnose heart
problems
1. Cardiac Arrhythmias
– Abnormal patterns of cardiac electrical
activity
2. Fibrillation
– Rapid, irregular, out of phase contractions
due to activity in areas other than SA node
– Defibrillation to stop all activity so SA
node can resume control
KEY CONCEPT
• Heart rate is normally established by cells of SA
node
• Rate can be modified by autonomic activity,
hormones, and other factors
• From the SA node, stimulus is conducted to AV
node, AV bundle, bundle branches, and Purkinje
fibers before reaching ventricular muscle cells
• Electrical events associated with the heartbeat
can be monitored in an electrocardiogram (ECG)
The events that take
place during an action
potential in cardiac muscle.
Contractile Cells
• Purkinje fibers distribute the stimulus
to the contractile cells, which make up
most of the muscle cells in the heart
Action Potentials in
Skeletal and Cardiac Muscle
Figure 20–15
The events that take place
during the cardiac cycle,
including atrial and ventricular
systole and diastole.
The Cardiac Cycle
• The period between the start of 1
heartbeat and the beginning of the
next
• Alternation contraction and relaxation
2 Phases of the Cardiac Cycle
• Within any 1 chamber:
– systole (contraction)
• Contraction
• High pressure
• Blood gets pushed to next chamber
– diastole (relaxation)
• Relaxation
• Low pressure
• Chamber fills with blood
Blood Flow
• Blood flows from high to low pressure:
– controlled by timing of contractions
– directed by one-way valves
Phases of the Cardiac Cycle
Figure 20–16
4 Phases of the Cardiac Cycle
1.
2.
3.
4.
Atrial systole
Atrial diastole
Ventricular systole
Ventricular diastole
Cardiac Cycle and Heart Rate
• At 75 beats per minute:
– cardiac cycle lasts about 800 msecs
• When heart rate increases:
– all phases of cardiac cycle shorten,
particularly diastole
Cardiac Cycle: Pressure and Volume
Figure 20–17
8 Steps in the Cardiac Cycle
1. Atrial systole begins:
–
–
atrial contraction begins
right and left AV valves are open
2. Atria eject blood into ventricles:
–
filling ventricles
3. Atrial systole ends:
–
–
AV valves close
ventricles contain maximum volume
8 Steps in the Cardiac Cycle
4. Ventricular systole: 1st phase
–
–
–
ventricular contraction
pressure in ventricles rises
AV valves shut
5. Ventricular systole (ejection): 2nd phase
–
–
–
Pressure in ventricle rises and exceeds
pressure in the arteries
semilunar valves open
blood flows into pulmonary and aortic trunks
8 Steps in the Cardiac Cycle
6. Ventricular pressure falls:
–
semilunar valves close
7. Ventricular diastole:
–
–
At start ventricular pressure is higher
than atrial pressure and all heart valves
are closed
Then ventricles relax
8 Steps in the Cardiac Cycle
8. Ventricular diastole: Atrial pressure is
higher than ventricular pressure
–
–
AV valves open
Blood fills the relaxed atria
•
–
Atria contraction only adds ~30% more to
ventricles (can live with bad atria)
cardiac cycle ends
Incr. heart rate 
decr. cycle time, decr. diastole time 
decr. time to fill
Heart sounds
relate to specific events
in the cardiac cycle.
Heart Sounds
Figure 20–18b
Heart Sounds
• S1: “lubb”
– produced by AV valves closing at start of
ventricular systole
• S2: “dubb”
– produced by semilunar valves closing at start of
ventricular diastole
• S3, S4:
– soft sounds
– blood flow into ventricles and atrial contraction
Cardiac output
and the factors that influence
it.
Cardiodynamics
• The movement and force generated by
cardiac contractions
• CO = cardiac output
– Amount of blood pumped by each
ventricle in one minute, depends on heart
rate and stroke volume
– CO = HR x SV
Important Cardiodynamics Terms
• End-diastolic volume (EDV) ~ 120 ml
– Volume of blood in ventricle before contraction
• End-systolic volume (ESV) ~ 50 ml
– Volume of blood in ventricle following a beat
– Typically 50 ml
• Stroke volume (SV): ~ 70 ml
– Amount of blood pumped by ventricle
SV = EDV — ESV
Usually SV is constant, you need to change HR to
increase CO as needed.
Stroke Volume
• Volume (ml) of blood ejected per beat
Figure 20–19
Cardiac Output
• Cardiac output (CO) ml/min =
• Heart rate (HR) beats/min 
• Stroke volume (SV) ml/beat
Heart rate effectors.
Overview: Control
of Cardiac Output
Figure 20–20 (Navigator)
HR affected by:
1. Autonomic nervous input
– Parasympathetic stimulation
• slows the heart
– Sympathetic stimulation
• speeds the heart
2. Hormones
3. Venous return
– More blood return = incr. HR
– Stretch receptors in right atrium
• trigger incr. heart rate through incr.
sympathetic activity
4. Other factors: ions, drugs
Cardiac Output
• At 160-180 bpm CO is at max:
– Incr. HR  decr. time to fill ventricles
– If ventricle is not full  decr. SV and CO
• Conditioning can incr. SV and decr. HR
• Fit athletes can
– incr. max CO by 700%
– decr. resting HR by 50% with same CO due
to incr. SV
Variables that influence stroke
volume and heart rate
effectors.
Heart Rate Effectors
1. Autonomic Innervations
– SA node, AV node and atrial myocardium
innervated by both sympathetic (NE) and
parasympathetic (Ach) nerve fibers equally
– Sympathetic dominated in ventricles
– Cardiac centers in medulla oblongata monitor
blood pressure and gasses to adjust HR
• A) Cardioacceleratory center – sympathetic
• B) Cardioinhibitory center parasympathetic
Heart Rate Effectors
1. Autonomic Innervations
– Parasympathetic tone reduce rate of SA node
• 72-80 bpm females
• 64-72 bpm males
• 40 bpm athletes
2. Hormones
– Epinephrine, Norepinephrine, thyroxine all
increase HR by acting at SA node
– Beta blockers: treats hypertension
• Blocks B-receptors for E/NE thus
preventing sympathetic stimulation
Other Heart Rate Effectors
• Caffeine
– Rapid depolarization of SA node, incr. HR
• Nicotine
– Stimulates sympathetic neurons, incr. HR
• Hyperkalemia
– High K+, inhibits repolarization
– Beats week, heart can stop
Other Heart Rate Effectors
• Hypercalcemia
– High Ca+
– Muscle cells excitable, incr. HR
– Can cause prolonged contraction  heart seizes
• Hypocalcemia
– Low Ca+
– Contractions weak  heart can stop
• Temperature
– Affects metabolic rate of cardiocytes
– High temp  incr. HR
– Low temp  decr. HR
What effect does drinking large
amounts of caffeinated drinks have
on the heart?
A.
B.
C.
D.
increased heart rate
decreased heart rate
erratic heart rate
bradycardia
If the cardioinhibitory center of the
medulla oblongata were damaged, which
part of the autonomic nervous system
would be affected, and how would the
heart be influenced?
A. parasympathetic; increased heart
rate
B. parasympathetic; decreased heart
rate
C. sympathetic; increased heart rate
D. sympathetic; decreased heart rate
Factors Affecting Stroke Volume
Figure 20–23 (Navigator)
Factors Affecting Heart
Rate and Stroke Volume
Figure 20–24
KEY CONCEPT
• Cardiac output:
– the amount of blood pumped by the left
ventricle each minute
– adjusted by the ANS in response to:
• circulating hormones
• changes in blood volume
• alterations in venous return
What effect would stimulating the
acetylcholine receptors of the
heart have on cardiac output?
A. It would
output.
B. It would
output.
C. It would
output.
D. It would
increase cardiac
decrease cardiac
cause erratic cardiac
have no effect.
What effect would an increase
in venous return have on the
stroke volume?
A. It would increase stroke volume.
B. It would decrease stroke volume.
C. It would cause erratic stroke
volume.
D. It would have no effect.
Joe’s end-systolic volume is 40 ml,
and his end-diastolic volume is 125
ml. What is Joe’s stroke volume?
A.
B.
C.
D.
165 ml
155 ml
85 ml
100 ml
SUMMARY
• Organization of cardiovascular system:
– pulmonary and systemic circuits
• 3 types of blood vessels:
– arteries, veins, and capillaries
• 4 chambers of the heart:
– left and right atria
– left and right ventricles
• Pericardium, mediastinum, and pericardial sac
• Coronary sulcus and superficial anatomy of the
heart
• Structures and cells of the heart wall
SUMMARY
• Internal anatomy and structures of the
heart:
– septa, muscles, and blood vessels
• Valves of the heart and direction of blood
flow
• Connective tissues of the heart
• Coronary blood supply
• Contractile cells and the conducting system:
– pacemaker calls, nodes, bundles, and
Purkinje fibers
SUMMARY
• Electrocardiogram and its wave forms
• Refractory period of cardiac cells
• Cardiac cycle:
– atrial and ventricular
– systole and diastole
• Cardiodynamics:
– stroke volume and cardiac output
• Control of cardiac output