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Chapter 12
The Heart
Introduction
• All cardiovascular
function depend on
the heart.
• It beats
100,000/day
• Pumps 8000 liters
of blood/day
The Heart and Circulatory System
• The circulatory system can be
subdivided into the pulmonary circuit
and the systemic circuit.
• Arteries carry blood away from the heart
and
• Veins return blood to the heart.
• Capillaries are tiny vessels between
arteries and veins.
• The heart has four chambers
– The right atrium
– The right ventricle
– The left atrium
– The left ventricle
The Anatomy and Organization of
the Heart
• The heart is surrounded by the
pericardial cavity
• The visceral pericardium (epicardium)
covers the heart’s outer surface.
• The parietal pericardium lines the inner
surface of the pericardial sac which
surrounds the heart.
Surface anatomy of the heart
• The coronary
sulcus, a deep
groove, marks the
boundary between
the atria and
ventricles.
The Heart Wall
• The bulk of the heart consists of the
muscular myocardium
• The endocardium lines the inner
surfaces of the heart
• The fibrous skeleton supports the
heart’s contractile cells and valves.
• Cardiac muscle cells
are interconnected
by intercalated
discs, which convey
the force of
contraction from cell
to cell and conduct
action potentials.
Internal Anatomy and Organization
• The atria are
separated by the
interatrial septum
and the ventricles
are divided by the
interventricular
septum
• The right atria receives blood from the
systemic circuit via two large veins, the
superior vena cava and inferior vena
cava
• Blood flows from the right atrium into
the right ventricle through the right
atrioventricular (AV) valve (tricuspid
valve.
• The opening is bounded by three cusps
of fibrous tissue braced by the
tendinous chordae tendineae with are
connected to papillary muscles.
• Blood leaving the right ventricle enters
the pulmonary trunk after passing
through the pulmonary semilunar valve.
• The pulmonary trunk divides to form the
left and right pulmonary arteries.
• The left and right pulmonary veins
return blood to the left atrium
• Blood leaving the
left atrium flows into
the left ventricle
through the left
atrioventicular valve
(bicuspid valve)
• Blood leaving the
left ventricle passes
through the aortic
semilunar valve and
into the systemic
circuit via the aorta.
• Anatomical difference between the ventricles
reflect the functional demands on them
• The walls of the right ventricle is relatively
thin, while the left ventricle has a massive
muscular wall.
• Valves normally permit blood flow in only one
direction preventing regurgitation(backflow) of
blood.
The Blood supply to the Heart
• Coronary circulation
meets the high
oxygen and nutrient
demands of cardiac
muscle cells
• The coronary
arteries originate at
the base of the aorta
THE HEARTBEAT
In a single heartbeat, the entire heart (atria
and ventricles) contract in a coordinated
manner so that blood flows in the correct
direction at the proper time.
Two general classes of cardiac cells are
involved in the normal heartbeat:
contractile cells and cells of the conducting
system
Contractile Cells
Uses a similar method of
contraction as skeletal muscle cells,
but Ca2+ ions enter the muscle fiber
for a longer period creating
delaying repolaization and
lengthening the muscle contraction.
The Conduction System
• The conducting system includes nodal
cells and conducting cells both of which
do not contract..
• The conduction system initiates and
distributes electrical impulses in the
heart.
• Nodal cells establish the rate of cardiac
contraction
• Pacemaker cells are nodal cells that
reach threshold first.
• Conducting cells distribute the
contractile stimulus to the general
myocardium
• Unlike skeletal muscle, cardiac muscle
contracts without neural or hormonal
stimulation.
• Pacemaker cells in the sinoatrial (SA)
node normally establish the rate of
contraction.
• From the SA node, the stimulus travels
to the atrioventricular (AV) node .
• From the AV node the stimulus travels
to the AV bundle, which divides into
bundle branches.
• From there Purkinje fibers convey the
impulses to the ventricular myocardium
The Electrocardiogram
• A recording of the electrical activities in
the heart is an electrocardiogram (ECG
or EKG)
• Important landmarks of an ECG include:
the P wave (atrial depolarization)
– QRS complex (ventricular depolarization
– T wave (ventricular repolarization)
The Cardiac Cycle
• The cardiac cycle consists of systole
(contraction) followed by diastole
(relaxation)
• Both ventricle contract at the same time,
and they eject equal volumes of blood.
• The closing of the heart valves and the
rushing of blood through the heart
cause characteristic heart sounds.
Heart Sounds
• The first heart sound (“lubb”) is
produced as the AV valves close and
the semilunar valves open.
• The second heart sound (“dupp”) occurs
when the semilunar valves close.
• What is going on in the heart during
each of these sounds?
HEART DYNAMICS
• Heart dynamics refers to the
movements and forces generated
during contractions.
• The amount of blood ejected by a
ventricle during a single heat is the
stroke volume (SV)
• The amount of blood pumped each
minute is the cardiac output (CO)
Factors Controlling Cardiac Output
• The major factors that affect cardiac
output are:
– blood volume reflexes ,
– autonomic innervation,
– and hormones.
• Blood volume reflexes are stimulated by
changes in venous return, the amount of
blood entering the heart.
• The atrial reflex accelerates the heart rate
when the walls of the right atrium are
stretched.
• Ventricular contractions become more
powerful and increase stroke volume when
the ventricular walls are stretched.
• The basic heart rate is established by
the pacemaker cells but it can be
modified by the autonomic nervous
system.
• Acetylcholine released by
parasympathetic motor neurons lowers
heart rate and stroke volume.
• Norepinephrine released by
sympathetic neurons increases the
heart rate and stroke volume
• Epinephrine and norpinephrine,
hormones released by the adrenal
medullae during sympathetic activation,
increase both heart rate and stroke
volume.
• Thyroid hormones and glucagon also
act to increase cardiac output.
• The cardioacceleratory center in the
medulla oblongata activates
sympathetic neurons;
• The cardioinhibitory center governs the
activities of the parasympathetic
neurons.
• These cardiac centers receive inputs
from higher centers and from receptors
monitoring blood pressure and the
levels of dissolved gases.