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Anatomy
of the Heart
The heart is located in a central
tissue mass that divides the ribcage into two cavities between the two
lungs. This central tissue consists of the heart, its associated blood
vessels, the esophagus, and the trachea.
The axis of the heart is tilted
so that the apex (tip) of the heart points toward the lower left. The
large arteries and veins running to and from the heart are connected to
the upper end or base of the heart.
Below is a cross section through
the body at the level indicated in the diagram above.
The heart functions as a dual pump with each half of the heart beating
together in unison during a single cardiac cycle.
That is, the atria contract
together and then the ventricles contract together.
Following a single erythrocyte
through the basic circuits of the cardiovascular system:
From the head, trunk, and extremities blood enters the right atrium through
the superior and inferior vena cavae.
The right atrium pumps blood through
the right atrioventricular (tricuspid) valve to the right ventricle.
From
the right ventricle, blood flows through the pulmonary valve to the lungs
via the pulmonary arteries (deoxygenated blood).
In the lungs the blood
becomes more red in color as it is oxygenated.
Blood returns to the heart
via the pulmonary veins (oxygenated blood) to the left atrium.
The left
atrium pumps blood through the left atrioventricular (bicuspid or mitral)
valve to the left ventricle.
From the left ventricle, blood flows through
the aortic valve to the arch of the aorta and out to the systemic arteries
of the body. These arteries pass the blood to capillaries where the tissues
of the body use the oxygen from the blood for aerobic cell respiration.
As blood passes out of the capillary beds of the body it is collected
by veins that return the deoxygenated blood to the heart.
The superior
and inferior vena cavae return deoxygenated venous blood to the right
atrium of the heart.
Heart
Valves
Heart valves function to ensure a one-way flow of
blood through the heart. The valves are not made of muscle, but rather
are composed of sheets of tough connective tissue (leaflets) that act
like flaps. The heart valves open and close passively because of pressure
differences on either side of the valve. When pressure is greater behind
the valve, the leaflets are blown open and the blood flows through the
valve. However, when pressure is greater in front of the valve, the leaflets
snap shut and blood flow is stopped. The motion of a heart valve is analogous
to the motion of the front door of your house. The door, which only opens
in one direction, opens and closes due to pressure on the door.
Atrioventricular
(AV) and Semilunar Valves
The atrioventricular valves (AV valves), which
separate the atria from the ventricles, allow blood to flow from the atria
to the ventricles, but prevent flow in the opposite direction. The right
AV valves is called the tricuspid valve. The left AV valve is called
the mitral valve. The opening and closing of the AV valves is dependent
on pressure differences between the atria and ventricles. When the ventricles
relax, atrial pressure exceeds ventricular pressure, the AV valves are
pushed open and blood flows into the ventricles. However, when the ventricles
contract, ventricular pressure exceeds atrial pressure causing the AV
valves to snap shut.
To ensure that the AV valves do not evert (turn inside-out),
they are attached to small papillary muscles by tough tendons called
the cordae tendineae. Papillary muscles contract in synchrony with
the ventricles, thus maintaining constant tension on the valve leaflets.
The semilunar valves (pulmonary valve and aortic
valve) are one-way valves that separate the ventricles from major arteries.
The aortic valve separates the left ventricle from the aorta, while
the pulmonary valve separates the right ventricle from the pulmonary
artery. As the ventricles contract, ventricular pressure exceeds arterial
pressure, the semilunar valves open and blood is pumped into the major
arteries. However, when the ventricles relax, arterial pressure exceeds
ventricular pressure and the semilunar valves snap shut.
Heart
Sounds are associated with Valve Closure
Normal heart sounds are caused by the closing of heart
valves. As valves snap shut, the walls of the chambers and major arteries
vibrate. We hear these vibrations as two distinct sounds; lub-dup. The
first sound, "lub", is associated with the closing of the AV
valves. The second sound, "dup", is associated with the closing
of the semilunar valves
CHAMBERS
OF THE HEART AND CIRCUITS
OF THE CARDIOVASCULAR SYSTEM
RIGHT ATRIUM
This chamber receives blood from the body via the:
superior vena cava = returns blood from the head and upper extremities
inferior vena cava = returns blood from the trunk and legs
RIGHT ATRIOVENTRICULAR VALVE (TRICUSPID VALVE)
This valve is found between the right atrium and the right ventricle
Both the right and the left atrioventricular valves are stabilized by
papillary muscles and the chordae tendineae.
RIGHT VENTRICLE
This chamber receives blood from the right atrium
The wall of this chamber is more muscular than the right atrium, but less
muscular than the left ventricle
PULMONARY SEMILUNAR VALVE
As compared to the atrioventricular valves, this smaller valve has self-supporting
cusps with no chordae tendinae or papillary muscles. When closed, the
3 symmetrical cusps support one another.
PULMONARY TRUNK & PULMONARY ARTERIES
These blood vessels carry blood from the heart to the lungs.
The pulmonary trunk branches to form the right and left pulmonary arteries.
PULMONARY VEINS
These four veins carry blood from the lungs into the left atrium.
LEFT ATRIUM
This chamber receives blood from the 4 pulmonary veins and pumps blood
through the left atrioventricular valve.
LEFT ATRIOVENTRICULAR VALVE (BICUSPID OR MITRAL VALVE)
This valve has two flaps and stronger papillary muscles and chordae tendinae
due to strength of the left ventricle.
LEFT VENTRICLE
This chamber has a much thicker muscular wall than the right ventricle
(blood has to travel farther through body) and it forms the apex (tip)
of the heart. This chamber pumps blood through the aortic semilunar valve
AORTIC SEMILUNAR VALVE
This valve keeps blood from flowing backward into the left ventricle as
blood is pumped from the left ventricle into the arch of the aorta.
ARCH OF AORTA
This is the largest artery in the body
It carries blood from the left ventricle to the upper and lower body.
OTHER IMPORTANT PARTS OF
THE HEART
papillary muscles = muscles in the ventricles connected to the
chordae tendinae. These muscles help stabilize the AV valves during ventricular
contraction.
chordae tendinae = fibrous cords that keep the AV valve cusps from
swinging into the atria & prevent regurgitation of blood.
CARDIAC MUSCLE CELLS
Cardiac muscle cells are uninucleate, somewhat striated cells, that contain
lots of mitochondria and are interconnected branching fibers that are
connected at specialized cell-to-cell junctions.
An electrical impulse generated
in one part of the heart spreads to the entire heart via the conduction
system of the heart. Cardiac cells can generate electrical impulses (action
potentials) without any input from the nervous system.
Electrical
Conduction System of the Heart
Action potentials that originate in the SA node
spread to the myocardial cells of the atria through gap junctions between
cells. Depolarization of the atria stimulates contraction of the atrial
myocardium. Action potentials cannot directly spread from the atrial myocardium
to the ventricular myocardium due to the presence of the non-conducting
fibrous skeleton that separates them. Rather, the impulse travels to the
ventricles through a system of specialized cells called the conduction
system.
The conduction system is composed of the:
- atrioventricular node (AV node)
- Bundle of His
- bundle branches
- Purkinje fibers.
Cardiac
Cycle
The heart undergoes a constant
cycle of contractions and relaxations. The period of ventricular contraction
is called systole. The period of ventricular relaxation is called
diastole.
Events
in Diastole
Diastole begins as the ventricles
start to relax. Soon the pressures within the aorta and pumonary artery
exceed ventricular pressures, causing the semilunar valves to close. As
the ventricular pressure falls below the atrial pressure the AV valves
open and the ventricles fill with blood. The ventricles fill to about
80% of capacity prior to contraction of the atria, the last event in diastole.
Atrial contraction forces the final 20% of the end-diastolic volume
(the volume of blood that exists in the ventricles at the end of diastole)
into the ventricles.
Summary of Diastole
- Ventricles relax
- pulmonary and aortic valves close
- AV valves open
- ventricles fill (about 80% of capacity)
- atria contract (ventricles fill another 20%)
Events
in Systole
As the ventricles start to
contract, the ventricular pressure soon exceeds the atrial pressure, causing
the AV valves to close. As the ventricles continue to contract, the ventricular
pressure exceeds the arterial pressures causing the semilunar valves open.
Blood is forcefully ejected out of the ventricles and into the aorta and
pulmonary artery.
Summary
of Systole
- ventricles contract
- AV valves close
- aortic and pulmonary valves
open
- blood is ejected
- atria relax and fill with
blood
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