What does it do?
The heart is the main organ in the cardiovascular system; its function is to distribute blood and its
many contents through the body to tissues. In addition, it helps to redistribute and eliminate
waste from the body. This system (made up of the heart and blood vessels), as you can imagine,
is vital for survival.
Structure of the Heart
The typical heart is approximately the size of a fist (14 cm x 9 cm on average). It is found
between the lungs, anterior to the spinal cord and posterior to the sternum.
The heart is covered by a thin, protective covering called the pericardium. It protects the heart,
adheres it to the diaphragm underneath, and releases fluid that helps lubricate the heart as it
moves within the pericardium.
The Heart Wall
The heart wall consists of three layers:
The epicardium, a thin, protective layer of connective and epithelial tissue
The myocardium, a thick layer that contains cardiac muscle tissue used to pump blood through
and out of the heart; in between sheets of muscle are layers of connective tissue with many
capillaries and neurons.
The endocardium, an inner layer of connective tissues and cardiac muscle fibers called Purkinje
fibers that help conduct cardiac impulses through the heart.
Chambers of the Heart
The heart consists of four internal chambers, two on the left and two on the right:
Atria: thin-walled upper chambers that receive blood returning to the heart. They contain dog-
eared projections called auricles that project anteriorly and increase their surface area
Ventricles: lower chambers with thicker, more muscular walls; force blood out of heart into
arteries (which lead to the lungs and later to body tissues)
Each pair of chambers is separated by a septum (the interatrial and interventricular septum,
Heart Valves: AV Valves
Blood travels between the atrium and ventricle on each side through an atrioventricular (AV)
orifice. This orifice is guarded by an atrioventricular (AV) valve on each side:
The tricuspid valve guards the AV orifice between the right atrium and right ventricle. It is
made up of three leaflets, or cusps.
The bicuspid valve guards the AV orifice between the left atrium and left ventricle. It is made
up of two cusps (hence the name).
How do AV valves work?
All valves in the heart are designed to promote blood flow in one direction only and to prevent
backup of blood into the atria. The AV valves do this in several ways:
The valves respond to blood pressure; they flap open against the AV orifice when blood
pressure is higher in the atria, and they flap closed when BP is higher in the ventricles.
Chordae tendinae are long strings of connective tissue that attach on the ventricular side. They
are controlled by papillary muscles, which contract when the ventricle contracts and help to
prevent the AV valves from swinging back into the atrium.
Heart Valves (cont.)
Blood also has to exit the heart through orifices; these orifices are guarded by semilunar valves.
The pulmonary valve guards the opening to the pulmonary trunk, which begins to bring blood
out to the lungs to receive oxygen. It opens as the right ventricle contracts, allowing blood to
enter the pulmonary trunk.
The aortic valve guards the opening to the aorta, the largest artery in the body. It opens as the
left ventricle contracts, allowing oxygenated blood to be distributed to tissues.
The semilunar valves lack chordae tendinae and papillary muscles.
Path of Blood Through the Heart: Where It All Begins
The best way to figure out the path of blood through the heart is to trace it--this will happen
soon. But for now, let’s explain the steps that are followed as blood makes its journey through
1. Blood enters the right atrium from two large veins: the superior and inferior venae
cavae (sing.: vena cava). This is deoxygenated blood; its oxygen has been distributed to tissues.
2. As blood pressure builds up in the right atrium, it contracts (both atria contract at the
same time, actually), pushing blood through the tricuspid valve into the right ventricle.
Getting Some Air
3. Once blood pressure becomes higher in the right ventricle, the tricuspid valve flaps
shut. The ventricle then contracts and sends blood through the pulmonary valve into the
4. The pulmonary trunk divides into two pulmonary arteries, one for each lung, that
carry the blood to the lungs to receive oxygen. In the lungs, the vessels gradually become thin,
narrow capillaries and absorb oxygen that diffuses in from the alveoli (air sacs). At the same
time, CO2 diffuses into the alveoli and is released from the body as we exhale.
Back in Business
5. The blood, with a fresh supply of O2 and relatively little CO2, returns to the heart
through four pulmonary veins, all of which empty into the left atrium.
6. The left atrium contracts, sending blood through the bicuspid valve into the left
7. When the left ventricle contracts, the aortic valve opens, and oxygenated blood is
pumped through the aorta into smaller arteries, then into capillaries which give oxygen off to
How does the heart get blood?
The tissues of the heart derive their blood supply from the first two branches of the aorta, the
right and left coronary arteries. These arteries have many branches that eventually branch out to
form many capillaries. These capillaries provide a virtually constant supply of oxygen and
nutrients to the myocardium.
Once the oxygen from the blood is used by tissues, it drains into a series of cardiac veins. These
veins join the coronary sinus, an enlarged vein behind the heart that eventually drains into the
The Cardiac Cycle
Many things need to happen in order for the heart to properly move blood through the body;
luckily, most of these events occur in response to differences in blood pressure from one
chamber/blood vessel to another. All of the events that occur to promote blood flow through the
heart, including the relaxation and contraction of both the atria and the ventricles, are part of a
Events of the cardiac cycle
The atria of the heart both contract at the same time, as do both the ventricles. When a pair of
chambers contracts, it is referred to as systole (contraction of the atria, for example, is atrial
systole). When one pair of chambers contracts, the other pair of chambers relaxes; this is referred
to as diastole (during atrial systole, the relaxation of the ventricles, or ventricular diastole,
Early in ventricular diastole, blood pressure is low; most of the blood gradually empties into the
ventricles before the atria contract. When the atria contract, they pump the rest of the blood into
the ventricles. As soon as ventricular pressure exceeds atrial pressure, the AV valves close, and
the papillary muscles contract to keep the AV valves from going back into the atria.
At this point, blood pressure in the atria is low, even lower than that in the veins. This causes
blood to drain from the veins into the atria in preparation for the next cardiac cycle.
Ventricular systole causes the thick muscular walls of the ventricles to contract, pushing blood
out of the ventricles. This makes the pressure in the ventricles higher than that in the pulmonary
trunk (right side) and aorta (left side). This causes the semilunar valves to open, and blood is
pushed out of the heart to the lungs (right side) and the aorta and general circulation (left side).
When you listen to someone’s heartbeat through a stethoscope, it sounds like “lub-dupp”.
These sounds correspond to the closing of the valves during the cardiac cycle:
The “lub” occurs during ventricular systole, when the AV valves are closing.
The “dupp” occurs during ventricular diastole, when the pulmonary and aortic valves are
closing. Sometimes, there is a short interval between these two events, and two distinct “dupps”
can be heard. In this case, the second heart sound is said to be split.
Blood pressure is measured in the arteries (usually in the brachial artery found near the elbow),
but its measurement has to do with the two principal phases of the cardiac cycle:
The maximum pressure in the ventricles (and also the arteries) occurs during ventricular
contraction (ventricular systole); this reading is taken as the systolic pressure.
The lowest pressure in the ventricles and the arteries occurs during ventricular relaxation
(diastole); this is known as the diastolic pressure.
The contraction and relaxation of the ventricles sends a surge of blood through the arteries.
This rhythmic expansion and contraction of the arteries produces a pulse that can be felt at
various points on the body (major arteries such as the brachial artery and the carotid artery on the
neck). Elevated blood pressure can cause an abnormally strong pulse, whereas weak blood
pressure can lead to a weak, almost undetectable pulse.
Normal blood pressure is below 120/80 (the first number is systolic); blood pressure above
140/90 indicates high blood pressure, or hypertension.