that the heart is a vital organ. We cannot live without our
heart. However, when you get right down to it, the heart is
just a pump. A complex and important one, yes, but still just
a pump. As with all other pumps it can become clogged, break
down and need repair. This is why it is critical that we know
how the heart works, With a little knowledge about your heart
and what is good or bad for it, you can significantly reduce
your risk for heart disease.
Heart disease is the leading cause of death in the U.S. In
1997, 725,000 persons died of heart disease (31% of all
deaths). Almost 2,000 Americans die of heart disease each day.
That is 1 death every 44 seconds. The good news is that the
death rate from heart disease has been steadily decreasing.
The age adjusted death rate decreased by 3% from 1996 to 1997
and 34.6% from 1979 to 1997. Unfortunately, heart disease
still causes sudden death and many people die before even
reaching the hospital.
The heart holds a special place in our collective psyche as
well. Of course the heart is synonymous with love. It has many
other associations -- What other word has as many meanings as
the word heart? Here are just a few examples:
Certainly no other bodily organ elicits this kind of
response. When was the last time you had a heavy pancreas?
- have a heart - be merciful
- change of heart - change your mind
- to know something by heart - memorize something
- broken heart - to lose love
- heartfelt - deeply felt
- have your heart in the right place - to be kind
- cry your heart out - to grieve
- heavy heart - sadness
- have your heart set on - to want something badly
In this edition of How Stuff
Works we will look at this most important -- and most
talked-about - organ so that you can understand exactly what
makes your heart tick!
Anatomy of the Heart
heart is a hollow, cone-shaped muscle located between the
lungs and behind the sternum (breastbone). Two-thirds of the
heart is located to the left of the midline of the body and
1/3 is to the right (see Figure 1).
[Please note - Medical
illustrations assume that the patient is
facing you so that
the right and left correspond to the patient's
left. That's why the left and right labels here seem
The apex (pointed end) points down and to the left. It is 5
inches (12 cm) long, 3.5 inches (8-9 cm) wide and 2.5 inches
(6 cm) from front to back, and is roughly the size of your
fist. The average weight of the heart in a female is 9 ounces
and in the male is 10.5 ounces. The heart comprises less than
0.5% of the total body weight.
The heart has 3 layers. The smooth inside lining of the
heart is called the endocardium. The middle layer of
heart muscle is called the myocardium. It is surrounded
by a fluid filled sac call the pericardium.
Chambers and Valves
heart is divided into 4 chambers: (see Figure 2)
- Right Atrium (RA)
- Right Ventricle (RV)
- Left Atrium (LA)
- Left Ventricle (LV)
Each chamber has a sort of one-way valve at its exit that
prevents blood from flowing backwards. When each chamber
contracts the valve at its exit opens. When it is finished
contracting the valve closes so that blood does not flow
When the heart muscle contracts or beats (called
systole) it pumps blood out of the heart. The
heart contracts in two stages. In the first stage the
Right and Left Atria contract at the same time, pumping blood
to the Right and Left Ventricles. Then the Ventricles
contract together to propel blood out of the heart. Then
the heart muscle relaxes (called diastole) before the
next heartbeat. This allows blood to fill up the heart
- Tricuspid valve- is at the exit of the Right Atrium.
- Pulmonary valve - is at the exit of the Right Ventricle.
- Mitral valve - is at the exit of the Left atrium.
- Aortic valve - is at the exit of the Left Ventricle.
The right and left sides of the heart have separate
functions. The right side of the heart collects oxygen-poor
blood from the body and pumps it to the lungs where it picks
up oxygen and releases carbon dioxide. The left side of the
heart then collects oxygen rich blood from the lungs and pumps
it to the body so that the cells throughout your body have the
oxygen they need to function properly.
Flow of Blood
blood enters the right side of the heart through two veins:
collects blood from the upper half of the body. The IVC
collects blood from the lower half of the body. Blood leaves
the SVC and the IVC and enters the Right Atrium (RA) (3).
- The Superior Vena Cava (SVC) and
- Inferior Vena Cava (IVC) (see figure 3).
When the RA contracts, the blood goes through the Tricuspid
Valve (4) and into the Right Ventricle (RV) (5). When the RV
contracts blood is pumped through the Pulmonary Valve (6),
into the Pulmonary Artery (PA) (7) and into the lungs where it
picks up oxygen.
Why does it happen this way? Because blood returning from
the body is relatively poor in oxygen. It needs to be full of
oxygen before being returned to the body. So the right side of
the heart pumps blood to the lungs first to pick up oxygen
before going to the left side of the heart where it is
returned to the body full of oxygen.
Blood now returns to the heart from the lungs by way of the
Pulmonary Veins (8) and goes into the Left Atrium (LA) (9).
When the LA contracts, blood travels through the Mitral Valve
(10) and into the Left Ventricle (LV) (11). The LV is a very
important chamber that pumps blood through the Aortic Valve
(12) and into the Aorta (13) to the rest of the body. The
Aorta is the main artery of the body. It receives all the
blood that the heart has pumped out and distributes it to the
rest of the body. The LV has a thicker muscle than any other
heart chamber because it must pump blood to the rest of the
body against much higher pressure in the general circulation
Here is a recap of what we just discussed. Blood from the
The blood picks up oxygen in the
lungs, and then flows from the lungs:
- to the Superior and Inferior Vena Cava,
- then to the Right Atrium
- through the Tricuspid Valve
- to the Right Ventricle
- through the Pulmonic Valve
- to the Pulmonary Artery
- to the Lungs
- to the Pulmonary Veins
- to the Left Atrium
- through Mitral valve
- to the Left Ventricle
- through the Aortic Valve
- to the Aorta
- to the body
you ever wondered what makes your heart beat? How does it do
it automatically, hour after hour, day after day?
The answer lies in a special group of cells that have the
ability to generate electrical activity on their own. These
cells separate charged particles. Then they spontaneously leak
certain charged particles into the cells. This produces
electrical impuses in the pacemaker cells which spread over
the heart, causing it to contract. These cells do this more
than once per second to produce a normal heart beat of 72
beats per minute.
The natural pacemaker of the heart is called the
Sinoatrial node (SA node). It is located in the Right
Atrium. The heart also contains specialized fibers that
conduct the electrical impulse from the pacemaker (SA node) to
the rest of the heart (see Figure 4).
The electrical impulse leaves the SA node (1) and
travels to the right and left Atria, causing them to contract
together. This takes .04 seconds. There is now a natural delay
to allow the Atria to contract and the Ventricles to fill up
with blood. The electrical impulse has now traveled to the
Atrioventricular Node (AV node) (2). The electrical
impulse now goes to the Bundle of His (3), then it
divides into the Right and Left Bundle Branches (4)
where it rapidly spreads using Purkinje Fibers (5) to
the muscles of the Right and Left Ventricle, causing them to
contract at the same time.
Any of the electrical tissue in the heart has the ability
to be a pacemaker. However, the SA node generates an electric
impulse faster than the other tissue so it is normally in
control. If the SA node should fail, the other parts of the
electrical system can take over, although usually at a slower
Although the pacemaker cells create the electrical impulse
that causes the heart to beat, other nerves can change the
rate at which the pacemaker cells fire and the how strongly
the heart contracts. These nerves are part of the Autonomic
Nervous System. The Autonomic Nervous System has 2 parts - The
Sympathetic Nervous System and the Parasympathetic Nervous
System. The Sympathetic Nerves increase the heart rate and
increase the force of contraction. The Parasympathetic Nerves
do the opposite.
All this activity produces electrical waves we can measure.
The measurement is typically represented as a graph called an
Electrocardiogram (EKG). Here is an example of three
heartbeats from an EKG (Figure 5):
Each part of the tracing has a lettered name:
Electrical system abnormalities can
range from minor premature beats (skipped beats) that do not
require treatment, to slow or irregular beats that require an
- P wave - coincides with the spread of electrical
activity over the Atria and the beginning of it's
- QRS complex - coincides with the spread of electrical
activity over the ventricles and the beginning of it's
- T wave - coincides with the recovery phase of the
arteries are the ones that we try to keep clear by eating a
healthy diet. If Coronary arteries are blocked a heart attack
The heart, just like any other organ, requires blood to
supply it with oxygen and other nutrients so that it can do
its work. The heart does not extract oxygen and other
nutrients from the blood flowing inside it. The heart gets its
blood from coronary arteries that eventually carry blood
within the heart muscle. Approximately 4-5% of the blood
output of the heart goes to the coronary arteries (7 ½
ounces/minute or 225 ml/min).
There are two main coronary arteries (figure 6) - The
Left Main Coronary artery (1) and the Right Coronary
Artery (2) which arise from the Aorta. The Left Main
Coronary Artery divides into the Left Anterior
Descending (3) and the Left Circumflex Arteries
(4). Each Artery supplies blood to different parts of the
heart muscle and the electrical system.
The heart also has veins that collect oxygen poor blood
from the heart muscle. Most of the major veins of the heart
(great cardiac vein, small cardiac vein, middle cardiac vein,
posterior vein of the Left Ventricle, and oblique vein of the
Left Atrium) drain into the coronary sinus which opens into
the Right Atrium.
Coronary artery disease is caused by a blockage in one of
the coronary arteries. When a coronary artery is partially
blocked that artery cannot supply enough blood to the heart
muscle to meet its needs during exertion. When someone with
coronary artery disease exerts himself or herself, it causes
chest pain. This is due to lack of blood and oxygen to that
part of the heart muscle. This is called angina. If the
angina worsens (more frequent angina episodes, with less
exertion) a condition called unstable angina exists. A
worsening of the obstruction causes this. A heart attack
occurs when a coronary artery is completely blocked and no
blood or oxygen is getting to the heart muscle served by that
artery. This also causes chest pain and causes death to the
heart muscle served by that artery.
What causes the sound your heart
When someone listens to your heart with a
stethoscope the sound is often described as - lub-dub lub-dub.
The 1st heart sound (lub) is caused by the acceleration and
deceleration of blood and vibration of the heart at the time
of the closure of the Tricuspid and Mitral Valves. The 2nd
heart sound (dub) is caused by the same acceleration and
deceleration of blood and vibrations at the time of closure of
the Pulmonic and Aortic Valves.
How may times does you heart beat?
heartbeat is 72 times per minute. In the course of one day it
beats over 100,000 times. In one year the heart beats almost
38 million times, and by the time you are 70 years old, on
average, it beats 2.5 billion times!
Does your heart rate change as we age?
pulse (average heart rate per minute) changes as we age. Here
is a chart of average pulse at different ages:
||60 - 100|
How much blood does your heart pump?
heart pumps 2.4 ounces (70 milliliters) per heartbeat. An
average heartbeat is 72 beats per minute. Therefore an average
heart pumps 1.3 gallons (5 Liters) per minute. In other words
it pumps 1,900 gallons (7,200 Liters) per day, almost 700,000
gallons (2,628,000 Liters) per year, or 48 million gallons
(184,086,000 liters) by the time someone is 70 years old.
That's not bad for a 10 ounce pump!
I hope this information has helped you to learn about the
heart in a fun way. This will serve as a good foundation
for future articles about heart disease.
About the Author
Bianco, M.D. is an Emergency Physician practicing at
Dorchester General Hospital in Cambridge, MD, located on the
Eastern Shore of Maryland. Dr. Bianco attended Medical school
at Georgetown University School of Medicine and he received
his undergraduate degree from Georgetown University majoring
in nursing and pre-med. He Completed an internship and
residency in Emergency Medicine at Akron City Hospital in
Dr. Bianco lives near Baltimore with his wonderful wife and
two wonderful children.