The human heart is a muscle about the size of your
fist that is located behind and slightly to the left of the breastbone. Its
function is to pump blood throughout the body to provide oxygen and nutrients,
remove waste products, pump deoxygenated blood through the lungs to release
carbon dioxide and collect oxygen. Every minute, it pumps 5L of blood around
the body.
The Heart's Structure
The heart has four chambers:
- Two smaller, upper chambers (the right and left atria)
- Two larger, lower pumping chambers (the right and
left ventricles)
The upper and lower chambers are separated by one-way
valves that ensure the forward motion of blood. The
heart is separated into right and left sides, with each side performing a unique
function:
- The right side
of the heart receives deoxygenated blood from the body and sends it to the
lungs to pick up oxygen.
- The left side of the heart receives oxygenated
blood from the lungs and sends it to the rest of the body where the oxygen
is delivered to organs such as the brain and kidneys.
Electrical Pathways within the Heart
In a healthy heart, each heartbeat begins in the sinus node (the heart's natural
pacemaker), which is located in the right atrium. The electrical signal from
the sinus node (sinoatrial or SA node) starts an electrical chain reaction that
spreads across both atria. This causes the atria to contract and pump blood into the ventricles.
This electrical chain reaction continues from the atria through an area between
the atria and ventricles called the atrioventricular (AV) node or junction.
The AV node connects to conduction pathways that relay the electrical
signal to both ventricles. The AV node acts as an electrical gateway to the
ventricles. The conduction pathways deliver the signal, which
causes the ventricles to contract and pump blood to the body.
An irregularity in the heart's electrical system is called an arrhythmia,
or heart rhythm disorder. Rhythm disorders can cause the heart to beat too slowly
(bradycardia), too fast (tachycardia),
or not in sequence (asynchrony). An arrhythmia can compromise the heart's ability
to pump and circulate blood through the body and brain.
The Heart's Function
A healthy heart is very sensitive to the body's needs. It adjusts its
rate of pumping blood to match the body's need for oxygen and nutrients. An
average person has approximately 120,000 heartbeats a day.
A normal heart rate can range from 60-140 beats per minute depending
upon age and whether the person is at rest or exercising. The younger the person,
the faster the heart rate. As you exercise or perform daily activities your
heart increases its rate to ensure that it is supplying enough oxygen to your
muscles to meet the needs of the activity. In a matter of seconds the heart
can increase its pumping volume up to four times the amount that it pumps when
at rest.
The pumping efficiency of the heart is evaluated using a measurement
called ejection
fraction (EF). The ejection fraction of the heart is defined as the
fraction or proportion of blood that the heart pumps (or ejects) out of the
left ventricular chamber with each heart beat. People with a normal healthy
heart have an ejection fraction of 55 percent or greater. This means that 55
percent of the blood that fills the left ventricle is pumped out to the body
with each contraction.
A low ejection fraction (EF) is one of the key indicators used to identify
heart failure patients. A below-normal EF means your heart is no longer pumping
efficiently and therefore may not be able to supply your organs and the rest
of your body with enough blood. If your EF is low you may need further tests
to determine if you are at risk of complications such as abnormal heart rhythms.
Last updated: 20 January 2005
