The heart weighs between 7 and 15 ounces (200 to 425 grams) and
is a little larger than the size of your fist. By the end of a
long life, a person's heart may have beat (expanded and contracted)
more than 3.5 billion times. In fact, each day, the average heart
beats 100,000 times, pumping about 2,000 gallons (7,571 liters)
of blood.
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Your
heart is located between your lungs in the middle of your
chest, behind and slightly to the left of your breastbone
(sternum). A double-layered membrane called the pericardium
surrounds your heart like a sac. The outer layer of the
pericardium surrounds the roots of your heart's major blood
vessels and is attached by ligaments to your spinal column,
diaphragm, and other parts of your body. The inner layer
of the pericardium is attached to the heart muscle. A coating
of fluid separates the two layers of membrane, letting the
heart move as it beats, yet still be attached to your body.
Your heart has 4 chambers. The upper chambers are called
the left and right atria, and the lower chambers are called
the left and right ventricles. A wall of muscle called the
septum separates the left and right atria and the left and
right ventricles. The left ventricle is the largest and
strongest chamber in your heart. The left ventricle's chamber
walls are only about a half-inch thick, but they have enough
force to push blood through the aortic valve and into your
body. |
The Heart Valves (illustration)
Four
types of valves regulate blood flow through your heart:
The
tricuspid valve regulates blood flow between the right atrium
and right ventricle.
The pulmonary valve controls blood flow from the right ventricle
into the pulmonary arteries, which carry blood to your lungs to
pick up oxygen.
The mitral valve lets oxygen-rich blood from your lungs pass from
the left atrium into the left ventricle.
The aortic valve opens the way for oxygen-rich blood to pass from
the left ventricle into the aorta, your body's largest artery,
where it is delivered to the rest of your body.
The Heartbeat
heartbeat is a two-part pumping action that takes
about a second. As blood collects in the upper chambers (the right
and left atria), the heart's natural pacemaker (the SA node) sends
out an electrical signal that causes the atria to contract. This
contraction pushes blood through the tricuspid and mitral valves
into the resting lower chambers (the right and left ventricles).
This part of the two-part pumping phase (the longer of the two)
is called the diastole.
The
second part of the pumping phase begins when the ventricles are
full of blood. The electrical signals from the SA node travel
along a pathway of cells to the ventricles, causing them to contract.
This is called systole. As the tricuspid and mitral valves shut
tight to prevent a back flow of blood, the pulmonary and aortic
valves are pushed open. While blood is pushed from the right ventricle
into the lungs to pick up oxygen, oxygen-rich blood flows from
the left ventricle to the heart and other parts of the body.
After
blood moves into the pulmonary artery and the aorta, the ventricles
relax, and the pulmonary and aortic valves close. The lower pressure
in the ventricles causes the tricuspid and mitral valves to open,
and the cycle begins again. This series of contractions is repeated
over and over again, increasing during times of exertion and decreasing
while you are at rest. The heart normally beats about 60 to 80
times a minute when you are at rest, but this can vary. As you
get older, your resting heart rate rises. Also, it is usually
lower in people who are physically fit.
Your
heart does not work alone, though. Your brain tracks the conditions
around you—climate, stress, and your level of physical activity—and
adjusts your cardiovascular system to meet those needs.
The
human heart is a muscle designed to remain strong and reliable
for a hundred years or longer. By reducing your risk factors for
cardiovascular disease, you may help your heart stay healthy longer.
The
Conduction System
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Electrical
impulses from your heart muscle (the myocardium) cause your
heart to contract. This electrical signal begins in the
sinoatrial (SA) node, located at the top of the right atrium.
The SA node is sometimes called the heart's "natural
pacemaker." An electrical impulse from this natural
pacemaker travels through the muscle fibers of the atria
and ventricles, causing them to contract. Although the SA
node sends electrical impulses at a certain rate, your heart
rate may still change depending on physical demands, stress,
or hormonal factors. |
The Circulatory System
Your
heart and circulatory system make up your cardiovascular system.
Your heart works as a pump that pushes blood to the organs, tissues,
and cells of your body. Blood delivers oxygen and nutrients to
every cell and removes the carbon dioxide and waste products made
by those cells. Blood is carried from your heart to the rest of
your body through a complex network of arteries, arterioles, and
capillaries. Blood is returned to your heart through venules and
veins. If all the vessels of this network in your body were laid
end-to-end, they would extend for about 60,000 miles (more than
96,500 kilometers), which is far enough to circle the earth more
than twice!
The
one-way circulatory system carries blood to all parts of your
body. This process of blood flow within your body is called circulation.
Arteries carry oxygen-rich blood away from your heart, and veins
carry oxygen-poor blood back to your heart.
In
pulmonary circulation, though, the roles are switched. It is the
pulmonary artery that brings oxygen-poor blood into your lungs
and the pulmonary vein that brings oxygen-rich blood back to your
heart.
In
the diagram, the vessels that carry oxygen-rich blood are colored
red, and the vessels that carry oxygen-poor blood are colored
blue.
Twenty
major arteries make a path through your tissues, where they branch
into smaller vessels called arterioles. Arterioles further branch
into capillaries, the true deliverers of oxygen and nutrients
to your cells. Most capillaries are thinner than a hair. In fact,
many are so tiny, only one blood cell can move through them at
a time. Once the capillaries deliver oxygen and nutrients and
pick up carbon dioxide and other waste, they move the blood back
through wider vessels called venules. Venules eventually join
to form veins, which deliver the blood back to your heart to pick
up oxygen.
