With each heartbeat, blood is sent throughout our bodies,
carrying oxygen and nutrients to all of our cells. Each day, 2,000
gallons of blood travel many times through about 60,000 miles of
blood vessels that branch and cross, linking the cells of our
organs and body parts.
From the hard-working heart to our thickest arteries to
capillaries so thin that they can only be seen through a
microscope, the heart and circulatory system (also called the
cardiovascular system) are our body's lifeline, delivering
blood to the body's tissues.
About the Heart and Circulatory System
The circulatory system is composed of the heart and blood
vessels, including arteries, veins, and capillaries. Our bodies
actually have two circulatory systems: The pulmonary circulation is
a short loop from the heart to the
lungs
and back again, and the systemic circulation (the system we usually
think of as our circulatory system) sends
blood
from the heart to all the other parts of our bodies and back
again.
The heart is the key organ in the circulatory system. As a
hollow, muscular pump, its main function is to propel blood
throughout the body. It usually beats from 60 to 100 times per
minute, but can go much faster when necessary. It beats about
100,000 times a day, more than 30 million times per year, and about
2.5 billion times in a 70-year lifetime.
The heart gets messages from the body that tell it when to pump
more or less blood depending on an individual's needs. When
we're sleeping, it pumps just enough to provide for the lower
amounts of oxygen needed by our bodies at rest. When we're
exercising or frightened, the heart pumps faster to increase the
delivery of oxygen.
The heart has four chambers that are enclosed by thick, muscular
walls. It lies between the lungs and just to the left of the middle
of the chest cavity. The bottom part of the heart is divided into
two chambers called the right and left ventricles, which pump blood
out of the heart. A wall called the interventricular septum divides
the ventricles.
The upper part of the heart is made up of the other two chambers
of the heart, the right and left atria. The right and left atria
receive the blood entering the heart. A wall called the interatrial
septum divides the right and left atria, which are separated from
the ventricles by the atrioventricular valves. The tricuspid valve
separates the right atrium from the right ventricle, and the mitral
valve separates the left atrium and the left ventricle.
Two other cardiac valves separate the ventricles and the large
blood vessels that carry blood leaving the heart. These are the
pulmonic valve, which separates the right ventricle from the
pulmonary artery leading to the lungs, and the aortic valve, which
separates the left ventricle from the aorta, the body's largest
blood vessel.
Arteries carry blood away from the heart. They are the
thickest blood vessels, with muscular walls that contract to keep
the blood moving away from the heart and through the body. In the
systemic circulation, oxygen-rich blood is pumped from the heart
into the aorta. This huge artery curves up and back from the left
ventricle, then heads down in front of the spinal column into the
abdomen. Two coronary arteries branch off at the beginning of the
aorta and divide into a network of smaller arteries that provide
oxygen and nourishment to the muscles of the heart.
Unlike the aorta, the body's other main artery, the
pulmonary artery, carries oxygen-poor blood. From the right
ventricle, the pulmonary artery divides into right and left
branches, on the way to the lungs where blood picks up oxygen.
Arterial walls have three layers:
- The
endothelium
is on the inside and provides a smooth lining for blood to flow
over as it moves through the artery.
- The
media
is the middle part of the artery, made up of a layer of muscle
and elastic tissue.
- The
adventitia
is the tough covering that protects the outside of the
artery.
As they get farther from the heart, the arteries branch out into
arterioles, which are smaller and less elastic.
Veins carry blood back to the heart. They're not as muscular
as arteries, but they contain valves that prevent blood from
flowing backward. Veins have the same three layers that arteries
do, but are thinner and less flexible. The two largest veins are
the superior and inferior vena cavae. The terms superior and
inferior don't mean that one vein is better than the other, but
that they're located above and below the heart.
A network of tiny capillaries connects the arteries and veins.
Though tiny, the capillaries are one of the most important parts of
the circulatory system because it's through them that nutrients
and oxygen are delivered to the cells. In addition, waste products
such as carbon dioxide are also removed by the capillaries.
What the Heart and Circulatory System Do
The circulatory system works closely with other systems in our
bodies. It supplies oxygen and nutrients to our bodies by working
with the respiratory system. At the same time, the circulatory
system helps carry waste and carbon dioxide out of the body.
Hormones - produced by the
endocrine system
- are also transported through the blood in the circulatory system.
As the body's chemical messengers, hormones transfer
information and instructions from one set of cells to another. For
example, one of the hormones produced by the heart helps control
the kidneys' release of salt from the body.
One complete heartbeat makes up a cardiac cycle, which consists
of two phases:
- In the first phase, the ventricles contract (this is called
systole), sending blood into the pulmonary and systemic
circulation. To prevent the flow of blood backwards into the
atria during systole, the atrioventricular valves close, creating
the first sound (the lub). When the ventricles finish
contracting, the aortic and pulmonary valves close to prevent
blood from flowing back into the ventricles. This is what creates
the second sound (the dub).
- Then the ventricles relax (this is called diastole) and fill
with blood from the atria, which makes up the second phase of the
cardiac cycle.
A unique electrical conduction system in the heart causes it to
beat in its regular rhythm. The sinoatrial or SA node, a small area
of tissue in the wall of the right atrium, sends out an electrical
signal to start the contracting of the heart muscle. This node is
called the pacemaker of the heart because it sets the rate of the
heartbeat and causes the rest of the heart to contract in its
rhythm.
These electrical impulses cause the atria to contract first, and
then travel down to the atrioventricular or AV node, which acts as
a kind of relay station. From here the electrical signal travels
through the right and left ventricles, causing them to contract and
forcing blood out into the major arteries.
In the systemic circulation, blood travels out of the left
ventricle, to the aorta, to every organ and tissue in the body, and
then back to the right atrium. The arteries, capillaries, and veins
of the systemic circulatory system are the channels through which
this long journey takes place.
Once in the arteries, blood flows to smaller arterioles and then
to capillaries. While in the capillaries, the bloodstream delivers
oxygen and nutrients to the body's cells and picks up waste
materials. Blood then goes back through the capillaries into
venules, and then to larger veins until it reaches the vena
cavae.
Blood from the head and arms returns to the heart through the
superior vena cava, and blood from the lower parts of the body
returns through the inferior vena cava. Both vena cavae deliver
this oxygen-depleted blood into the right atrium. From here the
blood exits to fill the right ventricle, ready to be pumped into
the pulmonary circulation for more oxygen.
In the pulmonary circulation, blood low in oxygen but high in
carbon dioxide is pumped out the right ventricle into the pulmonary
artery, which branches off in two directions. The right branch goes
to the right lung, and vice versa.
In the lungs, the branches divide further into capillaries.
Blood flows more slowly through these tiny vessels, allowing time
for gases to be exchanged between the capillary walls and the
millions of alveoli, the tiny air sacs in the lungs.
During the process called oxygenation, oxygen is taken up by the
bloodstream. Oxygen locks onto a molecule called hemoglobin in the
red blood cells. The newly oxygenated blood leaves the lungs
through the pulmonary veins and heads back to the heart. It enters
the heart in the left atrium, then fills the left ventricle so it
can be pumped into the systemic circulation.
Problems of the Heart and Circulatory System
Problems with the cardiovascular system are common - more than
64 million Americans have some type of cardiac problem. But
cardiovascular problems don't just affect older people - many
heart and circulatory system problems affect children and teens,
too.
Heart and circulatory problems are grouped into two categories:
congenital, which means the problems were present at birth, and
acquired, which means that the problems developed some time during
infancy, childhood, adolescence, or adulthood.
Congenital heart defects.
Congenital heart defects
are abnormalities in the heart's structure that are present at
birth. Approximately 8 out of every 1,000 newborns have congenital
heart defects ranging from mild to severe. These defects occur
while the fetus is developing in the mother's uterus and
it's not usually known why they occur. Some congenital heart
defects are caused by genetic disorders, but most are not. What all
congenital heart defects have in common, however, is that they
involve abnormal or incomplete development of the heart.
A common sign of a congenital heart defect is a
heart murmur
- an abnormal sound (like a blowing or whooshing sound)
that's heard when listening to the heart. Usually a heart
murmur is detected by a doctor who's listening to the heart
with a stethoscope during a routine exam. Murmurs are very common
in children and can be caused by congenital heart defects or other
heart conditions.
Arrhythmia.
Cardiac arrhythmias, also called dysrhythmias or rhythm disorders,
are abnormalities in the heart's rhythm. They may be caused by
a congenital heart defect or they may be acquired later. An
arrhythmia may cause the heart's rhythm to be irregular,
abnormally fast, or abnormally slow. Arrhythmias can occur at any
age and may be discovered during a routine physical examination.
Depending on the type of rhythm disorder, an arrhythmia may be
treated with medication, surgery, or pacemakers.
Cardiomyopathy.
Cardiomyopathy is a chronic disease that causes the heart muscle
(the myocardium) to become weakened. Usually, the disease first
affects the lower chambers of the heart, the ventricles, and then
progresses and damages the muscle cells and even the tissues
surrounding the heart. In its most severe forms, this condition may
lead to heart failure and even death. Cardiomyopathy is the
#1 reason for heart transplants in children.
Coronary artery disease.
The most common heart disorder in adults, coronary artery disease
is caused by atherosclerosis. Deposits of fat, calcium, and dead
cells, called atherosclerotic plaques, form on the inner walls of
the coronary arteries (the blood vessels that supply the heart) and
interfere with the smooth flow of blood. Blood flow to the heart
muscle may even stop if a thrombus, or clot, forms in a coronary
vessel, which may cause a heart attack. In a heart attack (or
myocardial infarction), the heart muscle becomes damaged by lack of
oxygen, and unless blood flow returns within minutes, muscle damage
increases and the heart's ability to pump blood is compromised.
If the clot can be dissolved within a few hours, damage to the
heart can be reduced. Heart attacks are rare in children and
teens.
Hyperlipidemia/hypercholesterolemia (high
cholesterol).
Cholesterol is a waxy substance that's found in the body's
cells, in the blood, and in some of the foods we eat. Having too
much cholesterol in the blood, also known as hypercholesterolemia
or hyperlipidemia, is a major risk factor for heart disease and can
lead to a heart attack.
Cholesterol is carried in the bloodstream by lipoproteins. Two
kinds - low-density lipoproteins (LDL) and high-density
lipoproteins (HDL) - are the most important. High levels of LDL
cholesterol (the bad cholesterol) increase a person's risk for
heart disease and stroke, whereas high levels of HDL cholesterol
(the good cholesterol) can protect against these.
A blood test can indicate if someone's cholesterol is too
high. A child's cholesterol level is borderline if it's 170
to 199 mg/dL, and it's considered high if it's above 200
mg/dL.
About 10% of teens between 12 and 19 have high cholesterol
levels that put them at increased risk of cardiovascular
disease.
High blood pressure (hypertension)
.
Over time, high blood pressure can cause damage to the heart
and arteries and other body organs. The symptoms of hypertension
can include headache, nosebleeds, dizziness, and lightheadedness.
Infants, kids, and teens can have high blood pressure, which may be
caused by genetic factors, excess body weight, diet, lack of
exercise, and diseases such as heart disease or kidney disease.
Kawasaki disease.
Kawasaki disease (also known as mucocutaneous lymph node syndrome)
affects the mucous membranes (the lining of the mouth and breathing
passages), the skin, and the lymph nodes (part of the immune
system). It can also lead to vasculitis, an inflammation of the
blood vessels. This can affect all major arteries in the body -
including the coronary arteries. It can also cause inflammation of
the heart muscle, called myocarditis. When coronary arteries become
inflamed, a child can develop aneurysms, which are weakened and
bulging spots on the walls of arteries. This increases the risk of
a blood clot forming in this weakened area, which can block the
artery, possibly leading to a heart attack. In addition to the
coronary arteries, the heart muscle, lining, valves, or the outer
membrane that surrounds the heart can become inflamed. Arrhythmias
or abnormal functioning of some heart valves can occur. Kawasaki
disease has surpassed rheumatic fever as the leading cause of
acquired heart disease in children in the United States.
Rheumatic heart disease.
Usually the complication of an untreated
strep throat
infection, rheumatic fever can lead to permanent heart damage and
even death. Most common in children between 5 and 15 years of age,
it begins when antibodies the body produces to fight the strep
infection begin to attack other parts of the body. They react to
tissues in the heart valves as though they were the strep bacteria
and cause the heart valves to thicken and scar. Inflammation and
weakening of the heart muscle may also occur. Usually, when strep
throat infections are promptly treated with antibiotics, this
condition can be avoided.
Stroke.
Strokes occur when the blood supply to the brain is cut off or when
a blood vessel in the brain bursts and spills blood into an area of
the brain, causing damage to brain cells. Children or infants who
have experienced stroke may be suddenly numb or weak, especially on
one side of the body, and they may experience a sudden severe
headache, nausea or vomiting, and difficulty seeing, speaking,
walking, or moving. During childhood, strokes are rare.
Getting plenty of exercise, eating a nutritious diet,
maintaining a healthy weight, and getting regular medical checkups
are the best ways to help keep the heart healthy and avoid
long-term problems like high blood pressure, high cholesterol, and
heart disease.
Reviewed by:
Steven Dowshen, MD
Date reviewed: July 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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