The heart and circulatory system make up
the cardiovascular system. A heart works as a pump that pushes blood
to the organs, tissues, and cells. Blood delivers oxygen and
nutrients to every cell and removes the carbon dioxide and waste
products made by those cells. Blood is carried from the heart to the
rest of the body through a network of arteries, arterioles, and
capillaries. Blood is returned to the heart through venules and
veins. If all the vessels of this network in a body were laid
end-to-end, they would extend for about 60,000 miles, which is far
enough to circle the earth more than twice!
The heart weighs between 7 and 15 ounces
and is a little larger than the size of an adult fist. By the end of
a long life, a person's heart may have beaten (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 of
blood.
The heart is located between the lungs
in the middle of the chest, behind and slightly to the left of the
breastbone (sternum). A double-layered membrane called the
pericardium surrounds the heart like a sack. The outer layer of the
pericardium surrounds the roots of the heart's major blood vessels.
It is attached by ligaments to the spinal column, diaphragm, and
other parts of the 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.
Hearts have 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 the heart. Its
half-inch thick chamber walls have enough force to push blood
through the aortic valve and into the body.
The Heart Valves
Four types of valves regulate blood flow through the 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 the lungs to pick up oxygen. The mitral valve lets oxygen-rich
blood from the 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, the body's largest
artery, where it is delivered to the rest of body body.
The Conduction System
Electrical impulses from a heart muscle (the myocardium) cause it 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, the heart rate may
still change depending on physical demands, stress, or hormonal
factors.
The Circulatory System
The one-way circulatory system carries blood to all parts of a body.
This process of blood flow within the body is called circulation.
Arteries carry oxygen-rich blood away from the heart, and veins
carry oxygen-depleted blood back to the heart. Twenty major arteries
make a path through body tissues, where they branch into smaller
vessels called arterioles. Arterioles further branch into
capillaries, the true deliverers of oxygen and nutrients to 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 the heart to pick up oxygen.
