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The heart, blood, and blood vessels are the three
basic elements that make up the circulatory system.
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to enlarge |
Heart
The heart is the engine of the circulatory
system. It is divided into four chambers: the right atrium, the
right ventricle, the left atrium, and the left ventricle.
The walls of these chambers are made of a special
muscle called myocardium, which continuously and rhythmically contracts
to pump blood.
The pumping action of the heart occurs in two
stages for each heartbeat: diastole, when the heart is at rest;
and systole, when the heart contracts to pump deoxygenated blood
toward the lungs and oxygenated blood to the body. During each heartbeat,
typically about 2 to 3 oz of blood is pumped out of the heart. If
the heart stops pumping, death usually occurs within four to five
minutes. |
Blood
Blood consists of three types of cells: oxygen-bearing
red blood cells, disease-fighting white-blood cells, and blood-clotting
platelets, all of which are carried through blood vessels in a liquid
called plasma. Plasma is yellowish and consists of water, salts, proteins,
vitamins, minerals, hormones, dissolved gases, and fats.
Blood vessels
Three
types of blood vessels form a complex network of tubes throughout
the body. Arteries carry blood away from the heart, and veins carry
it toward the heart Capillares are the tiny links between
the arteries and the veins.Oxygen and nutrients diffuse from the capillaries
to the body tissues.
All blood vessels are lined with endothelial cells that create a smooth
passage for the transit of blood. Connective tissue and smooth muscle
surround the inner layer of the blood vessels enabling the blood vessels
to expand or contract. When people exercise, blood vessels expand
to meet the increased demand for oxygen and to cool the body. Blood
vessels contract after an injury to reduce bleeding and also to conserve
body heat. |
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to enlarge |
Arteries have thicker walls than veins to withstand
the pressure of blood being pumped from the heart. Blood in the veins
is at a lower pressure, so veins have one-way valves to prevent blood
from flowing backward away from the heart. Capillaries, the smallest of
blood vessels, are only visible by microscope --ten capillaries lying
side by side are barely as thick as a human hair. If all the arteries,
veins, and capillaries in the human body were placed end to end, the total
length would equal more than 60,000 miles-- they could stretch around
the earth nearly two and a half times.
Circulation systems
The arteries, veins, and capillaries are
divided into two systems of circulation: systemic (or peripheral) and
pulmonary. The systemic circulation carries oxygenated blood from the
heart to all the tissues in the body except the lungs and returns deoxygenated
blood carrying waste products, such as the carbon dioxide, back to the
heart. The pulmonary circulation carries this spent blood from the heart
to the lungs. In the lungs, the blood releases its carbon dioxide and
absorbs oxygen. The oxygenated blood then returns to the heart before
transferring to the systemic circulation.
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