Circulatory System | Research & Encyclopedia Articles

Every living organism on Earth, from amoebas to redwoods to whales, has a circulatory system—a means of gathering and transporting nutrients and collecting and removing waste products.

Plants have an elegant system of strawlike tubes called phloem and xylem, which stretch from the roots to the topmost leaves. Stomata, tiny evaporative holes in the leaves, create suction that steadily draws water up the xylem from the roots, allowing plants hundreds of feet tall to circulate nutrients without a pump.

All cells of the simplest animals, such as single-celled amoebas and multicellular flatworms, are close to the surface. In these cells, nutrients wash through the cell fluid, and wastes pass out through a porous outer membrane between the cell and its environment. The cells of larger animals are buried many layers deep, so these animals require a system that connects each cell to the outer world. This system, which consists of the fluid that carries nutrients through vessels that reach every part of the body and the mechanism that powers the flow of nutrients, is called the circulatory system.

The simplest form of circulatory system is an open circulatory system. In an open circulatory system, blood flows through a network of open tubes and hollow spaces, and the movement of the animal itself keeps the blood flowing. In more complex systems, blood is pumped through the body by contractions of the blood vessels. Invertebrates, such as insects and other arthropods, have a central blood vessel that runs down the length of the back. A series of bulbous pumping centers slowly squeeze the blood through a maze of hollow spaces around the body past all the organs.

Vertebrates, including amphibians, reptiles, birds, and mammals, have increasingly complex, closed circulatory systems. Closed circulatory systems consist of an intricate network of vessels filled with blood that delivers nutrients, regulates internal temperature, and takes away waste products. The system is powered by the heart, a muscular pump that never stops working, which continually circulates the blood through the body.

In all vertebrates, the heart is made of involuntary muscle tissue, but the structure is very different in each group. Fish have a two-chambered, single-pump heart. Amphibians have a three-chambered heart that also acts as a single pump. Birds and mammals have a more sophisticated four-chambered, double-pump heart design. One chamber sends blood to the lungs to be purified and reoxygenated, the other sends the enriched blood out into the body. Interestingly, the human embryo goes through every stage of circulatory development, from a passive single-celled heart to a two-, three-, and four-chambered heart.

In adult humans, the circulatory system consists of blood, the heart, and a network of vessels through which the blood travels. Blood is plasma (a watery liquid) that contains billions of molecules of sugars, proteins, hormones, antibodies, and gases. The heart is a strong, muscular, double pump that pushes the blood continuously and automatically around the body through roughly 100,000 kilometers (62,000 miles) of arteries, veins, and capillaries. It takes the blood about one minute to complete a circuit around the body, and this happens about 1,000 times a day.

The human circulatory system has two loops. The shorter pulmonary circulation goes from the lower-right chamber of the heart (the ventricle) through the pulmonary artery to the lungs and back to the upper-left chamber (the atrium) through the pulmonary vein. From there, the newly oxygenated blood descends into the left ventricle through a one-way valve and is pumped into the longer systemic circulation through the main artery of the body, the aorta. The spent blood travels back to the right atrium in two main veins. The superior vena cava drains the upper body, the head, neck, and arms. The inferior vena cava handles the lower body. From the right atrium, the blood flows through the relaxed one-way valve into the right ventricle. Then another pulse of the powerful heart muscle closes the valve and spurts the blood into the pulmonary artery, beginning the cycle again. The sound known as the heart "beat" is the sound of the valves between the atria and ventricles and between the ventricles and arteries as they snap shut to keep the blood from flowing backward.

A healthy, relaxed, adult heart beats about seventy times a minute, pumping blood under high pressure into the thick-walled arteries. The elastic walls of the arteries stretch open to allow the blood to flow in, then squeeze back together to force it along. Arteries branch into narrower and more muscular arterioles. Arterioles branch into finer and finer capillaries, thin-walled, hairlike vessels that interact with surrounding body cells to exchange nutrients and wastes. Capillaries then enlarge into venules, which merge into veins, and carry the spent blood back to the heart. After the blood has traveled through the capillary network the pressure is greatly reduced, and the veins can afford to be much thinner than arteries with weaker muscle fiber. Small, one-way valves inside the veins keep the blood moving against gravity toward the heart.

As well as delivering the supplies that keep cells functioning, the bloodstream regulates body temperature by dissipating heat that builds up in the organs. The contraction or dilation of surface capillaries allows more or less heat to escape the system, depending on whether the body is too hot or too cold. The bloodstream also contains disease-and infection-fighting antibodies.

The lymphatic system is a one-way independent drainage network of fine capillaries primarily involved in fighting disease and infection. White blood cells used in neutralizing bacteria collect in lymph glands. Then normal muscle movement and one-way valves keep the lymph flowing toward the chest, where it drains into two large veins and reenters the blood stream.

Blood.

Ballard, Carol. Heart and Circulatory System. Austin, TX: Steck Vaughn, 1997.

Silverstein, Alvin, Virginia, and Robert Alvin. The Circulatory System. New York: Henry Holt & Co., 1994.

Unschuld, Paul U., trans. Nan Ching (Chinese Medicine). Berkeley: University of California Press, 1985.

This is the complete article, containing 1,005 words (approx. 3 pages at 300 words per page).