The Circulatory System

The function of blood. Blood carries not only food from the intestines to the cells of the body, but also carries oxygen from the lungs to the cells. These are its two most important functions. In addition to these functions, blood helps fight disease, supplies water to the cells, and carries waste products from the cells to the liver and the kidneys, where the waste products are removed.

The heart. The central part of the circulatory system is the heart. The heart is a special muscle that pumps blood through the circulatory system. It pumps blood through the pulmonary artery to the lungs, from the lungs via the pulmonary veins back to the heart itself, from the heart through the arteries, and from the arteries through the capillaries and veins back to the heart.

The action of the heart was discovered nearly three hundred and fifty years ago by William Harvey. He noticed that the blood circulates from the heart, through the blood vessels and back to the heart. His discovery was a milestone in the study of the human body, because it showed that the flow of vital fluids was explainable by logical, natural laws, and that it could be studied by scientists.

Once Harvey had discovered that blood circulates in a closed system, other men began to discover the details of the circulation. Harvey had known that a chamber called a right ventricle pumps blood into the arteries. He did not know how the blood passes from the arteries to the veins, which bring it back to the heart. He thought it passed through invisible spaces in the tissue.

What Harvey did not know was that the blood passes from arteries to veins through the capillaries. The missing link was discovered by Mar-cello Malpighi, who actually saw the tiny blood vessels that link the arteries and the veins.

The capillaries. The capillaries are more than junctions between the arteries and the veins. Because the walls of the capillaries are so thin, liquids and gases can pass through them into the spaces between the cells in body tissues.

Oxygen passes through these walls and is picked up by the cells in all tissues of the body. Carbon dioxide is given off by the cells and passes back through the capillary walls. This passage of gases is called cell respiration, which is described in the section on the respiratory system.

The lymph system. While the blood passes through the capillaries, part of the  liquid,   or  plasma, passes through the capillary walls and into the spaces between the cells. Here it bathes the cells, giving them food and picking up bacteria and solid wastes. Some of this fluid passes back into the blood. The remainder, called lymph, passes among the cells and is picked up by an intricate system of tiny lymph capillaries, which lace the tissue of the body.

The lymph capillaries carry the lymph into larger lymph vessels. The ever-larger lymph vessels carry the lymph toward the heart and finally empty it into the veins near the base of the neck.

Before the lymph passes into the veins, it is filtered by a series of bean-shaped nodes that are plentiful throughout the lymph system. These lymph nodes remove bacteria and solid wastes from the lymph and thus help keep bacteria from entering the blood stream. They form one of the body’s defenses against disease.

Veins. Once the blood has passed through the capillaries, it enters the veins. Here it takes on a bluish appearance because it has lost the oxygen that gave it its bright red color. The veins carry the blood back to the right auricle of the heart. From this chamber it flows into the right ventricle, or pumping section, of the heart.

The right ventricle pumps the blood through the capillaries of the lungs and back to the left auricle, from which it flows to the left ventricle. From here it is pumped into the arteries to begin its life-giving circuit once again.

 

Think for Yourself
What are the systems that are directly linked with the circulatory system?