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?

The Gastrointestinal System

Parts of the system. The gastrointestinal system might be thought of as a long passage through which food passes. It has glands attached to it that pour chemicals into the passage. These chemicals digest the food as it passes along the passageway. The purpose of this system is to convert the food you eat into substances your cells can use.

The mouth, stomach, intestines, and other parts of the gastrointestinal system digest food by both chemical and muscular action. The glands in the system secrete fluids that break down the foods into usable substances. The muscles in the mouth, esophagus, stomach, and intestines break the food into small particles and keep the food moving through the various stages of chemical digestion.

You will be able to understand all these processes when you read how foods pass through the digestive system. Every food is acted upon by one or more chemicals in the system.

Digestion in the mouth. The first step in digesting food takes place in the mouth, where you chew food. By chewing food, you break it into bits small enough for the digestive juices to work on. The saliva in the mouth is the first digestive juice. It breaks down some of the carbohydrates, or starches, and changes them to a simple sugar.

After food is chewed and mixed with saliva, it is forced down the esophagus by muscular activity called peristalsis. Peristalsis is the progressive contracting of ringlike muscles in the esophagus that forces the food down the canal into the stomach.

Digestion in the stomach. When the food gets to the stomach, it is mixed with hydrochloric acid and agitated by muscular action. The acid makes it possible for several special chemicals, called enzymes, to act on the food. These chemicals break down the protein, curdle milk, and convert fat into a substance the body can use.

In the intestine. When the liquid food passes into the small intestine, it is acted upon by still more enzymes. Actually, most of the digestion takes place in the intestine. In addition to the enzymes secreted by the walls of the small intestine, other secretions act on the liquified food. The food moves along the intestine by peristaltic action, and by the time it reaches the large intestine, or colon, the food value has been taken out of it.

Food enters the blood. As you have learned earlier, the food must enter the blood stream before it can feed the cells of the body. This is accomplished by tiny projections called villi that stick out into the intestines. The liquified foods pass through the thin walls of the villi and into the capillaries and lymph vessels, which are crowded into each projection.

The walls of the intestine are so lined with villi that they resemble the surface of a turkish towel. Once the digested protein and carbohydrates are passed into the blood and lymph, they are transported directly to the cells. These substances feed the cells and keep the body alive.

The Human Body

The study of the human body. Many years ago people explained some processes in the human body by saying that a "vital spirit" within the body controlled some body functions. They used this term to explain processes that they could not understand. They believed that this vital spirit could not be explained by the physical laws of science.

Today, scientists know that the human body functions according to the laws of nature. There is no mysterious vital spirit within the body that causes the heart to beat or the brain to operate. Scientists now explain the working of the body by discovering the physical principles behind its operation.

A physiologist is a modem scientist who studies the body and the ways Its parts function. He seeks scientific explanations for the ways the parts of the body operate. He is interested in the ways the parts of the human body work together to help a human to live.

The body is the most complicated organism known. Its many parts interact in so many ways that physiologists must simplify the organisation for purposes of study. In order to understand the functions of the major organs and tissues of the body, physiologists usually divide them into six major systems. These are the skeletal-muscular system, which controls the basic shape and movement of the body; the gastrointestinal system, which controls the progress of foods within the body; the respiratory system, which controls the oxidation of food in the body; the circulatory system, which is the blood circulating mechanism; the nervous system, which is the voluntary and involuntary control center for the body; and the endocrine system, which is a chemical control system for the other systems.

These systems are linked in many ways. For example, the blood in the circulatory system carries the oxygen from the respiratory system to the muscles in the skeletal-muscular system.

The organs. Each of these six systems is composed of organs, such as the heart, liver, stomach, and brain. Most of the important organs of the body are included in one of the six systems. Each of the organs carries out a special function, but it is so related to many other organs in the body that it often depends on them to help in the process.

Thus, the lungs cannot act without the brain, because the brain sends the impulses which regulate breathing. The stomach is worth nothing without the blood to carry the food it digests to the tissues. Virtually every organ depends on other organs to help it carry out its function.

Tissues and cells. The tissues that make up the organs are likewise dependent on other tissues and organs in the body. Tissues are made of cells, the smallest bits of living matter in a human body. The cells need nourishment and a way to cast off their wastes. This nourishment is provided by various organs of the body.

The tissues of the body, like the organs and the systems, are specialized. Each of the four kinds of tissue looks different from the others, and each makes up different organs. For instance, the epithelial cells in your skin are similar to the epithelial cells in your lungs. The muscle cells in your arm are similar to the muscle cells in your stomach. Look at the illustration to see the four shapes of tissue cells. These cells make up the tissues of the body.

When you read about each of the six systems of the body, you should keep one point in mind. The explanations of the six systems are simplifications. The systems are so complicated and so interrelated that they cannot be described in a few pages. In fact, doctors are still searching for functions of many parts of the six systems of the body.

Think for Yourself

What systems have structures that are dependent on the skin? You may find that there is more than one system.