Body's Systems

THE BLUEPRINT OF LIFE

Problem. What controls heredity ? What makes you grow up to look like you and not someone else? The answer to this question is the key to life.

Background. For centuries men have wondered why men look like men. Some people thought that the blood of a mother and father mixed and the characteristics carried in the blood of the parents were passed on to the children.This theory was proven false in 1902, when Walter Sutton discovered the chromosomes. The chromosomes are tiny threadlike structures in the nucleus of a cell that contain the heredity of the cell.

Each human cell has forty-six chromosomes, which are split into forty-six identical pairs whenever the cells divide. Each of the daughter cells receives one-half of the chromosomes. But scientists did not know why or how the chromosomes split into two identical groups.

Explanation. The key discovery in the study of heredity was made by molecular biologists who discovered the chemical make-up of the chromosomes. They found that the chromosomes are made of a certain nucleic acid, called deoxyribonucleic acid. Biologists usually refer to this by its abbreviation, DNA.

In 1953 Doctors Francis Crick and James Watson perfected a three-dimensional model that explained all the characteristics of DNA. The model showed that the molecule was shaped like a spiral staircase. The sides of the stair treads were made of two chemicals and the "treads" were made of four other chemicals.

Many experiments proved that it is the arrangement of these four kinds of "treads" that determines a person’s characteristics.

The cells of the body contain about three feet of DNA, if the molecules are laid end-to-end. There are three or four encyclopedias of information in these DNA molecules. They use another nucleic acid, RNA (ribonucleic acid), to translate this information into the proteins that make up new tissue.

Further research. For an excellent discussion of DNA, read The Cell, one of the books in the Life Science Library.

Activity for Health and Fitness

Exercise for Fitness

Tortoise and Hare.
You can practice the tortoise and hare by yourself by running in place and alternating between running fast and running slow. You can vary the activity by running on your toes part of the time.

Team Activity

Long Ball. This game is sometimes called long base. It combines many of the characteristics of the American game, baseball, and the English game, cricket. It is played by two teams of from three to ten players apiece.

You can begin this game by marking out a playing field with a home plate, a pitcher’s box, and a long base. Follow the dimensions given in the diagram. Be sure to mark the foul lines. The long base may be a gymnasium mat or a marked out space. The ball is a 12-inch Softball and the bats are softball bats.

The pitcher pitches underhand in the normal way. The batter runs when he hits the ball, whether or not it is foul. If it is foul, he waits at the long base until a teammate hits a fair ball. No batter can strike out.

More than one runner can remain on the long base at one time. No runner can return to long base once he has started running unless a fly ball is caught. In order to score a run, a player must return to home base and tag it. Any player may wait at long base as long as he likes. Any or all of the players at long base may run to home base when a fair ball is hit.

A side remains at bat until three outs are made. The game lasts for seven innings. Outs can be made by a player by holding the ball and tagging long base before the batter reaches it, by tagging a runner while holding the ball even if he is just "off base," or by catching a fly ball or foul tip. An out is also made when all players are held at long base because no one on the team has hit a fair ball.

The fielders may cover the entire playing area. They can run with the base runners in hopes of receiving a fielded ball and tagging the base runner.

All disputes should be settled by the umpires. One umpire from each team serves while his team is batting. It is a good idea to elect the umpires for the game before the start of the game.

Technorati Profile

The Endocrine System - Part 2

The pancreas. Another of the essential organs is the pancreas. Within it are the islets of Langerhans, groups of cells that secrete insulin, a hormone essential to the conversion of sugar to energy. The pancreas also controls the way fats are stored in the body.

The disease that results from a deficiency of insulin is called diabetes. This disease was fatal until Dr. Frederick Banting and four other Canadian  doctors  discovered  that secretion of the islets in the pancreas would stop the symptoms of diabetes They found that insulin controlled the way the body uses sugar. By injectiing insulin into patients, the disease could be arrested.

The thyroid gland. The thyroid is another part of the endocrine system The thyroid is a gland which straddles the trachea. The hormone thyroxin produced in this small gland, influences the body’s ability to use foods; consequently, it controls body weight, growth, development of intelligence, and certain other processes.

One of the important factors in the functioning of the thyroid is iodine. If sufficient iodine is not present in the diet, the thyroid enlarges and forms an ugly swelling in the neck, called a goiter. A goiter condition can be prevented by eating sufficient iodine in the form of seafood, iodized salt, or certain plants. People living in regions where iodine is especially common in foods do not need iodized salt.

The parathyroids. Buried in the thyroid gland are four parathyroid glands. These glands are tiny spots on the thyroid gland and are vital life. They secrete a hormone that controls the level of calcium in the blood This calcium controls muscle bility. A muscle deprived of calcium will twitch uncontrollably. Too much parathyroid hormone, on the other hand, will cause the bones to give up calcium to the blood, a condition equally undesirable.

The other endocrines. There are other endocrine glands of varying degrees of importance. These glands also work through the blood stream and control various body functions.

Balance in the body. The chemical controls in the body are just part of the interlocking systems that normally function so well together. Doctors long ago devised the concept of homeostasis, or automatic balance, to explain this balance. The body is such a complex series of dependent parts that damage
to one part will affect dozens of others. The body’s homeostasis automatically corrects for many of these imbalances, and brings the body parts into harmony again. In sickness, homeostasis may cure you without help from a doctor. It adjusts your functions to compensate for diet, temperature change and all manner of other upsetting factors in your environment. Without such a capacity for maintaining balance, you would not have a chance to lead a healthy, comfortable life.

The Endocrine System - Part 1

The control centers. The endocrine glands secrete chemicals which control the growth and development of the body. These chemicals are secreted into the blood stream and are carried to all parts of the body. Because they are in general circulation, these chemicals, called hormones, can act in areas far from the gland that secreted them. They may even act on several organs at once.

The endocrine glands have a certain similarity to the nervous system because they help to integrate the activities of widely separated organs. Emotional strain, for example, causes one of the endocrines to enlarge and secrete a chemical which speeds up the action of the heart.

The master gland. In the base of the skull is a tiny gland called the pituitary, or hypophysis. This gland is called the master gland because it controls growth and the action of the adrenal, thyroid, and male and female glands. Through these glands, it has an enormous effect on the other systems of the body.

The pituitary gland secretes at least eight hormones in order to exercise its control over body functions. Each hormone controls a particular body function. Examine the chart below to see what functions these eight hormones control.

The adrenals. The adrenal glands are two of the glands that are regulated by the pituitary. The adrenals are two glands located above the kidneys. Each adrenal is composed of two parts: an inner part, called the medulla, and an outer layer, called the cortex.

The hormone secreted by the medulla, called epinephrine, is probably the best known hormone in the body. Commonly called adrenalin, this hormone stimulates most of the emergency reactions within the body. It increases the heart beat, releases sugar from the liver into the blood stream, delays fatigue in the skeletal muscles, raises the blood pressure, and performs many other functions.

In view of these effects you would think that epinephrine is essential to the function of the body. Such is not the case, however. Parts of the autonomic nervous system can control these functions even when the adrenal medulla is removed from the body! It is likely that the medulla serves only to help the autonomic nervous system in times of danger, but that it is not necessary for the body’s functioning.

The adrenal cortex. The adrenal cortex, on the other hand, secretes substances which’ are vital to the body. A person deprived of his adrenal cortex would die within a few days. It is the secretion of the hormone from the adrenal cortex that is controlled by ACTH, a hormone secreted by the pituitary. Scientists are not sure why the cortex is essential, although they have found that cortisone is very useful in alleviating some symptoms of disease.

The Respiratory System

Respiration. You have learned that cell respiration takes place when the cells take in oxygen and give off carbon dioxide. This process maintains the life of the cell by providing oxygen for the burning of food in the cell.

Body respiration, or breathing, takes place in the lungs. When you breathe, you draw air into your lungs by moving a large sheet of tissue called the diaphragm, and by the action of the rib muscles. Look at the illustration to see how the diaphragm causes the lungs to expand and contract.

When air is drawn into your lungs, it passes through several parts of the respiratory system. It is first drawn through the nose and mouth. There it is cooled or warmed by the moist tissues that line the passages. Foreign bodies are filtered out by tiny hairs in the nose.

The air passes through the nasal passages and the throat and is drawn into the trachea, or wind-pipe, and the bronchial tubes. These tubes lead to air sacs called alveoli. In the alveoli, the transfer of gases into the blood takes place.

Oxygen in blood. The alveoli are surrounded by a dense concentration of capillaries that carry the blood from the veins. This blood has a high concentration of carbon dioxide and a low concentration of oxygen. Because the walls between the capillaries and the air sacs are so thin, both carbon dioxide and oxygen can pass through them.

Carbon dioxide passes out of the blood, and oxygen passes into it. The oxygen is picked up by a compound called hemoglobin. Hemoglobin gives the blood its red color.

After the blood obtains oxygen in the alveoli, it is pumped to the left side of the heart, then through the arteries to the capillaries, and then to the cells of the body, where cell respiration takes place, and the process begins again.

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 Nervous System

Control center. Your nervous system is like a general’s headquarters in the army. It has an intelligence division to find out information, a messenger division to deliver orders to the proper units, and a general headquarters, where all but the least important decisions are made.

The general headquarters of the brain is composed of the cerebrum and cerebral cortex. These two parts of the brain control your thinking and voluntary actions.  These structures and their associated nerves are called the central nervous system. This system controls your conscious actions. These parts are in control when you lift an ice cream cone to your mouth or when you think about the answer to a math problem.

The cerebellum and cerebrum are connected to two kinds of nerves. The first, afferent nerves, pick up information from the sense organs. The second, efferent nerves, relay impulses from the conscious center to the muscles. These nerve pathways enable you to sense your surroundings and carry out your activities.

Other parts of the brain control involuntary actions such as maintaining balance, focusing the eyes, and regulating internal organs. Look at the illustration to see what areas the parts of the brain control.

The spinal cord. Within the thirty-three bones of the spine is the spinal cord. This cord is the largest central nerve trunk of the body. The nerves begin at the base of the brain and branch off the spinal cord. They connect various internal organs with the brain.

Your spinal cord also takes part in body reactions. Whenever a quick but automatic reaction is required, the reaction is made in the spinal cord. It is the spinal cord, for instance, that directs your muscles to jerk your hand away from a flame or makes you lift your foot hurriedly if you step on a tack.

Your nerves are the spies and messengers of the body. They sense the temperatures, pressures, sights, sounds, and smells around you and send tiny electrical impulses to the brain or spinal cord to be interpreted. When the control center interprets the messages, it sends back an answer along other nerves to the muscles, which carry out the body’s activities.

Regulation of the body. Some nerves and parts of the brain are part of the autonomic nervous system. This system controls the internal organs. It regulates your breathing, digestion, rate of perspiration, and even the way in which you digest your food.

The autonomic system works in the same way that the central nervous system works. There are afferent nerves that are stimulated by heat, pressure, or pain; control centers in the spinal column and the base of the brain; and efferent nerves to send the messages to muscles that can carry out the necessary action.

Nerve cells. All nerve cells, whether they are in the nerves, the spinal cord, or the brain, are similar in structure. Nerve cells have a nucleus and cell body like all cells, and they have two special structures that allow them to send and receive electrical impulses. These are the dendrites and axons.

The dendrites receive impulses from a sensory organ and pass them through the cell body to the axon, which passes them to the dendrite of the next cell in the chain. Thus, a nerve fiber is like a long bucket brigade in which an impulse is transmitted from a sensory organ to a control center. However, the "bucket" travels at 300 feet per second, much faster than a bucket has ever traveled in any human bucket brigade.

These chains of nerve cells penetrate every part of the body. The nerves that make up the central nervous system, or conscious control centers, constitute the five senses and are connected to the skeletal muscles that you can control. The nerve chains that are part of the autonomic nervous system are concentrated in the organs inside the body. You seldom feel the effects of these nerves, but their activities are essential to the operation of your organs and systems.

Have you ever had perspiring palms when you were about to take a very important examination? This condition is evidence that there is a link between the sweat glands in your hand and the higher conscious control centers of your brain. Other evidence of the great complexity of the nervous system is seen in the many ways the rest of the organs react to emotional stress.

Think for Yourself
What internal organs are controlled by the autonomic nervous system? Which by the central nervous system?

The Skeletal System

The growth of bones. The system that is growing fastest at your age is probably your skeletal system. During the time between your tenth and eighteenth birthdays, some of your bones may actually double in length. Not only do they grow, but they complete the hardening process that occurs from the time you are born until you are twenty or twenty-five.

When you are born, your bones are mostly cartilage, or gristle. As you grow, calcium, phosphates and other minerals are deposited in the cartilage, and the bones become hard. However, there is always a cartilage covering of the ends of the bones, particularly where the bones end in movable joints.

Function of bones. Bones are not dead sticks that hold you upright and help to keep your internal organs in their place. They are living organs. All bones are laced with blood vessels, and the centers are filled with fatty tissue and red marrow. Besides serving as the support for the body, the bones are the body’s red-blood-cell factories. The red marrow produces millions of red blood cells per second. These cells are carried into the blood stream by the system of blood vessels, which penetrate the bones.

Holding the skeleton together. Another part of your skeletal system is the tissue that holds one bone to another. This tissue forms connections between bones that are called ligaments. If this tissue were to disappear, you would literally fall apart. The ligaments that hold bones together may hold them firmly, as in the spine, or flexibly as in the elbow joint. The ligaments also serve as cushions between bones, and as well-oiled ball bearings between bones in movable joints.

Muscles. Although the ligaments hold the joints together, movement ofthe arms and legs is entirely dependent upon muscles. Each movement generally involves two muscles. One muscle relaxes while another tightens. In the illustration you can see two of the muscles that relax and tighten whenever you raise and lower your forearm. The biceps contracts, and the triceps relaxes when you lift your forearm. When you lower it, the opposite occurs.

Many muscles increase in size as they are used. They become smaller if they are not often used. For this reason, a program of vigorous exercise will promote the tone and size of your skeletal muscles. Regular vigorous exercise also improves your circulation and respiration.

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.