2The levels of organization in the human body consist of cells, tissues, organs, and organ systems. A cell is the basic unit of structure and function in a living thing. Cells perform the basic processes that keep organisms alive. Most cells are too small to see without a microscope. In most animal cells, a cell membrane forms the outside boundary of the cell. Inside the cell membrane is a large structure called the nucleus. The nucleus is the control center that directs the cell’s activities and contains information that determines the cell’s characteristics. The area between the cell membrane and the nucleus is called the cytoplasm
3A tissue is a group of similar cells that perform the same function A tissue is a group of similar cells that perform the same function. The human body contains four basic types of tissue: muscle tissue, nerve tissue, connective tissue, and epithelial tissue. Muscle tissue can contract, or shorten. This tissue is what makes parts of your body move. Nervous tissue carries messages back and forth between the brain and every other part of the body. It directs and controls the body. Connective tissue provides support for your body and connects all its parts. Bone, fat, and blood ar e all connective tissues. Epithelial tissue covers the surfaces of your body. The skin and the lining of the digestive system are examples of epithelial tissue.
4An organ is a structure that is composed of different kinds of tissue An organ is a structure that is composed of different kinds of tissue. Like a tissue, an organ performs a specific job. An organ’s job is usually more complex than that of a tissue. The heart is an organ that pumps blood throughout the body.Each organ in your body is part of an organ system. An organ system is a group of organs that work together to perform a major function. Your heart is part of an organ system called the circulatory system. The blood vessels are also part of the circulatory system. The different organ systems work together and depend on one another.
5All the systems of the body work together to maintain homeostasis, the body’s tendency to keep an internal balance. Homeostasis is the process by which an organism’s internal environment is kept stable in spite of changes in the external environment. Sometimes, things can happen to throw off homeostasis. As a result, your heart may beat more rapidly or your breathing may quicken. These are signs of stress, the reaction of your body to potentially threatening, challenging, or disturbing events. When the stress is over, homeostasis is restored, and the body returns to its normal state.
6The Skeletal System Guide for Reading ■ What are the functions of the skeleton? ■ What role do joints play in the body? ■ What are the characteristics of bone, and how can you keep your bones strong and healthy?
7What the Skeletal System Does Your skeleton has five major functions. It provides shape and support, enables you to move, protects your organs, produces blood cells, and stores minerals and other materials until your body needs them.
8The Skeletal SystemThe skeleton is made up of all the bones in one’s body. Your skeleton has five major functions. It provides shape and support, enables you to move, protects your organs, produces blood cells, and stores minerals and other materials until your body needs them. The backbone, or vertebral column, is the center of the skeleton. The backbone is made up of 26 small bones, or vertebrae (singular vertebra). If your backbone were just one bone, you would not be able to bend or twist.
9Joints of the Skeleton - The Skeletal System A joint is a place in the body where two bones come together. Joints allow bones to move in different ways.
10The Skeletal SystemA joint is a place in the body where two bones come together. Joints allow bones to move in different ways. Immovable joints connect bones in a way that allows little or no movement. Movable joints allow the body to make a wide range of movements. The bones in movable joints are held together by a strong connective tissue called a ligament. Cartilage is a connective tissue that is more flexible than bone
11The Skeletal SystemBones are complex living structures that undergo growth and development. A thin, tough membrane covers all of a bone except the ends. Blood vessels and nerves enter and leave the bone through the membrane. Beneath the membrane is a layer of compact bone, which is hard and dense, but not solid.
12The Skeletal SystemSmall canals run through the compact bone, carrying blood vessels and nerves from the bone’s surface to the living cells within the bone. Just inside the compact bone is a layer of spongy bone, which has many small spaces within it. Spongy bone is also found at the ends of the bone. The spaces in bone contain a soft connective tissue called marrow. There are two types of marrow—red and yellow. Red bone marrow produces blood cells. Yellow marrow stores fat that serves as an energy reserve.
13The Skeletal SystemA combination of a balanced diet and regular exercise are important for a lifetime of healthy bones. As people become older, their bones begin to lose some minerals. Mineral loss can lead to osteoporosis, a condition in which the body’s bones become weak and break easily. Regular exercise and a diet rich in calcium can help prevent osteoporosis.
14Bones—Strong and Living - The Skeletal SystemBones—Strong and LivingBones are complex living structures that undergo growth and development.
15Asking Questions - The Skeletal System Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions.QuestionAnswerWhat does the skeleton do?The skeleton provides shape and support, helps you to move, protects organs, produces blood cells, and stores minerals and other materials.How do joints move?Joints can move forward or backward, in a circle, in a rotating motion, and in a gliding motion.How strong are bones?Bones can absorb more force without breaking than granite or concrete.What can I do to care for my bones?Eat a well balanced diet and getplenty of exercise.
16Diagnosing Bone and Joint Injuries Three common skeletal system injuries are fractures, dislocations, and sprains. A fracture, or a break in a bone, can occur when you fall so that all of your weight is placed on only a few bones. A dislocation occurs when a bone comes out of its joint. A sprain occurs when ligaments are stretched too far and tear in places. Sprains are the most common joint injuries.
17Diagnosing Bone and Joint Injuries Two ways to identify injuries of the skeletal system are X-rays and magnetic resonance imaging. X-ray images can determine whether bones have been broken. X-rays are a form of energy that travels in waves. Because most X-rays pass through the skin and other body tissues, the X-rays strike the photographic film beneath the area. Unlike other body tissues, bone absorbs X-rays. Absorbed X-rays do not reach the film. After the film is developed, it shows bones as clearly defined white areas. X-rays cannot be used to view injuries to soft tissues, such as muscle, and internal organs..
18Diagnosing Bone and Joint Injuries A more recently developed method for taking clear images of both the bones and soft tissues of the body is called magnetic resonance imaging, or MRI. In MRI, a person is exposed to short bursts of magnetic energy. This magnetic energy causes atoms within the body to vibrate, or resonate. A computer then analyzes the vibration patterns and produces an image of the area. MRI can show clear images of muscles and other soft tissues that anX-ray image cannot show.
19Diagnosing Bone and Joint Injuries If a doctor determines that you have a broken bone, you will usually have to wear a cast. In addition to wearing a cast, two other ways to treat skeletal system injuries include surgical procedures such as joint replacement and arthroscopy.
20The Muscular System Guide for Reading ■ What types of muscles are found in the body? ■ Why do skeletal muscles work in pairs?
21The Muscular System There are about 600 muscles in your body The Muscular System There are about 600 muscles in your body. The muscles that are not under your conscious control are called involuntary muscles. Involuntary muscles are responsible for activities such as breathing and digesting food. The muscles that are under your control are called voluntary muscles. Smiling and turning the pages in a book are actions of voluntary muscles.
22The Muscular SystemYour body has three types of muscle tissue—skeletal muscle, smooth muscle, and cardiac muscle. Some of these muscle tissues are involuntary, and some are voluntary. Skeletal muscles are attached to the bones of your skeleton. At the end of a skeletal muscle is a tendon. A tendon is a strong connective tissue that attaches muscle to bone. Because you have conscious control of skeletal muscles, they are classified as voluntary muscles. These muscles provide the force that moves your bones. Skeletal muscles react quickly and tire quickly. Skeletal muscle cells appear banded, or striated. For this reason, they are sometimes called striated muscles.
23The Muscular SystemSmooth muscles are called involuntary muscles because they work automatically. They are inside many internal organs of the body, and control many types of movements inside your body, such as those involved in the process of digestion. Smooth muscles react more slowly and tire more slowly than skeletal muscles. Cardiac muscles are involuntary muscles found only in the heart. Cardiac muscles do not get tired.
24The Muscular SystemMuscles work by contracting, or becoming shorter and thicker. Because muscle cells can only contract, not extend, skeletal muscles must work in pairs. While one muscle contracts, the other muscle in the pair relaxes to its original length. For example, in order to move the lower arm, the biceps muscle on the front of the upper arm contracts to bend the elbow. This lifts the forearm and hand. As the biceps contracts, the triceps on the back of the upper arm returns to its original length. To straighten the elbow, the triceps muscle contracts while the biceps returns to its original length.
25The Muscular SystemExercise is important for maintaining both muscular strength and flexibility. Exercise makes individual muscle cells grow wider, thicker, and stronger. Sometimes, muscle injuries such as strains and cramps, can occur. Resting the injured area can help it heal.
26The Skin Guide for Reading ■ What are the functions and the structures of skin? ■ What habits can help keep your skin healthy?
27The Skin The skin performs several major functions in the body The Skin The skin performs several major functions in the body. The skin covers and protects the body from injury, infection, and water loss. The skin also helps regulate body temperature, eliminate wastes, gather information about the environment, and produce vitamin D.
28The SkinThe skin is organized into two main layers, the epidermis and the dermis. The epidermis is the outermost layer of the skin. The epidermis does not have nerves or blood vessels. Cells in the epidermis have a definite life cycle. New cells form deep in the epidermis, gradually mature, and move upward. When these cells die, they become part of the surface layer of the epidermis. Soon, these cells are shed and replaced by the dead cells below them. Cells deep in the epidermis produce melanin, a pigment, or colored substance, that gives skin its color. The more melanin in your skin,the darker it is. Melanin production helps to protect the skin from burning.
29The SkinThe skin is organized into two main layers, the epidermis and the dermis. The epidermis is the outermost layer of the skin. The epidermis does not have nerves or blood vessels. Cells in the epidermis have a definite life cycle. New cells form deep in the epidermis, gradually mature, and move upward. When these cells die, they become part of the surface layer of the epidermis. Soon, these cells are shed and replaced by the dead cells below them. Cells deep in the epidermis produce melanin, a pigment, or colored substance, that gives skin its color. The more melanin in your skin,the darker it is. Melanin production helps to protect the skin from burning.
30The SkinThe dermis is the inner layer of the skin. It contains nerves, blood vessels, sweat glands, hairs, and oil glands. Sweat glands produce perspiration , which reaches the surface through openings called pores. Strands of hair grow within the dermis in structures called follicles. Oil produced in glands around the hair follicles waterproofs the hair. In addition, oil helps to keep the skin moist
31The SkinThree simple habits can help you keep your skin healthy. Eat a healthful diet. Keep your skin clean and dry. Limit your exposure to the sun.Eating a well-balanced diet and drinking plenty of water are important for healthy skin. Good washing habits can get rid of dirt and harmful bacteria, and can help control oiliness.
32The SkinRepeated exposure to sunlight can damage skin cells and cause them to become cancerous. Cancer is a disease in which some body cells divide uncontrollably. Wearing sunscreen and limiting sun exposure can protect skin from sun damage.
33Food and Energy Foods provide your body with materials for growing and for repairing tissues. Food also provides energy for everything you do.Your body breaks down the foods you eat into nutrients. Nutrients are the substances in food that provide the raw materials and energy the body needs to carry out all its essential processes. There are six kinds of nutrients necessary for human health carbohydrates, fats, proteins, vitamins, minerals, and water.
34Food and Energy The amount of energy released by nutrients can be measured in units called calories. One calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius
35Food and Energy Carbohydrates are nutrients composed of carbon, oxygen, and hydrogen. They are a major source of energy. In addition to providing energy, carbohydrates provide the raw materials to make parts of cells. Carbohydrates are either simple or complex. Simple carbohydrates are sugars. One sugar, glucose, is the major source of energy for your body’s cells. Complex carbohydrates are made up of many sugar molecules linked together in a chain. Starch is a complex carbohydrate found in some plant foods. Fiber is a complex carbohydrate found in plant foods. Fiber keeps the digestive system functioning properly.
36Food and Energy Like carbohydrates, fats are energy-containing nutrients that are composed of carbon, oxygen, and hydrogen. They contain twice as much energy as an equal amount of carbohydrates. In addition to providing energy, fats have other important functions. Fats form part of the cell membrane, the structure that forms the boundary of a cell. Fatty tissue protects and supports your internal organs and insulates your body.
37Food and Energy Unsaturated fats are usually liquid at room temperature. Saturated fats are usually solid at room temperature. trans fats are found in many commercially baked goods. Cholesterol is a waxy, fatlike substance found only in animal products. Large amounts of cholesterol, saturated fats, and trans fats can lead to heart disease.
38Food and Energy Proteins are nutrients that contain nitrogen as well as carbon, hydrogen, and oxygen. Proteins are needed for tissue growth and repair. They also play an important part in chemical reactions within cells. Proteins are made up of small units called amino acids
39Food and EnergyVitamins act as helper molecules in a variety of chemical reactions within the body. Minerals are nutrients that are not made by living things. You obtain minerals by eating plant foods or animals that have eaten plants. Both vitamins and minerals are needed by your body in small amounts to carry out chemical processes.Water is the most important nutrient because the body’s vital processes— including chemical reactions such as the breakdown of nutrients—take place in water. People cannot live without fresh water.
40Healthy Eating Guide for Reading ■ How can the Food Guide Pyramid help you plan a healthy diet? ■ What kind of information is included on food labels?
41Healthy Eating The Food Guide Pyramid was developed by nutritionists to help people plan a healthy diet. The Food Guide Pyramid classifies foods into six groups. It also indicates how many servings from each group should be eaten every day to maintain a healthy diet.
42Healthy Eating The pyramid has four levels Healthy Eating The pyramid has four levels. The base of the pyramid includes foods made from grains, such as bread, cereals, rice, and pasta. This bottom level is the widest part of the pyramid. This means that these foods should make up the largest part of the diet. The second level is made of two food groups, the Fruit group and the Vegetable group. People need fewer servings of these foods than of foods from the bottom level.
43Healthy Eating The third level contains the Milk, Yogurt, and Cheese group, and the Meat, Poultry, Fish, Dry Beans, Eggs, and Nuts group. People need still smaller amounts of food from this level. At the top of the pyramid are foods containing large amounts of fat, sugar, or both. Only small amounts of these foods should be eaten.
44Healthy EatingFood labels contain information that can help you to eat wisely. The serving size and the number of servings in the container are listed at the top of the label. Food labels allow you to evaluate a single food as well as to compare the nutritional value of two different foods.
45Healthy Eating A food label also has a column called Percent Daily Value. The Percent Daily Value indicates how the nutritional content of one serving fits into the diet of a person who consumes a total of 2,000 Calories a day. For example, suppose the label lists 12 percent for Total Carbohydrates
46Healthy EatingThis means that the food supplies 12 percent of the carbohydrates that an average person should eat each day. Food labels also list ingredients in order by weight, starting with the main ingredient. By reading ingredient lists, people can avoid foods that contain substances they should not eat.Dietary Reference Intakes (DRIs) are guidelines that show the amounts of nutrients needed every day. DRIs also show how the Calories that people eat each day should be split among carbohydrates, fats, and proteins.
47The Digestive Process Begins Guide for Reading ■ What functions are carried out in the digestive system? ■ What roles do the mouth, esophagus, and stomach play in digestion?
48The Digestive Process Begins The digestive system has three main functions. First, it breaks down food into molecules the body can use. Then, the molecules are absorbed into the blood and carried throughout the body. Finally, wastes are eliminated from the body.
49The Digestive Process Begins The process by which the body breaks down food into small nutrient molecules is called digestion. There are two kinds of digestion—mechanical and chemical. In mechanical digestion, foods are physically broken down into smaller pieces. In chemical digestion, chemicals produced by the body break foods into their smaller chemical building blocks.
50The Digestive Process Begins After food is digested, the molecules are ready to be transported throughout your body. Absorption is the process by which nutrient molecules pass through the wall of your digestive system into your blood. Materials that are not absorbed are eliminated as wastes.
51The Digestive Process Begins Both mechanical digestion and chemical digestion begin in the mouth. The fluid released when your mouth waters is called saliva. Saliva plays an important role in both mechanical and chemical digestion. Your teeth carry out the first stage of mechanical digestion. As the teeth break foods into smaller pieces, saliva mixes with the pieces of food and moistens them. Chemical digestion is accomplished by enzymes. An enzyme is a protein that speeds up chemical reactions in the body.
52The Digestive Process Begins As you swallow, muscles in your throat move the food downward. As this happens, a flap of tissue called the epiglottis seals off your windpipe, preventing the food from entering. Food moves into the esophagus, a muscular tube that connects the mouth to the stomach. The esophagus is lined with mucus. Mucus is a thick, slippery substance produced by the body. Mucus makes food easier to swallow. Food remains in the esophagus for only about 10 seconds. After food enters the esophagus, contractions of smooth muscles push the food toward the stomach. These involuntary waves of muscle contractions are called peristalsis.
53Final Digestion and Absorption Guide for Reading ■ What digestive processes occur in the small intestine, and how are other digestive organs involved? ■ What role does the large intestine play in digestion?
54Final Digestion and Absorption Once the food has been changed into a thick liquid, the stomach releases a little liquid at a time into the small intestine for further digestion. The small intestine is the part of the digestive system where most chemical digestion takes place. Almost all chemical digestion and absorption of nutrients takes place in the small intestine. Enzymes and secretions produced by the small intestine, the liver, and the pancreas finish the chemical digestion of food.
55Final Digestion and Absorption The liver, which is located in the upper right portion of the abdomen, is the largest organ inside the body. The role of the liver in the digestive system is to produce bile. Bile is a substance that breaks up fat particles. Bile flows from the liver into the gallbladder, the organ that stores bile. After you eat, bile passes through a tube from the gallbladder into the small intestine. The bile mixes with fats in food and breaks them into small droplets. These small droplets can then be broken down chemically by enzymes produced in the pancreas.
56Final Digestion and Absorption The pancreas is a triangular organ that lies between the stomach and the first part of the small intestine. As part of the digestive system, the pancreas produces enzymes that flow into the small intestine and help break down starches, proteins, and fats.
57Final Digestion and Absorption After chemical digestion takes place, the small nutrient molecules ar e ready to be absorbed by the body. The lining of the small intestine is covered with millions of tiny finger-shaped structures called villi (singular villus). The villi absorb nutrient molecules. Nutrient molecules pass from cells on the surface of a villus into blood vessels. The blood carries the nutrients to all of the body’s cells. The cells use them for many functions
58Final Digestion and Absorption After chemical digestion takes place, the small nutrient molecules ar e ready to be absorbed by the body. The lining of the small intestine is covered with millions of tiny finger-shaped structures called villi (singular villus). The villi absorb nutrient molecules. Nutrient molecules pass from cells on the surface of a villus into blood vessels. The blood carries the nutrients to all of the body’s cells. The cells use them for many functions
59Final Digestion and Absorption The large intestine is the last section of the digestive system. It contains helpful bacteria that feed on the material passing through. By the time material reaches the large intestine, most of the nutrients have been absorbed. The material entering the large intestine contains water and undigested food. As the material moves through the large intestine, water is absorbed into the bloodstream. The remaining material is readied for elimination from the body
60Final Digestion and Absorption The large intestine ends in a short tube called the rectum. Here, waste is compressed into a solid form. This waste is removed from the body through the anus, a muscular opening at the end of the rectum.