Chapter 15 - Food and Digestion

Section 1 - Food 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. The six nutrients necessary for human health are carbohydrates, fats, proteins, vitamins, minerals, and water. 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.

  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. 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.

  Fats are energy-containing nutrients 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.



  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.

  Vitamins act as helper molecules in a variety of chemical reactions in the body. Minerals are nutrients that are not made by living things; they are obtained by eating plants 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.

  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.

  Food labels contain information that can help you to eat wisely. Food labels allow you to evaluate a single food as well as to compare the nutritional value of two different foods. A food label also includes the 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. 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.




Section 2 - The 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.

  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.

  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.

  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.

  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.

  Food leaves the esophagus and enters the stomach. The stomach is a J-shaped muscular pouch located in the abdomen. Most mechanical digestion and some chemical digestion occur in the stomach. Mechanical digestion occurs as three strong layers of muscle contract to produce a churning motion. Chemical digestion is carried out in the stomach by digestive juice that contains enzymes and hydrochloric acid. When the food has been digested into a liquid form, it moves into the next part of the digestive system.




Section 3 - Final 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.

  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.

  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.

  After chemical digestion takes place, the small nutrient molecules are 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.

  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.

  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.




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