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Name____________________________ Class __________________ Date __________ Chapter 38 # Title Digestive and Excretory Systems Summary 38–1 Food and Nutrition One calorie is equal to the amount of energy needed to raise the temperature of one gram of water by one degree Celcius. The energy in food is measured in dietary Calories (with a capital C). One Calorie is equal to 1000 calories. The number of Calories you need each day depends on your age, sex, and activity level. Nutrients are substances in food that supply the body with energy and raw materials needed for growth, repair, and maintenance. The nutrients that the body needs are water, carbohydrates, fats, proteins, vitamins, and minerals. Every cell in the human body needs water because many of the body’s processes take place in water. Water makes up a large part of blood and other body fluids. Sweating removes water in order to cool the body by evaporation. Water must be replaced regularly. Simple and complex carbohydrates are the body’s main source of energy. Simple carbohydrates do not have to be digested or broken down. They provide quick energy for the body. Complex carbohydrates must be broken down into simple sugars to be used for energy. Fats are formed from fatty acids and glycerol. Saturated fats are usually solid at room temperature. Unsaturated fats are usually liquid at room temperature. Fats protect organs and joints. They help make up cell membranes, and insulate the body. Proteins supply raw materials for growth and repair of the body. Amino acids form proteins. The human body can produce twelve of the twenty amino acids. The other eight must be obtained from food. These amino acids are called essential amino acids. Vitamins are organic molecules that help regulate body processes. Fat-soluble vitamins can be stored in fatty tissues. Water-soluble vitamins cannot be stored and should be in the foods that a person eats every day. Some diseases result when the body does not receive a sufficient supply of vitamins. Vitamins can have serious effects on a person’s health. Minerals are inorganic nutrients. Your body usually needs minerals in small amounts. Examples of minerals include calcium and iron. The body loses minerals in sweat, urine, and other waste products, so they must be replaced by eating foods. The Food Guide Pyramid can help you choose a balanced diet. Foods at the base of the pyramid should make up most of your diet. Foods at the top of the pyramid should be used in small amounts. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 375 Name____________________________ Class __________________ Date __________ 38–2 The Process of Digestion The digestive system breaks down food into simpler molecules that can be absorbed and used by cells. The human digestive system is a one-way tube. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Other structures—salivary glands, pancreas, and liver—add secretions to the digestive system. 1. Digestion starts in the mouth. Teeth tear and crush food to begin mechanical digestion. Mechanical digestion is the physical breakdown of large chunks of food into smaller pieces. Salivary glands in the mouth secrete saliva, which contains enzymes that break down starches into sugars. This begins the process of chemical digestion. Chemical digestion breaks down large food molecules into smaller food molecules. 2. The swallowed clump of food passes through the pharynx and into the esophagus. A flap of skin, the epiglottis, keeps the food from entering the trachea. Muscle contractions, called peristalsis, squeeze food through the esophagus to the stomach. 3. Chemical and mechanical digestion take place in the stomach. Glands in the stomach lining make hydrochloric acid and the enzyme pepsin. The hydrochloric acid and pepsin start the chemical digestion of protein. Stomach muscles contract to churn and mix the stomach contents. This mechanical digestion forms a liquid mixture. 4. Most chemical digestion and absorption of food occurs in the small intestine. Enzymes from the pancreas help digest starch, protein, and fat. A liquid called bile from the liver dissolves and breaks up fat droplets. Several enzymes help break down carbohydrates and proteins. Tiny fingerlike projections called villi (singular: villus) increase the surface area of the small intestine. Cells at the small intestine’s surface absorb nutrients. 5. The large intestine removes water from the undigested material. The remaining waste passes out of the body. Peptic ulcers, diarrhea, and constipation are digestive system disorders. Bacteria cause most peptic ulcers. Diarrhea occurs when too little water is removed from waste in the large intestine. Constipation occurs when too much water is removed. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 376 Name____________________________ Class __________________ Date __________ 38–3 The Excretory System During metabolism, cells make wastes such as carbon dioxide and urea. Excretion is the process in which the body eliminates wastes. The main organs of excretion are the kidneys. The kidneys play an important role in maintaining homeostasis. They remove waste products from blood, maintain blood pH, and control the water content of blood. Two kidneys are located in the lower back. Blood containing wastes enters the kidneys. The kidneys remove urea, excess water, and other substances from the blood. This cleaned blood returns to circulation. The wastes are excreted. The wastes are removed and passed to the ureter. The basic unit of function in a kidney is the nephron. Each nephron is a small self-sufficient processing unit. As blood enters a nephron, impurities are filtered out and emptied into the collecting duct. The purified blood exits the nephron through the venule. The processes of filtration and reabsorption take place in the nephrons. • Filtration removes wastes from the blood. It occurs in a part of the nephron called the glomerulus. The glomerulus is enclosed within a structure called Bowman’s capsule. • Reabsorption returns some of the filtered materials back to the blood. These materials include nutrients and water. Fluid that remains in the kidneys is called urine. Urine contains urea, excess salts, and other substances. After some water is removed the urine leaves each kidney through a tube called the ureter. The ureters carry urine to the urinary bladder, where urine is stored. Urine leaves the body through a tube called the urethra. The kidney’s activity is controlled by hormones and by the composition of blood. Drinking excess water increases the amount of water in the blood. This causes the kidneys to decrease the amount of water they reabsorb and return to the blood. Similarly, salty foods result in excess salt in the blood. To keep the composition of blood the same, the kidneys excrete the excess salt in urine. A person can live with only one kidney. If both kidneys fail, the person must receive a kidney transplant or undergo dialysis. Dialysis purifies the blood by passing it through a filtering machine. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 377