Key Concepts:
Vocabulary: mastication, peristalsis, gastric juices, bile, peptic ulcer, appendicitis
The Digestive System
What Happens During Digestion
Main Idea: In digestion, foods are broken down and absorbed as nourishment or eliminated as waste.
The foods you eat provide nourishment. That food and drink, however, must be broken down into smaller nutrients to be absorbed into the blood and carried to the body’s cells. The digestive system functions can be divided into three main processes:
How Digestion Works
Main Idea: The digestive system consists of the mouth, esophagus, stomach, and intestines.
Digestion includes two processes. The mechanical process involves chewing, mashing, and breaking food down. The chemical process involves secretions produced by digestive organs. Figure 15.7 shows the organs involved in digestion.
Teeth. The teeth break the food you eat into smaller pieces. Mastication (mas-tih-KAY-shun) is the process ofchewing, which prepares food to be swallowed.
Salivary glands. These glands produce digestive juices. Saliva contains an enzyme that begins to break down the starches and sugars in food into smaller particles.
Tongue. The tongue prepares chewed food for swallowing by shaping it. The uvula, a small flap of tissue at the back of the mouth, prevents food from entering the nasal passages. The epiglottis, tissue covering the throat, prevents food from entering the respiratory system.
The Esophagus
When food is swallowed, it enters the esophagus, the muscular tube about ten inches long that connects to the pharynx: with the stomach. Food is moved through the esophagus, stomach, and intestine through peristalsis (pare-ih-STAWL- suhs), a series ofinvoluntary muscle contractions that moves food through the digestive tract. The action of peristalsis begins as soon as food is swallowed. A sphincter muscle-a circular muscle at the entrance to the stomach-allows food to move from the esophagus into the stomach.
The Stomach
The stomach is a hollow, sac-like organ enclosed in a wall of muscles. These muscles are flexible and allow the stomach to expand when you eat. The stomach, shown in Figure 15.9, has three tasks:
The Pancreas, Liver, and Gallbladder
In the small intestine, the juices of two other digestive organs mix with the food to continue the process ofdigestion. The pancreas produces enzymes that break down the carbohydrates, fats, and proteins in food. Glands in the wall of the intestine produce other enzymes that help this process.
The liver produces another digestive juice-bile, a yellow· green, bitter fluid important in the breakdown and absorptionof fats. Bile is stored in the gallbladder between meals. At mealtimes, it is secreted from the gallbladder into the bile duct to reach the intestine and mix with fats in food. Bile acids dissolve the fats into the watery contents of the intes- tine. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.
The Small and Large Intestines
The small intestine is 20 to 23 feet in length and 1 inch in diameter. It consists of three parts: the duodenum, the jejunum, and the ileum. As chyme enters the duodenum, it contains partially digested carbohydrates and proteins and undigested fats. This mixture is further dissolved by digestive juices secreted from the small intestine, liver, and pancreas. About 90 percent ofall nutrients are absorbed through the small intestine. The inner wall ofthe small intestine contains millions of fingerlike projections called villi. The villi are lined with capillaries that absorb the nutrients. Unabsorbed material leaves the small intestine in the form of liquid and fiber and moves by peristalsis into the large intestine. The undigested parts of the food-fiber, or roughage-pass into the colon, or large intestine. The large intestine is about 2.5 inches in diameter and 5 to 6 feet in length. Its function is to absorb water, vitamins, and salts, and to eliminate waste.
Digestive System Problems
Main Idea: Digestive problems range from indigestion to acute conditions that require immediate medical attention.
Taking care ofyour digestive system begins with the foods you eat and how you eat them. To maintain your digestive health, eat a variety of low-fat, high-fiber foods, wash your hands before preparing or eating meals, eat slowly and chewyour food thoroughly, drink at least eight 8-ounce glasses of water a day, and avoid using food as a way of dealing withyour emotions. Some digestive system problems may require medications and a visit to a health professional.
Functional Problems
The functioning of the digestive system may be affected by illness, stress, or eating a particular food. Functional problems of the digestive system include the following:
Structural Problems
The seriousness of structural problems of the digestive system can vary. Some problems are temporary or easily treated, others are serious and require immediate medical attention.
-Tooth decay may make it difficult to chew foods thor- oughly. Brushing and flossing teeth daily can prevent tooth decay, along with regular dental checkups.
-Gastritis is an inflammation of the mucous membrane that lines the stomach. An increase in the production of stomach acid, use of tobacco or alcohol, bacterial or viral infections, and some medications can cause gastritis. Symptoms include pain, indigestion, decreased appetite, and nausea and vomiting.
-A peptic ulcer is a sore in the lining of the digestive tract. Peptic ulcers can be caused by a bacterial infection or the overuse of aspirin. Common symptoms include abdominal pain that worsens when the stomach is empty, nausea, and vomiting. Ulcers can cause stomach bleeding.
-Gallstones form when cholesterol in bile crystallizes. Gallstones can block the bile duct between the gallbladder and the small intestine. Symptoms of a blockage include pain in the upper right portion of the abdomen, nausea, vomiting, and fever.
-Lactose intolerance results from an inability to digest lactose, a type of sugar found in milk and other dairy products. Lactose is normally broken down by the enzyme lactase. People who are lactose intolerant do not produce enough lactase. Symptoms include abdominal cramps, bloating, gas, and diarrhea. Soy products are a good replacement for milk or dairy products.
-Appendicitis is inflammation of the appendix, the 3- to 4-inch tube at the tip of the large intestine. It can be caused by a blockage or bacterial infection. Symptoms include pain in the lower right abdomen and a fever. Decreased appetite, nausea, and vomiting will also occur. The appendix may burst, spreading infection throughout the abdomen, which can lead to death.
-Colitis is the inflammation of the large intestine, or colon. It may be caused by bacterial or viral infections. Symp- toms can include fever, abdominal pain, and diarrhea that may contain blood.
-Colon cancer is the second leading cause of cancer death in the United States. It usually develops in the lowest part of the colon, near the rectum. A low-fat, high-fiber eating plan decreases the risk of colon cancer. Any rectal bleeding should be checked by a medical professional.
-Hemorrhoids are veins in the rectum and anus that may become swollen and inflamed. Hemorrhoids may occur with constipation, during pregnancy, and after childbirth. Signs of hemorrhoids include itching, pain, and bleeding.
-Crohn’s disease causes inflammation of the lining of the digestive tract. Symptoms include diarrhea, weight loss, fever, and abdominal pain. The cause is not known, but seems to be associated with immune system problems.
-Cirrhosis, or scarring of the liver tissue, is caused by prolonged heavy alcohol use. Cirrhosis can lead to liver failure and may cause death.
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