14.1 The Digestive Tract
The digestive system contributes to homeostasis by ingesting food, separating it into chemical nutrients that cells can use, absorbing those nutrients, and eliminating indigestible remains. Digestion takes place within a tube called the digestive tract, which begins with the mouth and ends with the anus. Digestion involves mechanical and chemical digestion.
The mouth ingests food and contains the sensory receptors, taste buds, that make people enjoy eating food. Salivary glands produce saliva, which contains an enzyme that starts digesting starch; this is the beginning of chemical digestion.
The teeth begin the process of mechanical digestion.
The pharynx is a region that receives air from the nasal cavities and food from the mouth. Swallowing occurs in the pharynx, at which point the epiglottis covers the opening to the larynx.
The esophagus is a muscular tube that passes from the pharynx to the stomach. Rhythmic muscular contractions called peristalsis push the food along the digestive tract.
The Wall of the Digestive Tract
The structure of the esophageal wall in the abdominal cavity is representative of that found in the stomach, small intestine, and large intestine; all of which are composed of four layers: mucosa, submucosa, muscularis, and serosa.
The stomach receives food from the esophagus, starts the digestion of proteins, and moves food into the small intestine.
The Small Intestine
The small intestine receives bile from the liver and pancreatic juice from the pancreas via the common bile duct that enters the duodenum, the first 25 cm of the small intestine. The enzymes in pancreatic juice and enzymes produced by the intestinal wall complete the process of food digestion. The wall of the small intestine contains fingerlike projections called villi that aid in the absorption of nutrients.
Regulation of Digestive Secretions
The secretion of digestive juices is promoted by the nervous system and by hormones. A hormone is a substance produced by one set of cells that affects a different set of cells.
The Large Intestine
The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated as feces.
14.2 Accessory Organs of Digestion
The pancreas, liver, and gallbladder are accessory digestive organs.
The pancreas is an endocrine gland that secretes insulin and glucagon to keep blood glucose levels within normal limits. As an exocrine gland, pancreatic cells produce pancreatic juice, which helps neutralize stomach acid, and digestive enzymes for all types of food.
The liver acts as the gatekeeper to the blood, it removes poisonous substances and detoxifies them. The liver also removes and stores iron and vitamins, makes many of the plasma proteins, helps regulate the quantity of cholesterol in the blood, and maintains the blood glucose level. It also makes bile, which emulsifies fat.
The gallbladder stores excess bile.
14.3 Digestive Enzymes
Digestive enzymes are proteins that speed up specific chemical reactions. They help break down carbohydrates, proteins, nucleic acids, and fats.
14.4 Human Nutrition
Nutrition is the intake of nutrients. A nutrient is a component of food that is utilized by the body. The six major classes of nutrients are carbohydrates, proteins, lipids, vitamins, minerals, and water.
Obesity is having a body mass index (BMI) of 30 or greater, but BMI does not take into account factors like fitness, bone structure, or gender.
Major Classes of Nutrients
Carbohydrates are present in food in the form of complex polysaccharides like starch and fiber, and simple sugars like glucose and sucrose. Glucose is the quickest, most readily available source of energy for the body. Complex sources of carbohydrates, such as whole-grain foods, are recommended because they are digested to sugars gradually and contain fiber. Fiber includes various indigestible carbohydrates derived from plants.
Foods rich in proteins include red meat, fish, poultry, dairy products, legumes, nuts, and cereals. Following digestion of protein, amino acids are incorporated into structural proteins found in muscles, skin, hair, and nails or used to synthesize such proteins as hemoglobin, plasma proteins, enzymes, and hormones.
Lipids include fats, oils, and cholesterol. Polyunsaturated oils are nutritionally essential because they contain two fatty acids the body cannot make called essential fatty acids.
Vitamins are organic compounds (other than carbohydrates, fats, and proteins) that the body needs for metabolic purposes but is unable to produce.
Minerals are inorganic chemical elements required by the body, which can be divided into major minerals and trace minerals. Major minerals are constituents of cells and body fluids and are structural components of tissues. The trace minerals are parts of larger molecules.
Diet and Osteoporosis
Many people take calcium supplements to counteract osteoporosis, a degenerative bone disease. Vitamin D is an essential companion to calcium in preventing osteoporosis because it promotes calcium absorption.
Sodium and Hypertension
High sodium intake has been linked to hypertension (high blood pressure) and the average American intakes more than the recommended amount per day.
Dietary supplements are nutrients and plant products that are used to enhance health. The U.S. government does not require dietary supplements to undergo the same safety and effectiveness testing that new prescription drugs must complete before they are approved. Therefore, many of these products have not been tested scientifically to determine their benefits. Ingesting high levels of certain nutrients can cause harm.
Food additives are substances that are added to foods as preservatives, or to enhance flavor or appearance. The influence of additives on health is not certain.
14.5 Eating Disorders
People with eating disorders have attitudes and behaviors toward food that are outside the norm.
Anorexia nervosa is a severe psychological disorder characterized by an irrational fear of getting fat. A self-imposed starvation diet is often accompanied by purging episodes involving self-induced vomiting and laxative abuse.
A person with bulimia nervosa binge-eats and then purges (vomits) to avoid gaining weight.
Eating Disorders Not Otherwise Specified (EDNOS)
Binge Eating Disorder
People with binge eating disorder frequently eat large amounts of food while feeling a loss of control over their eating, it is the most common eating disorder. Pica
Pica is defined as repeated ingestion of nonfood items.
14.6 Disorders of the Digestive System
Disorders of the digestive tract can be grouped into disorders of the tract itself, and disorders of the accessory organs.
Disorders of the Digestive Tract
A stomach ulcer is an open sore in the stomach wall caused by a gradual disintegration of the tissue. Most stomach ulcers are initiated by infection of the stomach by a bacterium.
The most common problems associated with the intestines are diarrhea and constipation. Diarrhea is loose, watery feces. The major causes of acute diarrhea are infections of the intestines, other diarrhea can be chronic. Severe diarrhea can lead to dehydration. Constipation is the opposite of diarrhea. Increasing the amount of water and fiber in the diet can help prevent constipation.
Polyps and Colon Cancer
The colon is subject to the development of polyps, small growths arising from the epithelial lining. Dietary fats are thought to increase the likelihood of colon cancer.
Disorders of the Accessory Organs
Disorders of the Pancreas
Pancreatitis is an inflammation of the pancreas. Pancreatic cancer is almost always fatal.
Disorders of the Liver and Gallbladder
Diseases that affect the liver can be life threatening. A person who has a liver ailment may develop jaundice, a yellowish coloring in the whites of the eyes, as well as in the skin. Hepatitis is inflammation of the liver and is caused by several viruses. Cirrhosis is a chronic disease of the liver, often seen in alcoholics. In some individuals, the cholesterol present in bile forms crystals that may grow to become gallstones.