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CHAPTER OUTLINE
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
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
The teeth begin the process of mechanical digestion.
The Pharynx
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
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
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
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
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
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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
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
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
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.
Proteins
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
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
Vitamins are organic compounds (other than carbohydrates, fats, and proteins)
that the body needs for metabolic purposes but is unable to produce.
Minerals
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
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
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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
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
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.
Bulimia Nervosa
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
Stomach Ulcers
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.
Intestinal Disorders
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.
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