Chapter 42 Digestion and Human Nutrition

I. Types of Digestive Systems and Their Functions
A. The digestive system is an internal space or tube with specialized regions for food transport,
processing, and storage.
1 An incomplete digestive system (for example, in a planarian) has one opening.
2 A complete digestive system is a tube with two openings allowing food to move in one direction
through the lumen.
a. Animals with discontinuous feeding habits may have organs for storage.
b. Ruminants (for example, cows) may eat grass continuously and have multiple stomachs to
digest cellulose.
B. The digestive system performs these four functions:
1 Movement is the breaking down, mixing, and transporting of ingested nutrients, and the
elimination of undigested residues.
2 Secretion is the release of needed enzymes and hormones.
3 Digestion is the chemical breakdown of food matter to molecules small enough to cross the gut
lining.
4 Absorption is the passage of digested nutrients into the blood and lymph.

II. Human Digestive System: An Overview
A. Components
1 Specialized regions include the mouth, pharynx, esophagus, stomach, small intestine, colon,
rectum, and anus.
2 Accessory glands include the salivary glands, liver (with gallbladder), and pancreas. B. Gut Structure and Motility
1 The mucosa is the epithelial lining that faces the lumen of the gut; connective tissue underlies it. 2 The submucosa contains blood and lymph vessels plus nerve networks.
3 A muscle layer (longitudinal and circular) is covered with the outermost serosa.
a. Muscular constrictions (sphincters) ensure the forward movement of the gut contents.
b. Peristalsis contractions move the food forward; repeated segmentation contractions create
an oscillating movement that mixes the food.
C. Control of the Digestive System
1 Distention of the gut wall after a meal stimulates mechanoreceptors and their neurons, which
respond with muscle action and enzyme secretion.
2 Four gastrointestinal hormones play roles.
a. Gastrin, produced by the stomach lining, stimulates secretion of acids into the stomach.
b. Secretin, from the intestinal lining, stimulates insulin secretions from the pancreas.
c. Cholecystokinin enhances the actions of secretion and stimulates gallbladder contractions. d. GIP (glucose insulinotropic peptide), released in the presence of glucose and fat, stimulates
insulin secretion also.

III. Into the Mouth, Down the Tube
A. Mouth and Salivary Glands
1 Mechanical breakdown of food and its mixing with saliva begin in the mouth.
a. Teeth chew the food.
1) Each has an enamel coat, a dentine core, and an inner pulp.
2) Incisors bite off chunks, cuspids tear, and molars grind food.
b. Saliva (from salivary glands) contains salivary amylase to begin carbohydrate digestion,
bicarbonate to neutralize acids, and mucins to lubricate.
2 The tongue pushes the ball of food into the pharynx where receptors initiate the swallowing reflex
into the esophagus and then into the stomach; the epiglottis closes off the trachea to prevent
choking.
B. The Stomach
1 The stomach is a muscular sac that stores and mixes food, secretes substances that
dissolve and degrade food, and controls the rate at which food enters the small
intestine.
2 Stomach Acidity
a. Gastric fluid includes hydrochloric acid, pepsinogens, and mucus.
1) HCl dissolves bits of food to form a soupy chyme; it also converts pepsinogen (inactive)
to pepsin (active).
2) Pepsin begins the digestion of proteins.
b. Stomach secretions begin in response to sensual perception of food and continue in response to
stretching of the stomach wall, presence of gastrin hormone, or caffeine in the stomach
contents.
c. Normally, mucus and bicarbonate ions protect the stomach lining; but if these are blocked,
hydrogen ions stimulate the release of histamine, which in turn stimulates release of
more HCl, which may result in a peptic ulcer.
3 Stomach Emptying
a. Peristaltic contractions churn the chyme and keep the sphincter of the stomach’s exit closed,
but small amounts are released at regular intervals into the small intestine.
b. A large meal hastens stomach emptying while increases in acidity, fat content, fear, and
depression slow it down.
C. The Small Intestine
1 Digestion is completed and most nutrients are absorbed in the small intestine.
2 Secretions from the pancreas and liver enter via a common duct.
3 Digestion Processes
a. Enzymes act on all the macromolecules of food.
1) Trypsin and chymotrypsin digest proteins to peptide fragments.
2) Carboxypeptidase and aminopeptidase degrade the fragments to amino acids.
b. Bicarbonate from the pancreas buffers the acid from the stomach.
c. Bile (stored in the gallbladder until used) is a secretion of bile salts, pigments, cholesterol,
and lecithin that assists in emulsification of fats.
4 Absorption Processes
a. Absorptive surface area is increased by fingerlike projections of the intestinal lining called
villi, the cells of which bear even smaller microvilli.
b. Monosaccharides (glucose) and amino acids cross the gut lining by active transport
and enter the bloodstream.
c. Free fatty acids diffuse into the gut epithelium and then into the lymph vessels.
d. Water and ions are also absorbed.
D. The Large Intestine
1 The large intestine stores and concentrates feces—undigested and unabsorbed material, water, and bacteria.
2 Sodium is actively transported out of the colon and water follows.
3 The large intestine begins as a cup-shaped pouch at its junction with the small intestine
(appendix attached here); it is draped across the lower abdomen and ends in a rectum
(feces storage) that opens to the outside through the anus.
4 Fiber (“bulk”) in the diet is important in moving material in the feces through the large
intestine at the proper speed.

IV. Human Nutritional Requirements
A. Energy Needs and Body Weight
1 To maintain acceptable weight, caloric intake must balance energy output.
2 Caloric requirements can be estimated by multiplying your desired weight by 10 (inactive
person), or 15 (moderately active), or 20 (very active) and then subtracting from 0 to 400
depending on age.
3 Obesity is an excess of fat in the body’s adipose tissue; by definition that term is applied to
persons who are 25 percent heavier than ideal.
B. Carbohydrates
1 Complex carbohydrates are the main source of energy taken into the body; they are degraded to
glucose, the main source of energy available to individual cells.
2 Fruits, vegetables, and grains should be at least 50 to 60 percent of daily caloric intake. C. Proteins
1 Proteins should make up about 12 percent of the total diet.
2 Of the twenty different amino acids in proteins, eight are essential (that is, must be supplied in
the diet).
3 Because some sources of protein (plants) are “incomplete,” nutritionists use the net protein
utilization (NPU) index to compare proteins from different sources.
D. Lipids
1 Phospholipids and cholesterol are important components of membranes; fats are energy reserves
and provide insulation and cushioning.
2 Fats make up 40 percent of the American diet—they should be less than 30 percent.
3 The body needs very little polyunsaturated fat to supply the essential fatty acids, those not made
by the body itself.
E. Vitamins and Minerals
1 Humans need small amounts of at least thirteen organic molecules called vitamins to assist in
cellular metabolism.
2 Inorganic substances called minerals (Ca, Mg, K, Fe, for example) are also needed.
3 A balanced diet will normally meet all requirements for these substances; excessive intake is at
least wasteful, and at worst harmful.

V. Nutrition and Metabolism
A. Storage and Interconversion of Nutrients
1 Nutrient molecules are shuffled and reshuffled once they have been absorbed.
2 Shortly after a meal, the level of carbohydrates rises; some are converted to fat for storage,
and others are converted to glycogen in the liver and muscle tissue.
3 Between meals, glucose levels are maintained by breakdown of glycogen reserves in the liver and
amino acids are converted to glucose; fatty acids from fats can be used directly by cells for
energy.
4 The liver is a valuable organ for conversion of nutrients and detoxification of chemicals. B. Controls Over Metabolism
1 Alpha cells secrete glucagon, which causes glycogen stored in the liver to be converted to
glucose, raising its levels in the blood.
2 Beta cells secrete insulin, which stimulates the uptake of glucose by liver, muscle, and adipose,
especially after a meal, lowering glucose levels in the blood.
3 Delta cells secrete somatostatin, which can inhibit the secretion of glucagon and insulin.