Digestive System

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Human Digestive System Adam

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Chapter Summary

Introduction (page 687)

Digestion is the process of mechanically and chemically breaking down foods so that they can be absorbed. The digestive system consists of an alimentary canal and several accessory organs.

General Characteristics of the Alimentary Canal (page 687)

Regions of the alimentary canal perform specific functions.

1. Structure of the wall a. The wall consists of four layers.

b. These layers include the mucosa, submucosa, muscular layer, and serosa.

2. Movements of the tube a. Motor functions include mixing and propelling movements.

b. Peristalsis is responsible for propelling movements.

c. The wall of the tube undergoes receptive relaxation just ahead of a peristaltic wave.

3. Innervation of the tube a. The tube is innervated by branches of the sympathetic and parasympathetic divisions of the autonomic nervous system.

b. Parasympathetic impulses generally increase digestive activities; sympathetic impulses generally inhibit digestive activities.

c. Sympathetic impulses contract certain sphincter muscles, controlling movement through the alimentary canal.

Mouth (page 689)

The mouth is adapted to receive food and begin preparing it for digestion. It also serves as an organ of speech and sensory perception.

1. Cheeks and lips a. Cheeks form the lateral walls of the mouth.

b. Lips are highly mobile and possess a variety of sensory receptors useful in judging the characteristics of food.

2. Tongue a. The tongue is a thick, muscular organ that mixes food with saliva and moves it toward the pharynx.

b. The rough surface of the tongue handles food and contains taste buds.

c. Lingual tonsils are located on the root of the tongue.

3. Palate a. The palate comprises the roof of the mouth and includes hard and soft portions.

b. The soft palate closes the opening to the nasal cavity during swallowing.

c. Palatine tonsils are located on either side of the tongue in the back of the mouth.

d. Tonsils consist of lymphatic tissues.

4. Teeth a. Two sets of teeth develop in sockets of the mandibular and maxillary bones.

b. There are twenty primary and thirty-two secondary teeth.

c. Teeth mechanically break food into smaller pieces, increasing the surface area exposed to digestive actions.

d. Different kinds of teeth are adapted to handle foods in different ways, such as biting, grasping, or grinding.

e. Each tooth consists of a crown and root and is composed of enamel, dentin, pulp, nerves, and blood vessels.

f. A tooth is attached to the alveolar process by collagenous fibers of the periodontal ligament.

Salivary Glands (page 694)

Salivary glands secrete saliva, which moistens food, helps bind food particles, begins chemical digestion of carbohydrates, makes taste possible, helps cleanse the mouth, and regulates pH in the mouth.

1. Salivary secretions a. Salivary glands include serous cells that secrete digestive enzymes and mucous cells that secrete mucus.

b. Parasympathetic impulses stimulate the secretion of serous fluid.

2. Major salivary glands a. The parotid glands are the largest, and they secrete saliva rich in amylase.

b. The submandibular glands in the floor of the mouth produce viscous saliva.

c. The sublingual glands in the floor of the mouth primarily secrete mucus.

Pharynx and Esophagus (page 695)

The pharynx and esophagus serve as passageways.

1. Structure of the pharynx a. The pharynx is divided into a nasopharynx, oropharynx, and laryngopharynx.

b. The muscular walls of the pharynx contain fibers in circular and longitudinal groups.

2. Swallowing mechanism a. Swallowing occurs in three stages.

(1) Food is mixed with saliva and forced into the pharynx.

(2) Involuntary reflex actions move the food into the esophagus.

(3) Peristalsis transports food to the stomach.

b. Swallowing reflexes momentarily inhibit breathing.

3. Esophagus a. The esophagus passes through the mediastinum and penetrates the diaphragm.

b. Circular muscle fibers at the distal end of the esophagus help prevent regurgitation of food from the stomach.

Stomach (page 700)

The stomach receives food, mixes it with gastric juice, carries on a limited amount of absorption, and moves food into the small intestine.

1. Parts of the stomach a. The stomach is divided into cardiac, fundic, body, and pyloric regions.

b. The lower esophageal sphincter serves as a valve between the esophagus and the stomach.

c. The pyloric sphincter serves as a valve between the stomach and the small intestine.

2. Gastric secretions a. Gastric glands secrete gastric juice.

b. Gastric juice contains pepsin, hydrochloric acid, lipase, and intrinsic factor.

3. Regulation of gastric secretions a. Parasympathetic impulses and the hormone gastrin enhance gastric secretion.

b. The three stages of gastric secretion are the cephalic, gastric, and intestinal phases.

c. The presence of food in the small intestine reflexly inhibits gastric secretions.

4. Gastric absorption a. The stomach is not well adapted for absorption.

b. A few substances such as water and other small molecules are absorbed through the stomach wall.

5. Mixing and emptying actions a. As the stomach fills, its wall stretches, but its internal pressure remains unchanged.

b. Mixing movements aid in producing chyme; peristaltic waves move chyme into the pyloric region.

c. The muscular wall of the pyloric region regulates chyme movement into the small intestine.

d. The rate of emptying depends on the fluidity of the chyme and the type of food present.

e. The upper part of the small intestine fills, and an enterogastric reflex inhibits peristalsis in the stomach.

f. Vomiting results from a complex reflex that has many stimuli.

Pancreas (page 707)

The pancreas is closely associated with the duodenum.

1. Structure of the pancreas a. It produces pancreatic juice that is secreted into a pancreatic duct.

b. The pancreatic duct leads to the duodenum.

2. Pancreatic juice a. Pancreatic juice contains enzymes that can split carbohydrates, proteins, fats, and nucleic acids.

b. Pancreatic juice has a high bicarbonate ion concentration that helps neutralize chyme and causes the intestinal contents to be alkaline.

3. Regulation of pancreatic secretion a. Secretin from the duodenum stimulates the release of pancreatic juice that contains few digestive enzymes but has a high bicarbonate ion concentration.

b. Cholecystokinin from the intestinal wall stimulates the release of pancreatic juice that has a high concentration of digestive enzymes.

Liver (page 709)

The liver is located in the upper right quadrant of the abdominal cavity.

1. Liver structure a. The liver is a highly vascular organ, enclosed in a fibrous capsule, and divided into lobes.

b. Each lobe consists of hepatic lobules, the functional units of the liver.

c. Bile from the lobules is carried by bile canals to hepatic ducts that unite to form the common bile duct.

2. Liver functions a. The liver has many functions. It metabolizes carbohydrates, lipids, and proteins; stores some substances; filters blood; destroys toxins; and secretes bile.

b. Bile is the only liver secretion that directly affects digestion.

3. Composition of bile a. Bile contains bile salts, bile pigments, cholesterol, and electrolytes.

b. Only the bile salts have digestive functions.

c. Bile pigments are products of red blood cell breakdown.

4. Gallbladder a. The gallbladder stores bile between meals.

b. A sphincter muscle controls release of bile from the common bile duct.

c. Gallstones may form within the gallbladder.

5. Regulation of bile release a. Cholecystokinin from the small intestine stimulates bile release.

b. The sphincter muscle at the base of the common bile duct relaxes as a peristaltic wave in the duodenal wall approaches.

6. Functions of bile salts a. Bile salts emulsify fats and aid in the absorption of fatty acids, cholesterol, and certain vitamins.

b. Bile salts are reabsorbed in the small intestine.

Small Intestine (page 715)

The small intestine extends from the pyloric sphincter to the large intestine. It receives secretions from the pancreas and liver, completes digestion of nutrients, absorbs the products of digestion, and transports the residues to the large intestine.

1. Parts of the small intestine a. The small intestine consists of the duodenum, jejunum, and ileum.

b. The small intestine is suspended from the posterior abdominal wall by mesentery.

2. Structure of the small intestinal wall a. The wall is lined with villi that greatly increase the surface area and aid in mixing and absorption.

b. Microvilli on the free ends of epithelial cells increase the surface area even more.

c. Intestinal glands are located between the villi.

d. Circular folds in the lining of the intestinal wall also increase its surface area.

3. Secretions of the small intestine a. Intestinal glands secrete a watery fluid that lacks digestive enzymes but provides a vehicle for moving chyme to the villi.

b. Digestive enzymes embedded in the surfaces of microvilli split molecules of sugars, proteins, and fats.

4. Regulation of small intestinal secretions a. Secretion is stimulated by gastric juice, chyme, and reflexes stimulated by distension of the small intestinal wall.

5. Absorption in the small intestine a. Blood capillaries in the villi absorb monosaccharides, amino acids, fatty acids, and glycerol.

b. Blood capillaries in the villi also absorb water and electrolytes.

c. Fat molecules with longer chains of carbon atoms enter the lacteals of the villi; fatty acids with short carbon chains enter the blood capillaries of the villi.

6. Movements of the small intestine a. Movements include mixing by segmentation and peristalsis.

b. Overdistension or irritation may stimulate a peristaltic rush and result in diarrhea.

c. The ileocecal sphincter controls movement of the intestinal contents from the small intestine into the large intestine.

Large Intestine (page 723)

The large intestine absorbs water and electrolytes and forms and stores feces.

1. Parts of the large intestine a. The large intestine consists of the cecum, colon, rectum, and anal canal.

b. The colon is divided into ascending, transverse, descending, and sigmoid portions.

2. Structure of the large intestinal wall a. The large intestine wall resembles the wall in other parts of the alimentary canal.

b. The large intestine wall has a unique layer of longitudinal muscle fibers arranged in distinct bands.

Critical Thinking Questions

3. Functions of the large intestine a. The large intestine has little or no digestive function, although it secretes mucus.

b. Mechanical stimulation and parasympathetic impulses control the rate of mucus secretion.

c. The large intestine absorbs water and electrolytes.

d. Many bacteria inhabit the large intestine, where they break down certain undigestible substances and synthesize certain vitamins.

4. Movements of the large intestine a. Movements are similar to those in the small intestine.

b. Mass movements occur two to three times each day.

c. A reflex stimulates defecation.

5. Feces a. The large intestine forms and stores feces.

b. Feces consist of water, undigested material, mucus, and bacteria.

c. The color of feces is due to bile pigments that have been altered by bacterial action.

Life-Span Changes (page 728)

1. Older people sometimes do not chew food thoroughly because thinning enamel makes teeth more sensitive to hot and cold foods, gums recede, and teeth may loosen.

2. Slowing peristalsis in the digestive tract may cause heartburn and constipation.

3. Aging affects nutrient absorption in the small intestine.

4. Accessory organs to digestion also age, but not necessarily in ways that affect health.

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