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Chapter Summary 17.1 General Characteristics of the Digestive System 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 that carry out the processes of ingestion, propulsion, digestion, absorption, and defecation. Structure of the alimentary canal wall The wall consists of four layers. These layers include the mucosa, submucosa, muscularis, and serosa. Movements of the alimentary canal wall Motor functions include mixing and propelling movements. Peristalsis is responsible for propelling movements. The wall of the tube undergoes receptive relaxation just ahead of a peristaltic wave. Innervation of the alimentary canal wall The tube is innervated by branches of the sympathetic and parasympathetic divisions of the autonomic nervous system. Parasympathetic impulses generally increase digestive activities; sympathetic impulses generally inhibit digestive activities. Sympathetic impulses contract certain sphincter muscles, controlling movement of digesting food through the alimentary canal.
17.2 Mouth The mouth is adapted to receive food and begin digestion by mechanically breaking up solid particles (mastication). It also serves as an organ of speech and sensory perception. Cheeks and lips Cheeks form the lateral walls of the mouth. Lips are highly mobile and have a variety of sensory receptors useful in judging the characteristics of food. Tongue The tongue is a thick, muscular organ that mixes food with saliva and moves it toward the pharynx. The rough surface of the tongue handles food and has taste buds. Lingual tonsils are located on the root of the tongue. Palate The palate comprises the roof of the mouth and includes hard and soft portions. The soft palate, including the uvula, closes the opening to the nasal cavity during swallowing. Palatine tonsils are located on either side of the tongue in the back of the mouth. Tonsils consist of lymphatic tissues.
The parotid glands are the largest, and they secrete saliva rich in amylase. The submandibular glands in the floor of the mouth produce viscous saliva containing amylase. The sublingual glands in the floor of the mouth primarily secrete mucus. 17.4 Pharynx and Esophagus The pharynx and esophagus serve as passageways. Structure of the pharynx The pharynx is divided into a nasopharynx, oropharynx, and laryngopharynx. The muscular walls of the pharynx contain fibers in circular and longitudinal groups. Swallowing mechanism Swallowing (deglutition) 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 in the esophagus to the stomach. Swallowing reflexes momentarily inhibit breathing. Esophagus
The esophagus passes through the mediastinum and penetrates the diaphragm. The lower esophageal sphincter, at the distal end of the esophagus, helps prevent regurgitation of food from the stomach. 17.5 Stomach The stomach receives food, mixes it with gastric juice, carries on a limited amount of absorption, and moves food into the small intestine. Parts of the stomach The stomach is divided into the cardia, fundus, body, and pylorus. The pyloric sphincter serves as a valve between the stomach and the small intestine. 712 Gastric secretions Gastric glands secrete gastric juice. Alkaline mucus protects the inner stomach wall. Gastric juice contains mucus, chemical messengers, pepsin (begins digestion of proteins), hydrochloric acid, lipase, and intrinsic factor. Regulation of gastric secretion Parasympathetic impulses and the hormone gastrin enhance gastric secretion.
The upper part of the small intestine fills, and an enterogastric reflex inhibits peristalsis in the stomach. Vomiting Vomiting results from a complex reflex that has many stimuli. Motor responses ultimately increase abdominal pressure to force contents upward and out. 17.6 Pancreas The pancreas is closely associated with the duodenum. Structure of the pancreas It produces pancreatic juice secreted into a pancreatic duct. The pancreatic duct leads to the duodenum. Pancreatic juice Pancreatic juice contains enzymes that can break down carbohydrates, proteins, fats, and nucleic acids. Pancreatic juice has a high bicarbonate ion concentration that helps neutralize chyme and causes the intestinal contents to be alkaline. Regulation of pancreatic secretion Secretin from the duodenum stimulates the release of pancreatic juice that contains few digestive enzymes but has a high bicarbonate ion concentration.
Cholecystokinin from the intestinal wall stimulates the release of pancreatic juice that has a high concentration of digestive enzymes. 17.7 Liver and Gallbladder The liver is located in the upper-right quadrant of the abdominal cavity. Liver structure The liver is a highly vascular organ, enclosed in a fibrous capsule, and divided into lobes. Each lobe consists of hepatic lobules, the functional units of the liver. Bile from the lobules is carried by bile ductules to hepatic ducts that unite to form the common hepatic duct. Liver functions The liver has many functions. It removes toxic substances from the blood (detoxifies); metabolizes carbohydrates, lipids, and proteins; stores some substances; and secretes bile. Bile is the only liver secretion that directly affects digestion. Composition of bile Bile contains bile salts, bile pigments, cholesterol, and electrolytes. Only the bile salts have digestive functions.
The small intestine consists of the duodenum, jejunum, and ileum. The small intestine is suspended from the posterior abdominal wall by mesentery. Structure of the small intestinal wall The wall is lined with villi that greatly increase the surface area of the intestinal lining, aiding absorption of digestive products. Microvilli on the free ends of epithelial cells increase the surface area even more. Intestinal glands are located between the villi. Circular folds in the lining of the intestinal wall also increase its surface area. 713 Secretions of the small intestine Intestinal glands secrete a watery fluid that does not have digestive enzymes but provides a vehicle for moving chyme to the villi. Digestive enzymes embedded in the surfaces of microvilli break down molecules of sugars, proteins, and fats. Regulation of small intestinal secretion Secretion is stimulated by chyme and parasympathetic reflexes stimulated by distension of the small intestinal wall. Absorption in the small intestine
Blood capillaries in the villi absorb monosaccharides and amino acids. Fatty acids diffuse into small intestinal epithelial cells where they are processed to form chylomicrons, which enter the lacteals of the villi. Blood capillaries in the villi also absorb electrolytes and water. Movements of the small intestine Movements include mixing by segmentation and peristalsis. Overdistension or irritation may stimulate a peristaltic rush and result in diarrhea. The ileocecal sphincter controls movement of the intestinal contents from the small intestine into the large intestine. 17.9 Large Intestine The large intestine absorbs water and electrolytes and forms and stores feces. Parts of the large intestine The large intestine consists of the cecum, colon, rectum, and anal canal. The colon is divided into ascending, transverse, descending, and sigmoid portions. Structure of the large intestinal wall The large intestinal wall resembles the wall in other parts of the alimentary canal. The large intestinal wall has a unique layer of longitudinal muscle fibers, arranged in distinct bands, that extend the entire length of the colon.
17.10 Life-Span Changes 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. Slowing peristalsis in the digestive tract may cause heartburn and constipation. Aging affects nutrient absorption in the small intestine. Accessory organs to digestion also age, but not necessarily in ways that affect health.
