Digestive System Help
The food we eat is utilized at the cellular level in chemical reactions involving: synthesis of proteins, carbohydrates, hormones, and enzymes; cellular division, growth, and repair; and production of heat. To become usable by the cells, most food must first be mechanically and chemically reduced to forms that can be absorbed through the intestinal wall and transported to the cells by the blood. The processes involved with digestion include:
|Ingestion:||Taking food into the mouth (mechanical process)|
|Mastication:||Chewing food (mechanical process)|
|Salivary action (chemical process)|
|Deglutition:||Swallowing (mechanical process)|
|Peristalsis:||Wavelike contractions that move food through the GI tract (mechanical process)|
|Absorption:||Passage of food molecules from the GI tract into the circulatory or lymphatic system (mechanical and chemical process)|
|Defecation:||Elimination of undigestible wastes (mechanical process)|
The digestive system can be divided into a tubular gastrointestinal tract (GI tract) and accessory digestive organs. The GI tract extends from the oral cavity (mouth) to the anus. The regions or organs of the GI tract include the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. The rectum and anal canal are located at the terminal end of the large intestine. The accessory digestive organs include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
Associated with the abdominal cavity and its viscera are serous membranes (see Chapter One), called peritoneum. The parietal peritoneum lines the wall of the abdominal cavity. The peritoneal covering continues around the abdominal viscera as the visceral peritoneum. The space between the parietal and visceral portions of the peritoneum is called the peritoneal cavity. Most of the digestive organs are located within the peritoneal cavity. A few are outside of the peritoneum, retroperitoneal.
Histology of the GI Tract
Structures and Functions of the GI Tract
Oral Cavity. The oral cavity ingests food; grinds and mixes it with saliva; initiates digestion of carbohydrates; forms bolus; swallows bolus.
- Teeth: Four types of teeth: Incisors (4 upper, 4 lower) for cutting and shearing food; Canines (2 upper, 2 lower) for holding and tearing; Premolars (4 upper, 4 lower) and Molars (6 upper, 6 lower) both for crushing and grinding food.
- Tongue: Moves food around mouth during mastication; assists in swallowing; formation of speech sounds; location of taste buds.
- Salivary glands: Three salivary glands: Parotid gland located over the masseter muscle; Submandibular gland inferior to the base of the tongue; Sublingual gland under the tongue, produces saliva and begins chemical digestion of carbohydrates.
- Palate: Roof of the oral cavity consisting of the bony hard palate anteriorly and the soft palate posteriorly. The uvula is suspended from the soft palate. The soft palate closes the nasopharynx during swallowing.
Pharynx: A funnel-shaped passageway that connects the oral and nasal cavity to the esophagus and trachea. Function is swallowing.
Esophagus: A muscular tube located in the thorax behind the trachea that connects the pharynx to the stomach. Transports bolus to the stomach via peristalsis.
Stomach: See Figure 19-1 for parts of the stomach. The stomach receives the bolus from esophagus; churns bolus with gastric juice to form chyme; initiates digestion of proteins; carries out limited absorption; and moves chyme into duodenum. Specializations of the tunics of the stomach include: an additional smooth muscle layer, the oblique layer; longitundinal folds of the mucosa called rugae and gastric glands, which secrete gastric juice.
Small intestine: The region between the stomach and the large intestine, approximately 10 feet long. It receives chyme from the stomach, bile and pancreatic secretions from the liver and pancreas respectively; chemically and mechanically breaks down chyme; absorbs nutrients; and transports wastes to the large intestine.
Modifications of the small intestine facilitate absorption. The plicae circulares and the villi, fingerlike projections of the mucosa, increase absorptive area. Each villus contains a capillary network and a lymph vessel, a lacteal. Absorption is accomplished as food molecules enter these vessels through microvilli, microprojections on the surface of the villi. At the bases of the villi are intestinal glands that secrete digestive enzymes.
Large intestine: Extends from the ileocecal valve to the anus, approx. 5 feet long. It receives wastes from the small intestine; absorbs water and electrolytes; forms, stores, and expels feces through defecation.
The appendix, consisting of lympatic tissue, is attached to the cecum. The tunics, mucosa, and submucosa have sacculations called haustra; the muscularis consists of longitunal bands called taeniae coli, and attached to the adventitia are fat-filled pouches called epiploic appendages.
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