Digestive System PDF

# Introduction The digestive system is responsible for the breakdown of complex organic nutrient molecules into simple organic and inorganic molecules that can then be absorbed into the blood or lymph to bransported to cells. The digestive system is divided into two parts: the alimentary tube a...

# Introduction The digestive system is responsible for the breakdown of complex organic nutrient molecules into simple organic and inorganic molecules that can then be absorbed into the blood or lymph to bransported to cells. The digestive system is divided into two parts: the alimentary tube and the accessory organs. The alimentary tube extends from the mouth to the anus. It consists of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. Digestion takes place within the oral cavity, stomach, and small intestine; most absorption of nutrients takes place in the small intestine. Undigestible material, primarily cellulose, is eliminated by the large intestine (also called the colon). The accessory organs of digestion are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas. # Oral Cavity Food enters the oral cavity (or buccal cavity) via the mouth. Boundaries of the oral cavity: * Superiorly: hard and soft palates. * Laterally: cheeks. Within the oral cavity are the teeth and tongue and the openings of the ducts of the salivary glands. # Teeth The function of the teeth is chewing. This is the process that mechanically breaks food into smaller pieces and mixes it with saliva. The *deciduous teeth* begin to erupt through the gums at about 6 months of age, and the set of 20 teeth is usually complete by the age of 2 years. These teeth are gradually lost throughout childhood and replaced by the *permanent teeth*, the first of which are molars that emerge around the age of 6 years. A complete set of permanent teeth consists of 32 teeth; the types of teeth are incisors, canines, premolars, and molars. The *crown* is visible above the gum (gingiva). The root is enclosed in a socket in the mandible or maxillae. The *periodontal membrane* lines the socket and produces a bone-like cement that anchors the tooth. The outermost layer of the crown is *enamel*, which is made by cells called *ameloblasts*. Enamel provides a hard chewing surface and is more resistant to decay than are other parts of the tooth. Within the enamel is *dentin*, which is very similar to bone and is produced by cells called *odontoblasts*. Dentin also forms the roots of a tooth. The innermost portion of a tooth is the *pulp cavity*, which contains blood vessels and nerve endings of the trigeminal nerve (5th cranial). Erosion of the enamel and dentin layers by bacterial aids (dental caries or cavities) may result in bacterial invasion of the pulp cavity and a very painful toothache. # Tongue The tongue is made of skeletal muscle that is innervated by the hypoglossal nerves (12th cranial). *Papillae* are small projections on the upper surface of the tongue. The nerve supply to the tongue includes facial nerve and glossopharyngeal nerve. Elevation of the tongue is the first step in swallowing. This is a voluntary action, in which the tongue contracts and meets the resistance of the hard palate. The mass of food, called a *bolus*, is thus pushed backward toward the pharynx. # Salivary Glands The digestive secretion in the oral cavity is saliva, produced by three pairs of salivary glands. The *parotid glands* are located posteriorly and anterior to the ears. The *submandibular* (also called submaxillary) glands are at the posterior corners of the mandible, and the *sublingual glands* are inferior to the floor of the mouth. Each gland has at least one duct that takes saliva to the oral cavity. Saliva secretion is continuous, but the amount varies in different situations. The presence of food (or anything else) in the mouth increases saliva secretion. This is a parasympathetic response mediated by the facial and glossopharyngeal nerves. The sight or smell of food also increases secretion of saliva. Sympathetic stimulation in stress situations decreases secretion, making the mouth dry and swallowing difficult. # Oesophagus The *oesophagus* is a muscular tube that takes food from the pharynx to the stomach; no digestion takes place here. Peristalsis of the esophagus propels food in one direction and ensures that food gets to the stomach even if the body is horizontal or upside down. At the junction with the stomach, the lumen (cavity) of the esophagus is surrounded by the *lower esophageal sphincter* (LES or cardiac sphincter), a circular smooth muscle. Relaxation of LES permits food to enter the stomach, while its contraction prevents the "backup" of stomach contents. If the LES does not close completely, gastric juice may splash up into the esophagus; this is a painful condition known as heartburn, or *gastroesophageal reflux disease (GERD)*. # Layers of the Alimentary Tube The *mucosa*, or lining, of the alimentary tube is made of epithelial tissue, areolar connective tissue, and two thin layers of smooth muscle. The esophagus has stratified squamous epithelium; The stomach and intestines has simple columnar epithelium. The *submucosa* is made of areolar connective tissue with many blood vessels and lymphatic vessels. Many millions of nerve fibers are also present. The nerve networks in the submucosa are called *Meissner's plexus* (or submucosal plexus), and they innervate the mucosa to regulate secretions. Parasympathetic impulses increase secretions, whereas sympathetic impulses decrease secretions. The external muscle layer typically contains two layers of smooth muscle: an inner, circular layer and an outer, longitudinal layer. In the esophagus, this layer is striated muscle in the upper third, which gradually changes to smooth muscle in the lower portions. The stomach has three layers of smooth muscle, rather than two. Contractions of this muscle layer help break up food and mix it with digestive juices. The one-way contractions of *peristalsis* move the food toward the anus. Sympathetic impulses decrease contractions and peristalsis, whereas parasympathetic impulses increase contractions and peristalsis, promoting normal digestion. The parasympathetic nerves are the vagus nerves. Above the diaphragm, for the esophagus, the *serosa*, the outermost layer, is fibrous connective tissue. Below the diaphragm, the serosa is the *mesentery* or visceral peritoneum, a serous membrane. Lining the abdominal cavity is the parietal peritoneum, usually simply called the *peritoneum*. The serous fluid between the peritoneum and mesentery prevents friction when the alimentary tube contracts and the organs slide against one another. # Stomach The stomach is located in the upper left quadrant of the abdominal cavity, to the left of the liver and anterior to the spleen. The stomach is not a tube, but rather a sac that extends from the esophagus to the small intestine. It serves as a reservoir for food, so that digestion proceeds gradually. Both mechanical and chemical digestion take place in the stomach. The *cardiac orifice* is the opening of the esophagus; *Fundus* is the portion above the level of this opening. The body of the stomach is the large central portion, bounded laterally by the greater curvature and medially by the lesser curvature. The *pylorus* is adjacent to the duodenum of the small intestine, and the *pyloric sphincter* surrounds the junction of the two organs. The fundus and body are mainly storage areas, whereas most digestion takes place in the pylorus. # Small Intestine The *small intestine* is about 1 inch (2.5 cm) in diameter and approximately 6 m long. It extends from the stomach to the cecum of the large intestine. The *duodenum* is the first 10 inches (25cm) of the small intestine. The common bile duct enters the duodenum at the ampulla of Vater (or hepatopancreatic ampulla). The *jejunum* is about 8 feet long; The *ileum* is about 11 feet in length. Digestion is completed in the small intestine, and the end products of digestion are absorbed into the blood and lymph. The mucosa has simple columnar epithelium that includes cells with microvilli and goblet cells that secrete mucus. *Enteroendocrine cells* secrete the hormones of the small intestine. *Lymph nodules* called Peyer's patches are especially abundant in the ileum to destroy absorbed pathogens. The external muscle layer has the typical circular and longitudinal smooth muscle layers that mix the chyme with digestive secretions and propel the chyme toward the colon. There are three sources of digestive secretions that function within the small intestine: the liver, the pancreas, and the small intestine itself.