Part 1: Anatomy and Physiology of the Digestive System
The organs of the digestive system can be separated into 2 main groups: those forming the alimentary canal, and the accessory digestive organs. The alimentary canal performs the whole menu of digestive functions. The accessory organs assist the process of digestive breakdown in various ways. Organs of the Alimentary Canal
The alimentary canal, also called the gastrointestinal (GI) tract, is a continuous, coiled, hollow, muscular tube that winds through the ventral body cavity and is open at both ends. Its organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The large intestine leads to the terminal opening, or anus. Food material within this tube is technically outside the body, because it has contact only with cells lining the tract and is open to the external environment at both ends. Mouth
Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity. The lips (labia) protect its anterior opening, the cheeks form its lateral walls, the hard palate forms its anterior roof, and the soft palate forms its posterior roof. The uvula is a fleshy fingerlike projection of the soft palate, which extends downward from its posterior edge. The space between the lips and cheeks externally and the teeth and gums internally is the vestibule. The area contained by the teeth is the oral cavity proper. The muscular tongue occupies the floor of the mouth. The tongue has several bony attachments – 2 of these are to the hyoid bone and the styloid processes of the skull. The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements. Pharynx
From the mouth, food passes posteriorly into the oropharynx and laryngopharynx, both of which are common passageways for food, fluids, and air. The pharynx is subdivided into the nasopharynx, part of the respiratory passageway; the oropharynx, posterior to the oral cavity; and the laryngopharynx, which is continuous with the esophagus below. The walls of the pharynx contain 2 skeletal muscle layers. The cells of the inner layer run longitudinally; those of the outer layer run around the wall in a circular fashion. Esophagus
The esophagus, or gullet, runs from the pharynx through the diaphragm to the stomach. About 25 cm long, it is essentially a passageway that conducts food to the stomach. The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same 4 basic tissue layers, or tunics: 1. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ. It consists primarily of a surface epithelium, plus a small amount of connective tissue and a scanty smooth muscle layer. Beyond the esophagus, which has a friction-resisting stratified squamous epithelium, the epithelium is mostly simple columnar. 2. The submucosa is found just beneath the mucosa. It is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels. 3. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells. 4. The serosa is the outermost layer of the wall. It consists of a single layer of flat serous fluid producing cells, the visceral peritoneum. The visceral peritoneum is continuous with the slick, slippery parietal peritoneum, which lines the abdominopelvic cavity by way of a membrane extension, the mesentery. The alimentary canal wall contains 2 important nerve plexuses – the submucosal nerve plexus and the myenteric nerve plexus. An additional small subserous plexus is associated with the serosa. These networks of nerve fibers are actually part of the autonomic nervous system. They help regulate the mobility and secretory activity of GI tract organs. Stomach
The C-shaped stomach...