Pathophysiology
Clinical meaning
The gastrointestinal (GI) system is responsible for the ingestion, digestion, absorption, and elimination of nutrients and waste products. The GI tract is a continuous muscular tube extending from the mouth to the anus, consisting of the oral cavity, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum), and anus. Accessory organs include the liver (bile production, detoxification, protein synthesis, glycogen storage), gallbladder (bile storage and concentration), and pancreas (exocrine function producing digestive enzymes; endocrine function producing insulin and glucagon). The GI wall has four layers: mucosa (innermost -- absorption and secretion), submucosa (blood vessels, lymphatics, nerve plexus), muscularis (smooth muscle for peristalsis -- inner circular and outer longitudinal layers), and serosa (outermost protective layer). Digestion involves both mechanical processes (chewing, churning, segmentation, peristalsis) and chemical processes (enzymatic breakdown of carbohydrates by amylase, proteins by pepsin and trypsin, and fats by lipase with bile salt emulsification). The enteric nervous system (sometimes called the 'second brain') contains approximately 100 million neurons and independently regulates motility, secretion, and blood flow, though it...