Clinical meaning
The gastrointestinal (GI) system is a continuous muscular tube extending from the mouth to the anus, approximately 9 meters in length, responsible for the ingestion, digestion, absorption, and elimination of nutrients and waste products. The GI tract is divided into the upper GI tract (mouth, pharynx, esophagus, stomach, and duodenum) and the lower GI tract (jejunum, ileum, cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus). The wall of the GI tract consists of four layers: the mucosa (innermost layer containing epithelial cells, glands, and the lamina propria), the submucosa (connective tissue with blood vessels, lymphatics, and the submucosal nerve plexus of Meissner), the muscularis (smooth muscle responsible for peristalsis, innervated by the myenteric nerve plexus of Auerbach), and the serosa or adventitia (outermost protective layer). Peristalsis is the coordinated wave-like contraction of smooth muscle that propels contents through the GI tract; it is regulated by the enteric nervous system, often referred to as the second brain, which operates semi-independently from the central nervous system. The stomach produces hydrochloric acid (HCl) from parietal cells, which activates pepsinogen...