Digestive system

Dr. Morton introduces the digestive system, outlining key questions about the organs, their functions, accessory organs, vascular supply, and innervation of the GI tract. The discussion begins with the oral cavity, focusing on physical digestion (mastication by teeth) and the role of salivary glands. Saliva, produced by submandibular, parotid, and sublingual glands, moistens food to form a bolus and initiates chemical digestion of carbohydrates via salivary amylase.

The esophagus transports food from the oral cavity to the stomach without metabolic changes. Its muscular wall transitions from skeletal muscle (voluntary swallowing) proximally to smooth muscle (involuntary peristalsis) distally. Peristalsis, an involuntary wave-like muscular contraction, moves food through the GI tract. The session then details the four layers of the GI tract: mucosa (epithelium and lamina propria responsible for protection, absorption, and secretion), submucosa (dense connective tissue with vessels and nerves), muscularis externa (inner circular and outer longitudinal smooth muscle layers for segmentation and peristalsis, containing the myenteric and submucosal plexuses), and serosa (mesothelial lining).

The stomach, located between the esophagus and duodenum, churns gastric contents to convert bolus into chyme, aiding physical breakdown and mixing with secretions. Goblet cells produce protective mucus, parietal cells secrete hydrochloric acid and intrinsic factor, and chief cells secrete pepsinogen for protein digestion. The stomach is divided into the cardia, fundus, body, and pylorus, featuring greater and lesser curvatures, and gastric rugae allowing for expansion.

The small intestine, approximately seven meters long with a small luminal diameter, is where digestion completes and nutrients are absorbed. It comprises three parts: the duodenum, jejunum, and ileum. The duodenum has Brunner's glands, four parts (including the duodenal cap, descending, transverse, and ascending parts), and is where bile and pancreatic enzymes enter via the major duodenal papilla. The jejunum holds most of the circular folds for maximum absorption. The ileum has fewer circular folds and is characterized by Peyer's patches (lymphatic nodules) and ends at the ileocecal valve.

The large intestine, shorter but with a larger luminal diameter, absorbs water, salts, and vitamins from intestinal flora, compacting and eliminating feces. Its distinguishing features include teniae coli, haustra, and epiploic appendages. The cecum, a blind-ended sac, is the start of the large intestine and includes the vermiform appendix. The colon consists of ascending, transverse (midgut to hindgut transition), descending, and sigmoid parts. The rectum is the terminal, straight portion, mostly retroperitoneal. The anus features an external (voluntary skeletal muscle) and internal (involuntary smooth muscle) anal sphincter.

The liver, located in the upper right quadrant, produces bile for fat emulsification and performs numerous metabolic functions. It has four lobes (right, left, quadrate, caudate) and a portal triad (proper hepatic artery, portal vein, common bile duct) at its hilum. The gallbladder stores and concentrates bile. The pancreas, deep to the stomach and mostly retroperitoneal, has both exocrine functions (secreting digestive enzymes: proteases, amylase, lipase into the duodenum) and endocrine functions (secreting insulin and glucagon from pancreatic islets of Langerhans into the bloodstream). The spleen, though not a digestive organ, is mentioned due to shared blood supply and its role in blood storage and filtration.

The GI tract developmental segments (foregut, midgut, hindgut) dictate its vascular supply. The foregut (stomach to proximal duodenum, and accessory glands) is supplied by the celiac trunk. The midgut (distal duodenum to two-thirds of transverse colon) is supplied by the superior mesenteric artery (SMA). The hindgut (distal transverse colon to rectum) is supplied by the inferior mesenteric artery (IMA). There are anastomotic connections between these arterial distributions. Lymphatic drainage follows these arterial supplies. Venous drainage of the GI tract leads to the hepatic portal system, where veins from all three segments (gastric, splenic, gastro-omental veins, SMV, IMV) converge to the portal vein, which delivers nutrient-rich blood to the liver for processing. Autonomic innervation consists of sympathetic (reducing peristalsis and secretions) and parasympathetic (increasing peristalsis and secretions) divisions, with specific pathways for foregut, midgut, and hindgut organs.