Overview
This lecture provides a comprehensive overview of the digestive system, covering the structure and function of digestive organs, accessory organs, histological layers, vascular supply, and innervation.
Digestive System Organs & Functions
- The oral cavity contains teeth for mastication (physical breakdown of food) and salivary glands for moistening food and starting carbohydrate digestion.
- Saliva, produced mainly by the submandibular gland, forms a bolus and contains salivary amylase to begin chemical digestion.
- The esophagus transports food from the mouth to the stomach, transitioning from voluntary (skeletal) to involuntary (smooth) muscle control.
- Peristalsis is the involuntary process that moves food through the GI tract.
- The stomach churns food via peristalsis, secretes acid (HCl), intrinsic factor, and pepsinogen for protein digestion.
- Small intestine (duodenum, jejunum, ileum) completes digestion and absorbs nutrients, with different regions specialized for various functions.
- Large intestine absorbs water and salts, compacts feces, and features structures like tenia coli, haustra, and epiploic appendages.
- The rectum and anus control defecation via involuntary (internal) and voluntary (external) sphincters.
Histological Layers of the GI Tract
- Mucosa: epithelium (simple columnar or stratified squamous), lamina propria (loose connective tissue for absorption), muscularis mucosae.
- Submucosa: dense connective tissue with blood vessels and nerves.
- Muscularis externa: inner circular and outer longitudinal muscle layers; responsible for peristalsis and segmentation.
- Serosa: visceral peritoneum; secretes serous fluid.
Accessory Digestive Organs
- Liver: located in the upper right quadrant, produces bile to emulsify fats, has multiple metabolic functions, and is divided into four lobes.
- Gallbladder: stores and concentrates bile, releases it into the duodenum via the common bile duct.
- Pancreas: mostly retroperitoneal; exocrine function produces digestive enzymes (proteases, amylase, lipase), endocrine function secretes insulin and glucagon.
- Spleen: not a digestive organ but shares vascular supply; stores blood, removes defective red blood cells.
Vascular Supply & Innervation
- Foregut organs supplied by celiac trunk; midgut by superior mesenteric artery; hindgut by inferior mesenteric artery.
- Venous drainage mirrors arterial supply and leads to the hepatic portal vein.
- Lymphatic drainage follows the arteries to celiac, superior, and inferior mesenteric nodes.
- Autonomic innervation: Sympathetic nerves decrease digestion, parasympathetic (mainly vagus and pelvic splanchnic nerves) increase digestion.
- Enteric nervous system (Meissner’s and Auerbach’s plexuses) regulates GI tract function.
Key Terms & Definitions
- Mastication — chewing, the physical breakdown of food.
- Peristalsis — involuntary wave-like muscle contractions moving food through the GI tract.
- Bolus — ball of chewed, moistened food ready to be swallowed.
- Chyme — semi-fluid mixture of food and digestive juices in the stomach.
- Brunner’s glands — duodenal glands that secrete bicarbonate for neutralizing stomach acid.
- Peyer’s patches — lymphatic nodules in the ileum.
- Tenia coli — longitudinal muscle bands of the colon.
- Portal triad — group of hepatic artery, portal vein, and bile duct in the liver.
- Enteric nervous system — network of neurons in the GI wall regulating digestive functions.
Action Items / Next Steps
- Review digestive system organ structures, functions, and histology.
- Study arterial supply and innervation for each GI region.
- Memorize the sequence of digestive tract organs and major accessory organs.
- Prepare for questions on organ identification, blood supply, and nervous regulation of digestion.