🫀Anatomy and Physiology II Unit 6 – Digestive System: Anatomy & Histology

Overview and Structure

• Digestive system consists of the gastrointestinal (GI) tract and accessory organs that aid in digestion
• GI tract is a continuous tube extending from the mouth to the anus, approximately 30 feet long
• Four main layers of the GI tract wall: mucosa, submucosa, muscularis externa, and serosa
  • Mucosa is the innermost layer that lines the lumen and contains epithelial cells, lamina propria, and muscularis mucosae
  • Submucosa contains blood vessels, lymphatic vessels, and submucosal plexus (Meissner's plexus)
• Digestive system functions include ingestion, propulsion, mechanical and chemical digestion, absorption, and defecation
• Mechanical digestion involves physical breakdown of food through chewing and peristalsis
• Chemical digestion involves enzymatic breakdown of macromolecules into smaller absorbable units
• Absorption occurs primarily in the small intestine, where nutrients are transported into the bloodstream or lymphatic system

Mouth and Esophagus

• Mouth (oral cavity) is the beginning of the digestive tract, where food is ingested and mechanical digestion begins
• Tongue manipulates food, assists in swallowing, and contains taste buds for sensory detection of food
• Salivary glands (parotid, submandibular, and sublingual) secrete saliva, which contains enzymes (salivary amylase) and lubricates food
• Teeth are specialized structures for mechanical digestion, with different types (incisors, canines, premolars, and molars) serving specific functions
• Pharynx is a common passageway for both the digestive and respiratory systems
• Epiglottis is a flap of cartilage that covers the glottis during swallowing to prevent aspiration
• Esophagus is a muscular tube that connects the pharynx to the stomach and propels food via peristalsis
  • Upper esophageal sphincter (UES) and lower esophageal sphincter (LES) regulate the passage of food and prevent reflux

Stomach

• Stomach is a J-shaped, muscular organ that serves as a temporary storage site for ingested food
• Four main regions of the stomach: cardia, fundus, body, and pylorus
• Stomach wall contains oblique, circular, and longitudinal smooth muscle layers for churning and mixing food
• Gastric glands in the stomach mucosa secrete gastric juice, which contains hydrochloric acid (HCl), pepsinogen, and intrinsic factor
  • HCl creates an acidic environment (pH 1.5-3.5) for protein denaturation and activation of pepsinogen to pepsin
  • Pepsin is an enzyme that breaks down proteins into smaller peptides
• Stomach also secretes mucus to protect the stomach lining from the acidic environment
• Pyloric sphincter regulates the passage of partially digested food (chyme) from the stomach to the small intestine

Small Intestine

• Small intestine is the longest part of the GI tract (approximately 20 feet long) and consists of three segments: duodenum, jejunum, and ileum
• Duodenum receives chyme from the stomach and is the site where most chemical digestion occurs
  • Receives secretions from the pancreas (pancreatic juice) and liver (bile) via the hepatopancreatic ampulla (ampulla of Vater)
• Jejunum and ileum are the primary sites of nutrient absorption
• Small intestine has a highly folded and specialized mucosa for increased surface area
  • Circular folds (plicae circulares), villi, and microvilli enhance absorption
• Brush border enzymes on the microvilli complete the final stages of chemical digestion
• Absorbed nutrients enter the bloodstream via capillaries in the villi or the lymphatic system via lacteals

Large Intestine

• Large intestine (colon) is the final segment of the GI tract, measuring approximately 5 feet long
• Four main regions of the large intestine: cecum, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal
• Primary functions include absorption of water and electrolytes, formation and storage of feces, and microbial fermentation
• Ileocecal valve (ICV) regulates the passage of contents from the ileum to the cecum
• Appendix is a small, finger-like projection attached to the cecum, containing lymphoid tissue and gut-associated lymphoid tissue (GALT)
• Haustra are sacculations in the colon wall that give it a segmented appearance and help in mixing and propulsion of contents
• Rectum stores feces prior to defecation
• Anal sphincters (internal and external) regulate the passage of feces during defecation

Accessory Organs

• Liver is the largest gland in the body and performs various functions, including bile production, detoxification, and nutrient metabolism
  • Bile is stored and concentrated in the gallbladder before being released into the duodenum
  • Bile aids in the emulsification and absorption of fats
• Pancreas is a mixed gland with both endocrine and exocrine functions
  • Exocrine portion secretes pancreatic juice containing digestive enzymes (proteases, lipases, and amylases) into the duodenum
  • Endocrine portion (islets of Langerhans) secretes hormones (insulin and glucagon) into the bloodstream for glucose regulation
• Salivary glands (parotid, submandibular, and sublingual) secrete saliva, which contains enzymes (salivary amylase) and lubricates food

Histology Highlights

• Epithelial lining of the GI tract varies depending on the region and function
  • Esophagus: non-keratinized stratified squamous epithelium
  • Stomach: simple columnar epithelium with gastric pits and glands
  • Small intestine: simple columnar epithelium with villi and microvilli (brush border)
  • Large intestine: simple columnar epithelium with abundant goblet cells
• Lamina propria contains loose connective tissue, blood vessels, lymphatic vessels, and immune cells (GALT)
• Muscularis mucosae is a thin layer of smooth muscle that separates the mucosa from the submucosa
• Submucosa contains dense irregular connective tissue, blood vessels, lymphatic vessels, and submucosal plexus (Meissner's plexus)
• Muscularis externa consists of an inner circular layer and an outer longitudinal layer of smooth muscle, with the myenteric plexus (Auerbach's plexus) between them
• Serosa is the outermost layer, consisting of a simple squamous epithelium (mesothelium) and a small amount of underlying connective tissue

Clinical Connections

• Gastroesophageal reflux disease (GERD) occurs when the lower esophageal sphincter (LES) fails to prevent stomach acid from refluxing into the esophagus, causing heartburn and potential damage to the esophageal lining
• Peptic ulcers are open sores that develop in the mucosa of the stomach or duodenum, often caused by Helicobacter pylori infection or prolonged use of nonsteroidal anti-inflammatory drugs (NSAIDs)
• Inflammatory bowel diseases (IBD), such as Crohn's disease and ulcerative colitis, are chronic conditions characterized by inflammation and damage to the GI tract
  • Crohn's disease can affect any part of the GI tract, most commonly the terminal ileum and colon, and involves transmural inflammation
  • Ulcerative colitis is limited to the colon and rectum and involves continuous mucosal inflammation
• Celiac disease is an autoimmune disorder triggered by the ingestion of gluten, leading to damage of the small intestinal mucosa and malabsorption
• Colorectal cancer is a malignant tumor that develops in the colon or rectum, often arising from precancerous polyps
  • Regular screening (colonoscopy) is recommended for early detection and prevention
• Gallstones are hardened deposits of bile that can form in the gallbladder, potentially causing obstruction and inflammation (cholecystitis)
• Pancreatitis is inflammation of the pancreas, which can be acute or chronic, and is often associated with gallstones or alcohol abuse