Anatomy and Physiology I Unit 23 ReviewThe Digestive System

Overview and Function

• The digestive system consists of the gastrointestinal tract (GI tract) and accessory organs that work together to break down food, absorb nutrients, and eliminate waste
• Ingested food undergoes mechanical and chemical digestion to convert it into absorbable nutrients (glucose, amino acids, fatty acids)
• The digestive process involves six main functions: ingestion, propulsion, mechanical breakdown, chemical digestion, absorption, and defecation
• Digestion begins in the mouth with mastication (chewing) and ends with the elimination of feces through the anus
• The digestive system is regulated by hormones, the nervous system, and the enteric nervous system (ENS) to coordinate its functions
  • The ENS is a subdivision of the autonomic nervous system that controls the GI tract
  • Hormones like gastrin, secretin, and cholecystokinin (CCK) regulate digestive processes

Anatomy of the Digestive Tract

• The digestive tract is a continuous tube extending from the mouth to the anus, consisting of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine
• The oral cavity contains the teeth, tongue, and salivary glands, which initiate mechanical and chemical digestion
• The pharynx is a common passageway for both the digestive and respiratory systems, connecting the oral cavity to the esophagus and larynx
• The esophagus is a muscular tube that propels food from the pharynx to the stomach through peristalsis
  • The lower esophageal sphincter (LES) controls the passage of food into the stomach and prevents reflux
• The stomach is a J-shaped organ that stores, mixes, and digests food with the help of gastric secretions
  • The stomach has four main regions: the cardia, fundus, body, and pylorus
• The small intestine is divided into three segments: the duodenum, jejunum, and ileum, where most digestion and absorption occur
• The large intestine consists of the cecum, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal, primarily responsible for water absorption and fecal storage

Key Digestive Organs

• The liver is the largest gland in the body, producing bile, which emulsifies fats and aids in their digestion and absorption
  • The liver also plays a crucial role in metabolism, detoxification, and nutrient storage
• The gallbladder is a small, pear-shaped organ that stores and concentrates bile produced by the liver, releasing it into the duodenum when needed
• The pancreas is a mixed gland with both endocrine and exocrine functions
  • The exocrine portion secretes pancreatic juice containing digestive enzymes (lipase, amylase, trypsin) into the duodenum
  • The endocrine portion (islets of Langerhans) secretes hormones like insulin and glucagon to regulate blood sugar levels
• The salivary glands (parotid, submandibular, and sublingual) secrete saliva, which contains enzymes (salivary amylase) and lubricates food for easier swallowing
• The appendix is a small, finger-like projection attached to the cecum, which may play a role in maintaining gut flora and immune function

Digestive Processes

• Mastication (chewing) in the oral cavity mechanically breaks down food into smaller particles, increasing surface area for enzymatic action
• Deglutition (swallowing) involves the coordinated action of the tongue, pharynx, and esophagus to move food from the mouth to the stomach
• Peristalsis is a series of wave-like muscle contractions that propel food through the digestive tract
• Segmentation contractions in the small intestine mix food with digestive secretions and increase contact with the absorptive surface
• Chemical digestion involves the action of enzymes secreted by the salivary glands, stomach, pancreas, and small intestine to break down macronutrients
  • Carbohydrates are digested by salivary and pancreatic amylase into disaccharides, then further broken down by brush border enzymes (lactase, sucrase, maltase)
  • Proteins are denatured by stomach acid and digested by pepsin, trypsin, and chymotrypsin into peptides and amino acids
  • Fats are emulsified by bile and digested by pancreatic lipase into fatty acids and monoglycerides
• Absorption occurs primarily in the small intestine, where nutrients are transported across the intestinal wall into the bloodstream or lymphatic system
• Defecation is the process of eliminating undigested food and waste products from the body through the rectum and anus

Enzymes and Secretions

• Saliva contains salivary amylase, which initiates the digestion of carbohydrates, and mucus, which lubricates food
• Gastric juice secreted by the stomach contains hydrochloric acid (HCl) and pepsinogen
  • HCl denatures proteins and activates pepsinogen to pepsin, which digests proteins
  • Intrinsic factor, secreted by parietal cells, is essential for vitamin B12 absorption
• Pancreatic juice contains a variety of enzymes, including amylase (carbohydrates), lipase (fats), and trypsin and chymotrypsin (proteins)
• Bile, produced by the liver and stored in the gallbladder, emulsifies fats and facilitates their digestion and absorption
• Brush border enzymes, such as lactase, sucrase, and maltase, are located on the surface of small intestine epithelial cells and complete carbohydrate digestion
• Enterokinase, secreted by the duodenum, activates trypsinogen to trypsin, which then activates other pancreatic enzymes

Absorption and Nutrient Transport

• Absorption primarily occurs in the small intestine, where nutrients are transported across the intestinal wall into the bloodstream or lymphatic system
• The small intestine has a large surface area due to the presence of villi and microvilli, which increase the absorptive capacity
• Simple diffusion allows the passive movement of small, non-polar molecules (water, vitamins, minerals) across the intestinal wall
• Facilitated diffusion uses carrier proteins to transport specific molecules (glucose, amino acids) down their concentration gradient
• Active transport requires energy (ATP) to move molecules (calcium, iron) against their concentration gradient
• Glucose and amino acids are absorbed into the bloodstream and transported to the liver via the hepatic portal vein
• Fatty acids and monoglycerides are reassembled into triglycerides and packaged into chylomicrons, which enter the lymphatic system before joining the bloodstream

Common Disorders and Diseases

• Gastroesophageal reflux disease (GERD) occurs when the lower esophageal sphincter (LES) fails to prevent stomach acid from flowing back into the esophagus, causing heartburn and damage to the esophageal lining
• Peptic ulcers are open sores that develop in the lining of the stomach or duodenum, often caused by Helicobacter pylori infection or excessive use of nonsteroidal anti-inflammatory drugs (NSAIDs)
• Inflammatory bowel disease (IBD) includes Crohn's disease and ulcerative colitis, which cause chronic inflammation and damage to the digestive tract
• Celiac disease is an autoimmune disorder triggered by the ingestion of gluten, leading to damage of the small intestine's lining and malabsorption of nutrients
• Gallstones are hardened deposits of bile that can cause pain, inflammation, and obstruction of the bile ducts
• Pancreatitis is inflammation of the pancreas, which can be acute or chronic, leading to pain, digestive problems, and potential complications
• Colorectal cancer is a malignant growth in the colon or rectum, often developing from precancerous polyps
• Hemorrhoids are swollen veins in the rectum or anus, causing discomfort, itching, and bleeding

Clinical Applications

• Endoscopy is a diagnostic procedure that uses a flexible tube with a camera (endoscope) to visualize the inside of the digestive tract, allowing for the detection of abnormalities, biopsy, and minor surgical procedures
  • Examples include esophagogastroduodenoscopy (EGD), colonoscopy, and capsule endoscopy
• Barium swallow and barium enema are imaging tests that use barium contrast to visualize the upper and lower digestive tract, respectively, helping to diagnose structural abnormalities
• Fecal occult blood tests (FOBT) and fecal immunochemical tests (FIT) are used to screen for colorectal cancer by detecting hidden blood in the stool
• Proton pump inhibitors (PPIs) and H2 receptor antagonists are medications that reduce stomach acid production, used to treat GERD and peptic ulcers
• Probiotics are beneficial bacteria that can be consumed to maintain a healthy gut flora balance, potentially alleviating digestive issues and supporting immune function
• Bariatric surgery, such as gastric bypass or sleeve gastrectomy, is a weight-loss procedure that alters the digestive tract to limit food intake and absorption, used to treat severe obesity and related comorbidities
• Fecal microbiota transplantation (FMT) involves the transfer of healthy donor feces to the gut of a patient with a dysbiotic or diseased gut microbiome, such as in cases of recurrent Clostridium difficile infection