🫁Honors Anatomy and Physiology Unit 13 – Digestive System

Key Digestive Organs and Structures

• Mouth initiates the digestive process through chewing (mastication) and salivary enzymes
  • Teeth mechanically break down food into smaller pieces
  • Tongue manipulates food and aids in swallowing (deglutition)
• Esophagus connects the pharynx to the stomach, using peristaltic contractions to move food
• Stomach stores and breaks down food through muscular contractions and gastric secretions
  • Rugae, folds in the stomach lining, allow for expansion and contraction
  • Gastric glands secrete hydrochloric acid (HCl) and pepsinogen
• Small intestine, divided into duodenum, jejunum, and ileum, is the primary site of nutrient absorption
  • Villi and microvilli increase surface area for efficient absorption
• Large intestine (colon) absorbs water and electrolytes, forming and storing feces
• Liver produces bile, which emulsifies fats and aids in their digestion
• Gallbladder stores and concentrates bile, releasing it into the duodenum when needed
• Pancreas secretes digestive enzymes and bicarbonate to neutralize stomach acid

Digestive System Functions

• Ingestion involves taking food into the mouth for processing
• Mechanical digestion breaks down food into smaller pieces through chewing and stomach contractions
• Chemical digestion uses enzymes to break down macronutrients (carbohydrates, proteins, and fats)
  • Salivary amylase begins carbohydrate digestion in the mouth
  • Pepsin initiates protein digestion in the stomach
  • Pancreatic enzymes (lipase, amylase, trypsin) continue digestion in the small intestine
• Absorption moves nutrients from the digestive tract into the bloodstream and lymphatic system
  • Villi and microvilli in the small intestine maximize surface area for absorption
• Secretion releases digestive enzymes, mucus, and other substances into the digestive tract
• Motility propels food through the digestive system using smooth muscle contractions (peristalsis)
• Elimination removes undigested material (feces) from the body through defecation

Stages of Digestion

• Oral phase occurs in the mouth, involving chewing and salivary enzyme action
  • Saliva moistens food and contains salivary amylase to begin carbohydrate digestion
• Gastric phase takes place in the stomach, where food is mixed with gastric secretions
  • Stomach churning (peristalsis) mechanically breaks down food
  • Pepsin and hydrochloric acid (HCl) initiate protein digestion
• Intestinal phase begins in the duodenum and continues through the small intestine
  • Pancreatic enzymes and bile enter the duodenum to further digest macronutrients
  • Brush border enzymes on the surface of enterocytes complete digestion
• Absorption phase occurs primarily in the small intestine, with some absorption in the large intestine
  • Nutrients pass through the intestinal wall and enter the bloodstream or lymphatic system
• Elimination phase removes undigested material from the body as feces
  • Peristalsis moves waste through the large intestine
  • Defecation expels feces from the rectum through the anus

Enzymes and Chemical Processes

• Salivary amylase (ptyalin) begins carbohydrate digestion in the mouth, breaking down starch into maltose
• Pepsin, activated by hydrochloric acid (HCl) in the stomach, initiates protein digestion
  • Pepsinogen, the inactive form, is converted to active pepsin in the presence of HCl
• Pancreatic enzymes continue digestion in the small intestine
  • Pancreatic amylase further digests carbohydrates into disaccharides
  • Trypsin, chymotrypsin, and carboxypeptidase break down proteins into peptides
  • Pancreatic lipase digests fats (triglycerides) into fatty acids and monoglycerides
• Brush border enzymes on the surface of enterocytes complete digestion
  • Maltase, sucrase, and lactase break down disaccharides into monosaccharides
  • Aminopeptidases and dipeptidases break down peptides into amino acids
• Bile, produced by the liver and stored in the gallbladder, emulsifies fats to increase surface area for lipase action

Nutrient Absorption

• Carbohydrates are absorbed as monosaccharides (glucose, fructose, galactose) via specific transporters
  • Glucose and galactose use the sodium-glucose cotransporter (SGLT1)
  • Fructose uses the glucose transporter (GLUT5)
• Proteins are absorbed as amino acids via active transport and facilitated diffusion
  • Sodium-dependent amino acid transporters (e.g., B0AT1) use the sodium gradient
  • Sodium-independent transporters (e.g., LAT1) facilitate diffusion
• Fats are absorbed as monoglycerides and fatty acids, which form micelles with bile salts
  • Micelles transport lipids to the brush border, where they diffuse into enterocytes
  • In enterocytes, monoglycerides and fatty acids are reassembled into triglycerides
  • Triglycerides combine with proteins to form chylomicrons, which enter the lymphatic system
• Vitamins and minerals are absorbed through various mechanisms
  • Fat-soluble vitamins (A, D, E, K) are absorbed with dietary fats
  • Water-soluble vitamins (B-complex, C) are absorbed via specific transporters or passive diffusion
  • Minerals (e.g., calcium, iron) use specific transporters or ion channels

Common Digestive Disorders

• Gastroesophageal reflux disease (GERD) occurs when stomach acid frequently flows back into the esophagus
  • Symptoms include heartburn, chest pain, and difficulty swallowing
• Peptic ulcers are open sores in the lining of the stomach or duodenum
  • Caused by Helicobacter pylori (H. pylori) infection or long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs)
• Inflammatory bowel disease (IBD) includes Crohn's disease and ulcerative colitis
  • Characterized by chronic inflammation of the digestive tract
  • Symptoms include abdominal pain, diarrhea, and weight loss
• Celiac disease is an autoimmune disorder triggered by gluten consumption
  • Leads to damage of the small intestine lining and impaired nutrient absorption
• Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder affecting bowel motility
  • Symptoms include abdominal pain, bloating, and changes in bowel habits (diarrhea or constipation)
• Gallstones are hardened deposits of bile that can cause pain and obstruct the bile ducts
  • Risk factors include obesity, rapid weight loss, and high-fat diets
• Diverticulosis involves the formation of small pouches (diverticula) in the colon wall
  • Diverticulitis occurs when these pouches become inflamed or infected

Maintaining Digestive Health

• Eat a balanced diet rich in fiber, fruits, vegetables, and whole grains
  • Fiber promotes regular bowel movements and maintains colon health
  • Fruits and vegetables provide essential vitamins, minerals, and antioxidants
• Stay hydrated by drinking adequate water throughout the day
  • Water helps maintain proper digestive function and prevents constipation
• Engage in regular physical activity to promote digestive motility and maintain a healthy weight
• Manage stress through relaxation techniques, as stress can exacerbate digestive issues
  • Deep breathing, meditation, and yoga can help reduce stress
• Avoid excessive alcohol consumption and smoking, which can irritate the digestive tract
• Maintain good oral hygiene by brushing and flossing regularly
  • Oral bacteria can contribute to digestive issues if left unchecked
• Eat slowly and chew food thoroughly to facilitate proper digestion
• Listen to your body and seek medical attention for persistent digestive symptoms

Digestive System Fun Facts

• The digestive system is approximately 30 feet long, spanning from the mouth to the anus
• The stomach can expand to hold up to 4 liters of food and liquid
• The small intestine is the longest part of the digestive system, measuring about 20 feet in length
  • Despite its name, the small intestine is three times longer than the large intestine
• The surface area of the small intestine is about 250 square meters, roughly the size of a tennis court
  • This large surface area is due to the presence of villi and microvilli
• The liver is the largest internal organ and the only one capable of regeneration
• The gallbladder can store up to 50 mL of bile, which is released when fatty foods enter the duodenum
• The human body produces about 1.5 liters of saliva per day
  • Saliva not only moistens food but also contains antibacterial properties
• The digestive system is home to trillions of bacteria, known as the gut microbiome
  • These bacteria play crucial roles in digestion, immunity, and overall health
• The average person produces about 1-2 liters of gas per day, primarily due to bacterial fermentation in the colon