Digestive System

• The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste.
• o Ingestion: Taking in food.

  o Digestion: Breakdown of food into smaller molecules.

  o Absorption: Nutrients are absorbed into the bloodstream.

  o Defecation: Elimination of indigestible substances and waste
• Structure of the Digestive System

  • Alimentary Canal (Gastrointestinal Tract):

  • Accessory Organs:
• Mouth: Entry point for food; teeth mechanically break down food, and saliva contains enzymes (amylase) that begin carbohydrate digestion.
• Esophagus: Tube that connects the pharynx to the stomach. Uses peristalsis (rhythmic contractions) to move food.
• o Pharynx: Muscular passage that connects the mouth to the esophagus.
• Stomach: J-shaped organ that stores food, secretes gastric juices (including hydrochloric acid and pepsin), and begins protein digestion.
• Small Intestine: Consists of three parts—duodenum, jejunum, and ileum. The site of most digestion and nutrient absorption.
• Large Intestine (Colon): Absorbs water, electrolytes, and some vitamins; forms and stores feces.
• Rectum and Anus: Store and eliminate waste
• • Accessory Organs: Assist in digestion but are not part of the alimentary canal.
• Salivary Glands: Produce saliva, which contains enzymes like amylase for starch digestion.
• o Liver: Produces bile, which emulsifies fats and aids in fat digestion.
• Gallbladder: Stores bile produced by the liver and releases it into the small intestine to aid in fat digestion
• Pancreas: Produces digestive enzymes (lipase, amylase, proteases) and bicarbonate to neutralize stomach acid entering the small intestine.
• Digestion Processes

  • Mechanical Digestion:

  • Chemical Digestion:
• Mechanical Digestion:

  o Chewing (mastication) in the mouth breaks down food into smaller pieces.

  o Churning and mixing in the stomach help break down food into a semi-liquid form called chyme.

  o In the small intestine, peristalsis and segmentation mix and move food.
• Chemical Digestion:

  o Carbohydrates: Amylase (from saliva and pancreas) breaks down starch into sugars (maltose, glucose).

  o Proteins: Pepsin (in the stomach) and proteases (from the pancreas) break proteins down into amino acids.

  o Lipids: Bile emulsifies fats, and lipases (from the pancreas) break fats down into fatty acids and glycerol.

  o Nucleic Acids: Nucleases (from the pancreas) break down nucleic acids into nucleotides.
• Nutrients absorbed:

  ▪ Carbohydrates: Simple sugars like glucose.

  ▪ Proteins: Amino acids.

  ▪ Lipids: Fatty acids and glycerol.

  ▪ Vitamins and Minerals: Various vitamins and minerals are absorbed along the small intestine.
• Absorption of Nutrients

  • Small Intestine: The majority of nutrient absorption occurs here.

  o The inner wall is lined with villi and microvilli to increase surface area for absorption.

  o Nutrients are absorbed into the bloodstream via capillaries and into the lymphatic system via lacteals
• . Motility of the Digestive Tract

  • Peristalsis: Rhythmic contractions of smooth muscle that propel food through the digestive tract.

  • Segmentation: Localized contractions that mix food and digestive juices, primarily in the small intestine.

  • Sphincters: Circular muscles that regulate the passage of food between different sections of the digestive system (e.g., lower esophageal sphincter, pyloric sphincter, ileocecal valve, anal sphincters).
• Control of Digestion

  • Nervous Regulation:

  o The enteric nervous system (ENS), also known as the "gut brain," regulates digestive processes through reflexes.

  o The vagus nerve stimulates the digestive system, particularly during the cephalic phase (before food enters the stomach).
• Control of Digestion

  Hormonal Regulation:

  o Gastrin: Stimulates the secretion of gastric juices in the stomach.

  o Secretin: Stimulates the pancreas to release bicarbonate to neutralize stomach acid.

  o Cholecystokinin (CCK): Stimulates the release of bile from the gallbladder and digestive enzymes from the pancreas.

  o GIP (Gastric Inhibitory Peptide): Inhibits gastric motility and secretion
• Liver Function in Digestion

  • Bile Production: The liver produces bile, which emulsifies fats and enhances digestion and absorption of lipids.

  • Storage and Detoxification: The liver stores glucose as glycogen, detoxifies harmful substances, and processes nutrients absorbed from the intestines.
• Gastroesophageal Reflux Disease (GERD): Acid from the stomach flows back into the esophagus, causing heartburn and damage to the esophageal lining.
• Peptic Ulcers: Sores in the lining of the stomach or duodenum, often caused by Helicobacter pylori infection or excessive use of NSAIDs.
• Irritable Bowel Syndrome (IBS): A functional disorder causing abdominal pain, bloating, and changes in bowel movements.
• Celiac Disease: An autoimmune disorder in which the ingestion of gluten causes damage to the small intestine lining
• • Crohn’s Disease: A chronic inflammatory bowel disease (IBD) that affects the gastrointestinal tract, leading to abdominal pain, diarrhea, and malnutrition.
• Gallstones: Solid particles that form in the gallbladder and can obstruct bile flow, leading to pain and possible inflammation of the gallbladder (cholecystitis).
• Hepatitis: Inflammation of the liver, often due to viral infections, alcohol abuse, or autoimmune disorders.
• Ingestion is the entry of food, which is mechanically and chemically broken down in the mouth, stomach, and small intestine.
• • Nutrients are absorbed primarily in the small intestine, where digestion is completed.
• • The liver and pancreas provide essential digestive secretions (bile, enzymes).
• The digestive system is regulated by the nervous system and hormones, ensuring the efficient breakdown and absorption of nutrients.
• Waste products are eliminated through the large intestine and eventually the rectum