Digestive System Functions

Functions of the Digestive System

• Ingestion: The process of taking in food and liquids through the mouth.
• Propulsion: The movement of food through the digestive tract, including swallowing and peristalsis.
• Mechanical Digestion: The physical breakdown of food into smaller pieces, including chewing, churning, and segmentation.
• Chemical Digestion: The breakdown of complex molecules into simpler molecules by enzymes, which occurs in the mouth, stomach, and small intestine.
• Absorption: The process of taking nutrients into the bloodstream or lymphatic system from the digestive tract, primarily in the small intestine.
• Defecation: The elimination of indigestible substances and waste products from the body in the form of feces.
• Secretion: The release of digestive juices and enzymes by various organs, such as saliva from salivary glands, gastric juice from the stomach, bile from the liver, and pancreatic juice from the pancreas.
• Regulation: Coordination of the digestive process through neural and hormonal signals to ensure efficient digestion and nutrient absorption.

Organs of the GI Tract and Their Accessory Organs

• GI Tract Organs:
  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine (duodenum, jejunum, ileum)
  • Large intestine (cecum, colon, rectum, anal canal)
• Accessory Organs:
  • Salivary glands (parotid, submandibular, sublingual)
  • Liver
  • Gallbladder
  • Pancreas

Layers of the Alimentary Canal

• Mucosa: Innermost layer, contains epithelium, lamina propria, and muscularis mucosae. Functions in secretion, absorption, and protection.
• Submucosa: Connective tissue layer containing blood vessels, lymphatics, and nerves. Supports the mucosa and allows it to move flexibly during peristalsis.
• Muscularis Externa: Layers of smooth muscle (circular and longitudinal). Responsible for peristalsis and segmentation.
• Serosa (or Adventitia): Outermost layer. The serosa is the visceral peritoneum, and the adventitia is a connective tissue layer (in regions not covered by the peritoneum).

Mechanical vs. Chemical Digestion

• Mechanical Digestion: Physical breakdown of food into smaller pieces, such as chewing, churning, and segmentation.
• Chemical Digestion: Breakdown of food by enzymes into simpler molecules that can be absorbed, such as amylase breaking down starch into sugars.

Peristalsis

• Definition: A series of wave-like muscle contractions that move food through the digestive tract.
• Muscularis contractions move food through the digestive tract.

Myenteric and Submucosal Plexus

• Myenteric Plexus: Located between the layers of the muscularis externa, controls gastrointestinal motility.
• Submucosal Plexus: Found in the submucosa, regulates enzyme secretion, blood flow, and water and ion transport.

Covering of the Organs in the Abdomen

• Peritoneum: Serous membrane covering abdominal organs.
• Visceral Peritoneum: Covers the external surfaces of most digestive organs.
• Parietal Peritoneum: Lines the body wall.
• Small intestine has mesentary that keeps it in place.

Retroperitoneal

• Definition: Organs located behind the peritoneum.
• Examples: Kidneys, Duodenum, pancreas, ascending colon, and descending colon.

Mastication and Deglutition

• Mastication: The process of chewing food, begins the mechanical breakdown of food.
• Deglutition: The act of swallowing, voluntary phase, and involuntary phase.

Salivary Glands

• Types:
  • Parotid glands behind ears
  • Submandibular glands under mandible
  • Sublingual glands under lingual

Esophagus

• Function: Transports food from the pharynx to the stomach using peristalsis.
• Anatomy: A muscular tube lined with stratified squamous epithelium, containing the upper and lower esophageal sphincters.

Stomach

• Functions:
  • Stores food
  • Mixes food with gastric juices to form chyme
  • Begins protein digestion
  • Secretes intrinsic factor for vitamin B12 absorption
  • Regulates passage of food into the small intestine
• Anatomy:
  • Lower Esophageal Sphincter: Prevents acid reflux
  • Body: Main part of the stomach
  • Fundus: Upper part of the stomach
  • Cardia: Area surrounding the opening of the esophagus
  • Pylorus: Region leading to the duodenum of small intestine
  • Rugae: Folds in the stomach lining that allow expansion
  • Pyloric Sphincter: Regulates passage of food from stomach to duodenum

Gastric Juice

• Components:
  • Hydrochloric Acid (HCl): Lowers pH, kills bacteria, activates pepsin
  • Pepsinogen: Inactive form of pepsin, begins protein digestion when activated
  • Mucus neck cells secrete mucus, protects stomach lining from acid and digestive enzymes

Parietal Cells and Intrinsic Factor

• Parietal Cells: Secrete HCl and intrinsic factor
• Intrinsic Factor: Necessary for vitamin B12 absorption in the small intestine, needed for RBC production

Chief Cells

• Secrete pepsinogen, which is activated to pepsin by HCl

Chyme

• Definition: Semi-liquid mixture of partially digested food and gastric juices

Mucosal Barrier in Stomach

• Protects stomach lining from self-digestion by HCl and enzymes

Small Intestine

• Regions:
  • Duodenum
  • Jejunum
  • Ileum
• Functions:
  • Major site of digestion and nutrient absorption
  • Absorbs 90% of nutrients

Plica Circulares, Villi, and Microvilli

• Increase surface area for nutrient absorption
• PLICA: Folds of submucosa
• Villi: Folds of mucosa, contains capillaries and lacteals for absorption of nutrients
• Microvilli: Folds of cell membrane of mucosa for absorption across the microvilli, increases surface tension

Peyer's Patches

• Location: Ileum
• Function: Immune surveillance and response in the small intestine

Large Intestine

• Functions:
  • Absorbs water and electrolytes
  • Forms and stores feces
  • Houses gut microbiota
• Parts:
  • Cecum
  • Appendix
  • Ascending Colon
  • Transverse Colon
  • Descending Colon
  • Sigmoid Colon
  • Rectum
  • Anal Canal
  • Haustra: Pouches of the colon
  • Internal and External Anal Sphincter: Control defecation

Accessory Organs of the Digestive System

• Salivary Glands
• Liver
• Gallbladder
• Pancreas

Role of Liver in Hepatic Portal Circulation

• Processes nutrients and toxins from the digestive tract
• Receives nutrient-rich blood from the digestive organs via the hepatic portal vein
• Processes blood, filtering out toxins and storing nutrients, before releasing purified blood into the general circulation through the hepatic veins

Bile and Fat Digestion

• Bile: Produced by the liver, stored in the gallbladder, emulsifies fats in the small intestine

Gallbladder

• Stores and concentrates bile, releases it into the small intestine

Pancreas as Endocrine and Exocrine Gland

• Endocrine: Secretes insulin and glucagon
• Exocrine: Produces digestive enzymes and buffers (bicarbonate) to neutralize stomach acid

Pancreatic Secretions

• Digestive Enzymes: Break down proteins, fats, and carbohydrates
• Bicarbonate: Neutralizes stomach acid

Functions of Digestive Enzymes

• Salivary and Pancreatic Amylase: Break down starch into sugars
• Lingual, Gastric, and Pancreatic Lipase: Break down lipids
• Pepsin: Breaks down proteins into smaller peptides
• Nucleases: Break down nucleic acids (DNA and RNA) into nucleotides