Digestive System

Chapter 21: Digestive System

Key Definitions and Overview

  • Digestive System: Also referred to as the gastrointestinal (GI) system.

  • Digestive Tract: Begins with the oral cavity, including the mouth and pharynx.

    • **Pathway of Food: **

    • Swallowed food travels from oral cavity to esophagus, stomach, small intestine, large intestine, and finally to the rectum.

    • Gastrointestinal Tract: From the stomach to the rectum, referred to as the gastrointestinal tract (GI tract).

Digestion Process

  1. Oral Cavities: Digestion begins here with chewing and the secretion of saliva by salivary glands.

    • Salivary Glands: Include parotid, sublingual, and submandibular glands.

  2. Esophagus: Consists of skeletal muscle in the upper part and smooth muscle in the lower.

  3. Stomach: A bag-like organ with a capacity of up to 2-4 liters of fluid and food. Contains three sections:

    • Fundus

    • Central body

    • Antrum

    • Function in Digestion: Mixes food with acids and enzymes, leading to the formation of chyme.

  4. Large Intestine (Colon): Proximal portion is known as antrum where water and electrolytes are absorbed, turning chyme into semi-solid feces.

    • Rectum: Feces are propelled into the rectum, where stretching of the rectal wall leads to the defecation reflex.

  5. Pylorus: The passage between the stomach and the small intestine, regulated by the pyloric valve, allowing only small amounts of chyme to enter the small intestine at a time.

Digestive Processes

  • Primary Function: The digestive system transfers water, nutrients, and electrolytes from the external environment to the internal environment of the body.

  • Basic Processes:

    • Digestion: The chemical and mechanical breakdown of food moving across the intestinal epithelium.

    • Absorption: Movement from the lumen of the GI tract to the extracellular fluid.

    • Defense Mechanisms: Mucus secretion, digestive enzymes, and fluid movement protect against harmful substances.

    • Motility: Movement facilitated by muscle contractions for digestion and excretion.

Mass Balance

  • Definition: What comes in must also come out; matching fluid input with fluid output.

  • Fluid Flow: An adult's digestive tract handles approximately 9 liters of fluid per day, which includes:

    • 2-3 liters from ingested food and drink.

    • 6-7 liters from various secretions in the digestive system (saliva, gastric, pancreatic secretions).

  • Fluid Absorption: Most fluid is absorbed in the small intestine (7.5 L) and 1.5 L in the large intestine, with minimal excretion in feces.

Motility

  • Definition: Movement of substances through the GI tract.

    • Functions:

    1. Propels food from mouth to anus.

    2. Mechanically breaks down food, aiding digestion.

  • Smooth Muscle: Primary muscle type involved in motility, integrated with the autonomic nervous system.

  • Types of Contractions:

    • Tonic contractions (sustained) found in sphincters and stomach.

    • Phasic contractions (alternating) primarily aid in movement and are linked with autorhythmic cells (interstitial cells of Cajal).

    • Behavior of muscle contractions is linked to the frequency of action potentials (APs).

    • Patterns of contraction:

      • Migrating motor complex (engagement between meals).

      • Peristaltic contractions (push food forward during meals).

      • Segmental contractions (mix food and aid absorption).

Digestive Reflexes

  • Motility and Secretion Regulation: Both are modulated to optimize nutrient absorption and digestion times.

  • Enteric Nervous System (ENS): Regulates gut reflexes, independent of central nervous system (CNS), impacts motility and secretion.

  • Cephalic Phase: Anticipatory response in digestion when food is expected; activates neurons in the medulla leading to increased salivation and gut activity.

The Mouth and Saliva

  • Saliva Functions:

    1. Softens and moistens food.

    2. Initiates starch digestion via salivary amylase (breaks starch into maltose).

    3. Facilitates taste perception.

    4. Provides defense with antibacterial enzymes and immunoglobulins.

  • Swallowing (Deglutition): Reflex action that transfers a bolus into the esophagus.

    • Initiated by pressure from food against soft palate; signals sent through the glossopharyngeal nerve.

    • Involves voluntary (upper esophagus) and involuntary (lower esophagus) muscle control.

Gastric Phase

  • Functions of Stomach: Storage, digestion, and defense.

  • Actions Upon Food Entry:

    • Stomach distension triggers hormone and neurotransmitter release, influencing motility and secretion.

    • Paristaltic waves mix food with gastric acids and enzymes, forming chyme, which is released into the duodenum.

Stomach Wall Structure

  • Gut Wall Layers:

    1. Mucosa (with submucosa and muscularis externa components).

      • Epithelium varies across digestive sections, facilitating nutrient transfer and secretion.

    2. Submucosa contains a nerve network (submucosal plexus).

    3. Muscularis externa made of inner circular and outer longitudinal layers assisting in motor activity.

    4. Serosa serves as the outer covering of the digestive tract.

Gastric Secretions

  • Gastric Mucosa: Contains mucous-producing cells that form gastric pits, which house various secretory cells.

  • Key Secretions:

    • Gastric acid (HCl), enzymes, hormones, and paracrine signaling molecules.

    • G-Cells: Release gastrin, influencing HCl secretion.

    • Parietal cells: Secrete HCl, achieving a stomach pH as low as 1.

    • Chief cells: Secrete pepsinogen (activated to pepsin) and gastric lipase.

Cephalic-Gastric Integration

  • Paracrine Secretions: Histamine, somatostatin, and gastrin are released during the digestive process and modulate gastric functions.

  • Mucus Barrier: Protects gastric mucosa from autodigestion, with a bicarbonate layer neutralizing pH.

Intestinal Phase

  • Chyme's Journey: Once chyme exits the stomach, the intestinal phase begins with limited digestion due to stomach's earlier preparation.

  • Intestinal Mucosa Structure: Projects into the lumen forming villi to enhance absorption. Crypts secrete various molecules necessary for digestion.

  • Hepatic Portal System: Nutrient-rich blood from the intestines is filtered through the liver before general circulation.

Intestinal Secretions

  • Secretions from: Liver, pancreas, and intestinal lining.

    • Bile: Released from gallbladder, emulsifies fats for digestion.

    • Bicarbonate: Neutralizes acidic chyme entering the small intestine What's secreted from the pancreas (NaHCO3 and digestive enzymes).

  • Saline Secretion: Involves Cl- ions moving into intestinal crypts, increasing NaCl and drawing in H2O.

Pancreas Functionality

  • Exocrine and Endocrine Roles: Contains both endocrine cells (insulin, glucagon) and digestive enzyme-secreting exocrine cells.

    • Zymogens are secreted and converted into active enzymes in the intestinal lumen.

  • Activation Mechanism: Entrokinase is important for the activation of trypsin from trypsinogen, facilitating further enzyme activation.

Liver Functions

  • Bile Secretion: Key components include bile salts (for fat absorption), bile pigments, and cholesterol.

  • Bile Pathway: Travels through hepatic duct, stored in gallbladder, and released into the duodenum upon fat digestion.

  • Bile Salts: Amphipathic molecules assisting in emulsification of fats to increase surface area for digestion.

Fat Digestion and Absorption

  • Triglcerides: Main fat form; digested by lipases, separating into fatty acids and monoglycerides.

  • Micelle Formation: Fatty acids and monoglycerides facilitate absorption through the brush border of intestinal cells.

  • Chylomicrons Formation: Far in the intestinal cell combines with cholesterol and proteins, released into lymphatic system.

Carbohydrate Digestion and Absorption

  • Carbohydrate Breakdown: By amylase into smaller chains (e.g. maltose); facilitated by intestinal enzymes.

  • Monosaccharide Absorption: Glucose and galactose absorbed via sodium-dependent transporters while fructose uses GLUT transporters.

Protein Digestion and Absorption

  • Enzymatic Classification on Proteins: Endopeptidases (internal peptide bond breakdown) and exopeptidases (AA removal at ends).

  • Transport Mechanisms: Free amino acids transported via Na-dependent transport systems; some peptides absorbed intact by oligopeptide transporters.

Water and Electrolyte Absorption

  • Absorption Sites: Primarily occurs in small intestine, with lesser amounts in large intestine.

  • Nutrient Absorption and Osmotic Gradient: Water follows nutrients absorption, mainly through paracellular pathways.

Integration of Digestion Phases

  • Gastric and Intestinal Phases: Both regulated to enhance absorption and adjustment of secretions respectively.

  • Role of Chyme: Determines digestion pace based on content, enhancing enzyme secretion through feedback mechanisms.

Large Intestine Overview

  • Chyme Handling: Gradually moves from cecum through colon, where significant water and ion absorption occurs.

  • Peristalsis and Defecation Reflex: Controlled by sphincters, allowing regulated elimination of solid waste.