Digestion

The Digestive System

Overview

  • Chapters: 21 and 22

Mass Balance in the Digestive System

Homeostasis

  • To maintain homeostasis, the volume of fluid entering the GI tract by intake or secretion must equal the volume leaving the lumen.

Fluid Input into the Digestive System

  • Total input into lumen: 9.0 L
      - Ingestion: 2.0 L (food and drink)
      - Secretion:
        - 1.5 L intestinal secretions
        - 1.5 L pancreatic secretions
        - 2.0 L gastric secretions
        - 0.5 L bile (from liver)
        - 1.5 L saliva (from salivary glands)

Fluid Removed from the Digestive System

  • Total removed from lumen: 9.0 L
      - 7.5 L from small intestine
      - 1.4 L from large intestine
      - 0.1 L in feces

Four Processes of the Digestive System

Key Processes

  1. Digestion: Chemical and mechanical breakdown of food into absorbable units.

  2. Absorption: Movement of material from GI lumen to extracellular fluid (ECF).

  3. Motility: Movement of material through the GI tract as a result of muscle contraction.

  4. Secretion: Movement of material from cells into lumen or ECF.

Anatomy of the Digestive System

Components

  • Oral Cavity: Includes salivary glands.

  • Esophagus: Connects throat to stomach.

  • Stomach: Divided into fundus, body, and antrum; ends with pylorus and pyloric valve.

  • Small Intestine: Divided into duodenum, jejunum, and ileum.

  • Large Intestine: Colon and rectum; ends with anus and external anal sphincter.

  • Accessory Organs: Pancreas and liver; gallbladder stores bile.

Anatomy Summary: Salivary Glands

  • Types:
      - Parotid
      - Sublingual
      - Submandibular

Digestive System Structure

Layers of the Gut Wall

  1. Mucosa:
       - Composed of epithelial cells, lamina propria, and muscularis mucosae.
       - Modifications to increase surface area include:
         - Rugae (stomach): Surface folds.
         - Plicae (intestine): Circular folds.
         - Villi (small intestine): Finger-like projections.
         - Gastric glands, crypts, submucosal glands, and Peyer’s patches (GALT).

  2. Submucosa:
       - Connective tissue with the submucosal plexus of the enteric nervous system.

  3. Muscularis Externa:
       - Two layers of smooth muscle containing the myenteric plexus.

  4. Serosa:
       - Continuation of the peritoneal membrane, which forms sheets of mesentery.

Motility in the Digestive System

Types of Contractions

  1. Tonic Contractions: Sustained for minutes or hours, occur in smooth muscle sphincters and anterior stomach, preventing backward movement of the bolus.

  2. Phasic Contractions: Last a few seconds, occur predominantly in the posterior stomach and small intestine. Includes:
       - Peristalsis: Moves the bolus forward.
       - Segmentation: Mixes the bolus without significant forward movement.

  3. Slow-Wave Potentials: Originated by interstitial cells of Cajal, important for the rhythmic contractions.

Secretion in the Digestive System

Secretion Components

  • Digestive enzymes released in the mouth, stomach, and intestine.

  • Mucous cells in stomach, serous cells in salivary glands, and goblet cells in intestine produce mucins.

  • Saliva is an exocrine secretion comprising:
      - Liver (hepatocytes): Secrete bile, which contains bile salts, pigments, and cholesterol.
      - Gallbladder: Stores and concentrates bile.
      - Pancreas: Acinar cells secrete digestive enzymes, duct cells secrete bicarbonate.

The Cephalic Phase of Digestion

Mechanisms of Digestion in the Mouth

  • Chemical and mechanical digestion initiated by:
      - Salivary secretion, which is under autonomic control; helps to soften and lubricate food.
      - Salivary Enzymes: Salivary amylase and some lipase initiate the digestion of carbohydrates and fats, respectively.
      - Saliva also serves a protective function.

  • Mastication: The process of chewing food.

Reflexes in Digestion

Overview of Long and Short Reflexes

  • Long reflexes controlled by the CNS are initiated by sensory stimuli such as sight, smell, or taste of food.

  • Short reflexes occur due to distension or presence of peptides and amino acids, leading to secretion and motility in the stomach.

  • The medulla oblongata integrates swallowing, while mechanical actions push the bolus into the esophagus, triggering the swallowing reflex.

The Gastric Phase

Functions of the Stomach

  1. Storage: Holding food before digestion.

  2. Digestion: Involves acid, enzymes, paracrines, and hormones for breaking down food.

  3. Protection: Against pathogens and harmful substances.

Digestive Hormones

Categories

  1. Gastrin Family:
       - Gastrin (from stomach) and cholecystokinin (CCK) (from intestine).

  2. Secretin Family:
       - Secretin, vasoactive intestinal peptide (VIP), glucose-dependent insulinotropic peptide (GIP), and glucagon-like peptide-1 (GLP-1).

  3. Others:
       - Motilin: secreted from M cells in the small intestine; initiates the migrating motor complex.

Table of Digestive Hormones

  • Gastrin:
      - Stimulus: Peptides and amino acids.
      - Target: ECL cells, parietal cells.
      - Effect: Stimulates secretion of gastric acid and mucosal growth.
      - Inhibited by somatostatin.

  • Cholecystokinin (CCK):
      - Stimulus: Fatty acids and some amino acids.
      - Target: Gallbladder, pancreas, stomach.
      - Effect: Stimulates gallbladder contraction and pancreatic enzyme secretion; inhibits gastric emptying and acid secretion.

Modifications to GLP-1

Amendments to Native GLP-1

  • Enhancements include substitutions at specific positions to alter function and efficacy, promoting insulin secretion and lowering glycogen levels.

Secretion Types and Mechanisms

Components of Gastric Secretion

  • Key Secretion Cells:
      - Mucous Neck Cells: Mucus.
      - Parietal Cells: Secrete gastric acid (HCl) and intrinsic factors for vitamin B12 absorption.
      - Chief Cells: Secrete pepsinogen.
      - D Cells: Secrete somatostatin, which inhibits gastric acid secretion.
      - G Cells: Secrete gastrin, stimulating gastric acid secretion.

Gastric Mucosa Cell Types

  • Essential Functions:
      - Mucous: Protection.
      - Bicarbonate: Buffering of gastric acid.
      - Gastric acid: Digestion and pathogen control.

  

Integration of Digestive Phases

Integration of Cephalic and Gastric Phases

  • The stages of gastric secretion begin with sensory input from food, leading to increased gastrin release and subsequent acid and pepsin secretion in response to food intake.

Intestinal Phase of Digestion

Overview

  • Begins when chyme moves into the duodenum, triggering:
      1. Enzyme and bicarbonate secretion.
      2. Feedback mechanisms to slow gastric digestion and emptying.
      3. Feed-forward mechanisms to initiate insulin secretion.

Key Processes in the Small Intestine

  • Bicarbonate: Neutralizes gastric acid.

  • Goblet Cells: Secrete mucus for protection and lubrication.

  • Bile: Aids in fat digestion, primarily secreted into the small intestine.

Bicarbonate and Enzyme Secretion in the Pancreas

Exocrine Function of the Pancreas

  • Duct Cells: Secrete sodium bicarbonate into the digestive tract.

  • Acinar Cells: Secrete digestive enzymes crucial for digestion.

Activation of Pancreatic Zymogens

  • Inactive Enzymes: Secreted by the pancreas, activated in a cascade through enteropeptidase in the brush border to ensure proper function in digesting proteins.

Hepatic Portal System

  • Function: Most nutrients absorbed by the intestine pass through the liver, which filters potentially harmful substances before reaching systemic circulation.

Components

  • Includes veins that transport nutrients from digestive tract to the liver for processing.

The Role of Villi and Crypts in Absorption

  • Villi and Crypts enhance the surface area for absorption and secretion in the small intestine.

Types of Cells in Villi

  • Enterocytes: Transport nutrients.

  • Goblet Cells: Secrete mucus.

  • Lacteals: Transport fats to lymphatic system.

Digestion and Absorption of Nutrients

Carbohydrates and Proteins

  • Breakdown of carbohydrates into monosaccharides occurs via enzymes like amylase, maltase, sucrase, and lactase.

  • Protein digestion involves endopeptidases and exopeptidases that release individual amino acids or small peptides for absorption.

Digestion and Absorption of Fats

  • Bile Salts: Emulsify fats to enhance absorption.

  • Lipase: Breaks down triglycerides into fatty acids and monoglycerides.

Immune Functions of the GI Tract

  • M Cells: Sample gut contents, triggering immune responses such as cytokine release to attract more immune cells, modulating the inflammatory response.

Regulation of GI Function

Reflexes in Digestion

  • Long Reflexes: Integrated in the CNS, responding to external stimuli.

  • Short Reflexes: Integrated in the enteric nervous system for localized gut responses.

Metabolism and Energy Balance

Overview

  • Climate and Energy Balance: Integrates food intake control, metabolic rate, and stored energy.

Appetite and Satiety Mechanisms

  • Neural Inputs: From cerebral cortex and limbic system; influenced by GI peptide hormones, adipocytokines from adipose tissue, and sensory information.

Theories of Food Intake Regulation

  1. Glucostatic Theory: Hypothalamic glucose metabolism regulates food intake.

  2. Lipostatic Theory: Signals from fat stores regulate eating behavior.

Metabolic Rate Influencing Factors

  • Basal Metabolic Rate (BMR): The body's rate of energy expenditure at rest, influenced by age, gender, muscle mass, activity level, diet, hormones, and genetics.

Energy Utilization and Storage

  • Ingested biomolecules can be converted to energy, used for cellular maintenance, or stored as glycogen and fat.

Control of Metabolism Through Hormones

  • Insulin and Glucagon: Secreted by the pancreas; regulate metabolic activity in fed and fasted states.

Diabetes Mellitus

  • Type 1 and Type 2: Characterized by hyperglycemia due to insufficient insulin. Affected by lifestyle and potential for medication intervention.

Metabolic Syndrome

  • A cluster of conditions increasing heart disease risk, typical features include obesity, hypertension, dyslipidemia, and insulin resistance.

Summary

  • Understanding the intricate functions and interconnections of the digestive system is crucial for recognizing how the body processes food and maintains metabolic balance. This includes an appreciation of the role of hormones, neural control, and the physiological differences in different phases of digestion.