Digestive System Processes and Regulatory Mechanisms

The Phases of Digestion

Pharyngeal Phase

• The bolus of food triggers receptors in the smooth muscle lining of the pharynx.
• These receptors initiate a contraction, moving the bolus through the pharynx into the esophagus.

Esophageal Phase

• The presence of the bolus in the esophagus triggers receptors in the esophageal lining.
• Peristaltic contractions move the bolus down towards the stomach.
• Glands in the pharynx and esophagus secrete mucus to lubricate the bolus, facilitating smooth transition into the stomach.

Small Intestine

• Food (chyme) enters the small intestine from the stomach, first into the duodenum, then the jejunum, and finally the ileum.

Unique Features of the Small Intestine

1. Circular Folds of the Mucosa:

   • Transverse folds predominantly in the duodenum and jejunum.
   • Increase surface area for absorption.

2. Villi:

   • Finger-like projections on the mucosa that increase surface area.

3. Microvilli:

   • Smaller projections on the surface of intestinal cells (enterocytes).
   • Enterocytes produce brush border enzymes important for carbohydrate and protein digestion.

Large Intestine

• The first part of the large intestine is the cecum, followed by the colon, rectum, and anus.

Major Functions

• Finishing absorption of nutrients missed by the small intestine.
• Reabsorbing water.

Differences from Small Intestine

• Fewer enzyme-secreting cells.
• Fewer or simpler circular folds.
• More enterocytes for water and salt absorption.
• More goblet cells for mucus secretion to ease the movement of feces.

Unique Features

1. Taenia Coli:

   • Longitudinal muscle fibers.

2. Haustra:

   • Pouches or sacculations in the colon.

3. Epiploic Appendages:

   • Small, fat-filled pouches attached to the colon.

GI Tract as Continuation of External Environment

• The GI tract is a continuous tube from the mouth to the anus, including the pharynx, esophagus, stomach, small intestine, and large intestine.

Digestive Processes and Regulatory Mechanisms

Six Processes of the Digestive System

1. Ingestion:

   • Entry of food into the mouth.

2. Propulsion:

   • Voluntary (swallowing) and involuntary (peristalsis) movement of food.
   • Peristaltic contractions: Sequential alternating waves of muscle contraction/relaxation that propel food throughout the GI system (pharynx, esophagus, stomach, small intestine, large intestine, anus).

3. Mechanical Digestion:

   • Physical processes that break down food into smaller pieces without changing its chemical nature.

   • Increases surface area for digestive enzymes.

   • Includes:

     • Chewing (mouth).
     • Churning (stomach): The stomach's oblique muscle layer churns food into chyme.
     • Segmentation contractions (small intestine): Localized contractions of circular muscle that break up chyme and mix it with digestive enzymes.

4. Chemical Digestion:

   • Starts in the mouth with salivary amylase breaking down carbohydrates.
   • Hydrochloric acid in the stomach contributes to chemical digestion.

5. Absorption:

   • Movement of nutrients from the GI lumen into blood vessels.
   • The majority of nutrients and water are absorbed in the small intestine.
   • Water absorption occurs in the large intestine, firming the feces.

6. Defecation:

   • Removal of undigested materials from the body.

Regulatory Mechanisms of Digestion

• Neural and hormonal mechanisms work together to maintain optimal conditions for digestion and absorption.

Neuroregulation

• Nerve plexuses in the GI system interact with the central nervous system to promote reflexes.

  • Long Reflexes:

    • Involve the central and autonomic nervous systems.
    • Respond to stimuli outside the digestive system (e.g., smelling bread triggers salivation).

  • Short Reflexes:

    • Orchestrated by factors intrinsic to the GI system.
    • E.g., food entering the stomach causes distension, triggering mechanoreceptors to increase gastric acid release.

• Parasympathetic nervous system:

  • Increases secretion of gastric juices and stomach motility.
  • Decreases movement through the small intestine (rest and digest).

• Sympathetic nervous system:

  • Inhibits the gastrointestinal system (fight or flight).

• Receptors in GI walls:

  • Chemoreceptors, mechanoreceptors, and osmoreceptors respond to distension, pH changes, and the presence of food.

Hormonal Regulation

• Hormones control the speed at which food passes through the GI system.

  • Gastrin:

    • Secreted by the stomach in response to food.
    • Stimulates gastric acid secretion by parietal cells.

  • Cholecystokinin (CCK) and Secretin:

    • Released by the small intestine (duodenum).
    • Stimulate the release of pancreatic enzymes and bile from the gallbladder.

  • Gastric Inhibitory Peptide:

    • Inhibits gastric secretion, regulating the movement of food from the stomach to the small intestine.

Three Phases of Digestion

1. Cephalic Phase:

   • Reflex phase of gastric secretion, occurring before food enters the stomach.
   • Triggered by sight, smell, or taste of food.
   • The parasympathetic nervous system increases activity through the vagus nerve, stimulating stomach-secreting activity and hydrochloric acid release.

2. Gastric Phase:

   • Triggered by short reflexes and hormones upon food entry into the stomach.
   • Distension activates stretch receptors, stimulating parasympathetic neurons via the vagus nerve to increase gastric juice release.
   • The presence of proteins increases the pH, stimulating gastrin release from G cells, which triggers hydrochloric acid release.

3. Intestinal Phase:

   • Initiated by the entrance of chyme into the small intestine.
   • Distension of the duodenum triggers mechanoreceptors, inhibiting stomach motility via a short reflex.
   • Distension also triggers the pyloric center to contract, stopping food entry into the small intestine.
   • Fatty acids and partially digested foods trigger the release of intestinal hormones, stimulating the release of pancreatic enzymes and bile from the gallbladder.

Absorption Mechanisms

Co-transport with Sodium

• Sodium is transported across the cell membrane, bringing monosaccharides like glucose and galactose with it.
• Once inside the cell, transport proteins facilitate the diffusion of glucose into the interstitial fluid.

Stomach Cells and Secretions

• Cheek Cells: Release pepsinogen (inactive proenzyme).
• Parietal Cells: Release hydrochloric acid in response to gastrin.

Liver Functions and Bile Production

• Hepatocytes filter blood, exchanging nutrients between blood and liver via peg sinusoids.
• Hepatocytes produce about one liter of bile per day, secreted into canaliculi, which then flows into bile ducts.
• Bile release from the gallbladder is important for fat digestion.

Metabolic Processes

Metabolism:

• The sum of every chemical reaction happening inside the body, involved in breaking down food and converting that food into energy and smaller building blocks, as well as using that energy and those smaller building blocks to produce more tissues and organs around the body.

• Scientists categorize these reactions into:

Catabolic Reactions (Catabolism)

• Breaking down macromolecules into smaller components, producing energy.

Anabolic Reactions (Anabolism)

• Using energy from catabolism and building blocks to build tissues or organs.

Carbohydrate Metabolism

• Breakdown of carbs into monosaccharides like glucose.
• Glycolysis: Breakdown of glucose to form energy.

Lipid Metabolism

• Lipids are stored as triglycerides.
• Catabolic reactions break down triglycerides into energy (ATP).

Protein Metabolism (Proteolysis)

• Breakdown of proteins into smaller peptides or amino acids.
• Occurs during digestion and in situations of extended starvation.

Anabolic Reactions of Carbohydrates

• Glycogenesis: Formation of glycogen from glucose for storage.
• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources (amino acids, lactate).

Insulin Release

• Beta cells in the pancreas store insulin.
• When blood glucose levels rise, beta cells release insulin into the bloodstream.
• Insulin facilitates the removal of glucose from the blood into cells.
• Parasympathetic activity can also trigger insulin release.