Notes on Digestion: Overview, Processes, and Control

Overview of Digestive System

• The digestive system is divided into two main groups:

  1. Alimentary Canal (GI Tract or Gut):
  • A muscular tube approximately 9 m long, running from the mouth to the anus.
  • Functions:
    • Digests food by breaking it down into smaller fragments.
    • Absorbs these fragments through the lining into the blood.
  • Organs include:
    • Mouth
    • Pharynx
    • Esophagus
    • Stomach
    • Small intestine
    • Large intestine
    • Anus
  1. Accessory Digestive Organs:
  • Includes:
    • Teeth
    • Tongue
    • Gallbladder
    • Digestive glands that produce secretions for food breakdown:
      • Salivary glands
      • Liver
      • Pancreas

Digestive Processes

• The processing of food involves six essential activities:
  1. Ingestion: Eating food.
  2. Propulsion: The movement of food through the alimentary canal.
  • Includes Peristalsis: The major means of propulsion involving alternating contractions and relaxations to push food along.
  1. Mechanical Breakdown:
  • Involves:
    • Chewing (mouth)
    • Mixing food with saliva
    • Churning food in the stomach
    • Segmentation (local constriction of the intestine mixes food with digestive juices).
  1. Digestion: A series of catabolic steps that involve enzymes breaking down complex food molecules into chemical building blocks.
  2. Absorption: The passage of digested fragments from the lumen of the GI tract into blood or lymph (mainly water absorption occurs in the large intestine).
  3. Defecation: The elimination of indigestible substances via the anus as feces.

Control of the Digestive System: Enteric Nervous System

• The GI tract has its own nervous system, known as the Enteric Nervous System (ENS), often referred to as the "gut brain."

  • Key Features:
  • Contains more neurons than the spinal cord.
  • Made up of enteric neurons that communicate extensively with each other.

• Reflex Arcs:

  • Short Reflexes: Mediated by enteric nerve plexuses; they respond to stimuli within the GI tract.
  • Long Reflexes: Involve responses to stimuli inside or outside the gut.
  • The Parasympathetic nervous system enhances digestive processes, while the Sympathetic system inhibits digestion.

Regulation of Digestive Activity

• Three key concepts regulating GI activity:
  1. Stimuli: Digestive activity is provoked by mechanical and chemical stimuli such as stretch, osmolarity, pH, and presence of substrates/end products of digestion.
  2. Effectors: Smooth muscles and glands initiate reflexes that mix and move lumen contents; they may also activate or inhibit digestive glands secreting digestive juices or hormones.
  3. Control Mechanisms:
  • Neurons (intrinsic and extrinsic) and hormones control digestive activity.
  • Hormonal controls involve hormones from stomach and small intestine affecting target cells in same/different organs.
  • Nervous system controls:
    • Intrinsic Controls: Short reflexes via the enteric nervous system.
    • Extrinsic Controls: Long reflexes via the autonomic nervous system.

Mouth and Salivary Glands

• Functions of Saliva:
  • Cleanses the mouth.
  • Dissolves food chemicals for taste.
  • Moistens food and compacts it into a bolus.
  • Begins starch breakdown with the enzyme amylase.
• Composition of Saliva:
  • Mostly water (97–99.5%); slightly acidic (pH 6.75-7.00).
  • Contains electrolytes, salivary amylase, lingual lipase, and components of the immune system (e.g., IgA antibodies, defensins).
• Control of Salivation:
  • Up to 1500 ml/day can be produced.
  • Minor glands keep the mouth continuously moist.
  • Major glands activated by parasympathetic NS through chemoreceptors and mechanoreceptors; sympathetic stimulation inhibits salivation.

Swallowing Process

• The pharynx and esophagus are conduits for food passage from the mouth to the stomach.
• Phases of Swallowing (Deglutition):
  • Buccal Phase: Voluntary phase where the tongue contracts to push food into the oropharynx.
  • Pharyngeal-Esophageal Phase: Involuntary phase, involving:
  • The rise of the soft palate and uvula, blocking the nasopharynx.
  • The larynx rises, and the epiglottis blocks the trachea.
  • Relaxation of the upper esophageal sphincter allows food into the esophagus.
  • Peristalsis: Occurs in the esophagus to move food to the stomach, with the gastroesophageal sphincter closing to prevent regurgitation after food entry.