Digestive Physiology: Study Guide for College ANP Students

Digestive Physiology

Primary and Secondary Digestive Organs

The digestive system consists of primary organs, which form the alimentary canal, and secondary organs, which assist in digestion but are not part of the canal.

• Primary digestive organs: Mouth, esophagus, stomach, small intestine, large intestine.

• Secondary digestive organs: Salivary glands, liver, pancreas, gallbladder.

• Example: The pancreas is a secondary organ that secretes digestive enzymes into the small intestine.

Mouth

The mouth is the entry point for food and initiates both mechanical and chemical digestion.

• Digestive processes: Mastication (chewing), mixing food with saliva.

• Enzymes: Salivary amylase (breaks down starch), lingual lipase (begins fat digestion).

• Deglutition: The process of swallowing, moving food from the mouth to the esophagus.

Esophagus

The esophagus transports food from the mouth to the stomach via coordinated muscular contractions.

• Digestive role: Propulsion of food by peristalsis; no significant chemical digestion occurs here.

Stomach

The stomach is a muscular organ that continues mechanical and chemical digestion, producing chyme.

• Gastroesophageal sphincter: Prevents reflux of stomach contents into the esophagus.

• Functions: Storage, mixing, and breakdown of food; secretion of gastric juices.

• Gastric Mucosal Barrier (GMB): Protects stomach lining from acidic environment.

  • Significance: Prevents self-digestion.

  • What can pass through: Alcohol, some drugs.

  • Physical risks if injured: Ulcers, bleeding.

• Role of HCl: Denatures proteins, activates pepsinogen, kills bacteria.

• Contractile activity: Mixing and propulsion of chyme.

• Important cells:

  • Parietal cells: Secrete HCl and intrinsic factor (IF).

  • Chief cells: Secrete pepsinogen.

  • Paracrine cells: Secrete histamine and other signaling molecules.

• Regulation of stomach activity: Neural and hormonal mechanisms.

• Intrinsic Factor (IF): Essential for vitamin B12 absorption in the small intestine.

• Phases of gastric activity:

  • Cephalic phase: Triggered by sight, smell, or thought of food.

  • Gastric phase: Initiated by food in the stomach.

  • Intestinal phase: Begins as chyme enters the small intestine.

• Peristalsis vs. Segmentation:

  • Peristalsis: Propels food forward.

  • Segmentation: Mixes food with digestive juices.

• Regulation of gastric emptying: Controlled by neural and hormonal signals, meal composition affects timing.

Small Intestine

The small intestine is the primary site for digestion and absorption of nutrients.

• Main digestive roles: Enzymatic digestion, absorption of nutrients.

• Arrival of chyme: Triggers release of digestive enzymes and hormones.

• Bile:

  • Origin: Liver.

  • Storage site: Gallbladder.

  • Stimulus for secretion: Presence of fatty chyme in duodenum.

  • Role: Emulsifies fats for digestion.

• Cholecystokinin (CCK): Stimulates release of bile and pancreatic enzymes.

• Protein digestion and absorption:

  • Three major inactive proteases: Trypsinogen, chymotrypsinogen, procarboxypeptidase.

  • Activating enzyme: Enteropeptidase.

  • Active proteases: Trypsin, chymotrypsinv, carboxypeptidase.

  • Brush border proteases: Complete protein digestion at the epithelial surface.

• Carbohydrate digestion and absorption:

  • Major enzymes: Pancreatic amylase, brush border enzymes (e.g., maltase, sucrase, lactase).

  • Examples of sugars: Maltose, sucrose, lactose.

• Fat digestion:

  • Key enzymes: Pancreatic lipase.

  • Ideal pH: Slightly alkaline (pH 7-8).

  • Reabsorption difference: Fats are absorbed into lymphatics (lacteals), proteins and carbs into blood.

  • Micelles: Transport lipids to intestinal cells.

  • Chylomicrons: Lipoprotein particles formed in intestinal cells, transported via lacteals.

  • Lacteals: Specialized lymphatic vessels for fat absorption.

Large Intestine

The large intestine absorbs water and electrolytes, forms feces, and houses gut microbiota.

• Digestive roles: Water absorption, formation of feces, bacterial fermentation.

Motility

Motility refers to the movement of contents through the digestive tract, regulated by neural and hormonal signals.

• Regulation of chyme movement: Controlled by feedback from the duodenum.

• Meal type impact: High-fat meals delay gastric emptying.

• Ileocecal valve: Controls passage from small to large intestine.

• Haustral contractions: Segmental movements in the colon.

• Mass movements: Powerful contractions moving feces toward rectum.

• Defecation reflex: Initiated by rectal distension, leading to expulsion of feces.