The Basics of Digestion

Major Organs of Digestion and Their Functions

The digestive system is composed of several organs, each with specialized roles in breaking down food and absorbing nutrients.

• Mouth: Begins mechanical digestion (chewing) and chemical digestion (salivary amylase breaks down carbohydrates).

• Esophagus: Transports food from the mouth to the stomach via peristalsis.

• Stomach: Mixes food with gastric juices, initiates protein digestion with pepsin, and forms chyme.

• Small Intestine: Main site for digestion and absorption; enzymes from the pancreas and bile from the liver aid in nutrient breakdown.

• Large Intestine: Absorbs water and electrolytes; forms and stores feces.

• Liver: Produces bile, processes nutrients, detoxifies substances.

• Gall Bladder: Stores and concentrates bile, releases it into the small intestine.

• Pancreas: Secretes digestive enzymes and bicarbonate into the small intestine; regulates blood glucose via insulin and glucagon.

Example: The pancreas releases amylase to digest carbohydrates, lipase for fats, and proteases for proteins.

Sphincters

Sphincters are muscular rings that regulate the passage of food and prevent backflow in the digestive tract.

• Lower Esophageal Sphincter (LES): Prevents stomach contents from refluxing into the esophagus.

• Pyloric Sphincter: Controls the release of chyme from the stomach to the small intestine.

• Ileocecal Valve: Regulates flow from the small to the large intestine.

Peristalsis

Peristalsis is the coordinated, rhythmic contraction of smooth muscles that propels food through the digestive tract.

• Occurs in the esophagus, stomach, and intestines.

• Essential for moving food and mixing digestive contents.

Mechanical vs. Chemical Digestion

Digestion involves both mechanical and chemical processes:

• Mechanical Digestion: Physical breakdown of food (chewing, churning in stomach).

• Chemical Digestion: Enzymatic breakdown of macromolecules into absorbable units.

Example: Salivary amylase chemically digests starch in the mouth, while chewing is mechanical digestion.

Digestive Hormones

Hormones regulate digestive processes:

• Gastrin: Stimulates secretion of gastric acid in the stomach.

• Ghrelin: Signals hunger to the brain, stimulates appetite.

• Cholecystokinin (CCK): Stimulates gall bladder contraction and pancreatic enzyme secretion.

Bicarbonate

Bicarbonate is secreted by the pancreas to neutralize acidic chyme entering the small intestine from the stomach.

Bolus and Chyme

• Bolus: The mass of chewed food mixed with saliva, swallowed into the esophagus.

• Chyme: The semi-liquid mixture of partially digested food and gastric juices in the stomach and small intestine.

Digestive Enzymes

• Amylase: Breaks down carbohydrates (salivary and pancreatic forms).

• Lipase: Digests fats (mainly from the pancreas).

• Proteases: Break down proteins (e.g., trypsin, chymotrypsin from the pancreas).

• Pepsin: Initiates protein digestion in the stomach.

Hydrochloric Acid (HCl)

Secreted by parietal cells in the stomach, HCl denatures proteins, activates pepsin, and kills pathogens.

Villi and Microvilli

The small intestine is lined with villi and microvilli to increase surface area for nutrient absorption.

• Villi: Finger-like projections containing blood and lymph vessels.

• Microvilli: Tiny projections on villi cells, forming the "brush border" with digestive enzymes.

Liver, Gall Bladder, and Pancreas Importance

• Liver: Produces bile, processes absorbed nutrients, detoxifies chemicals.

• Gall Bladder: Stores and releases bile for fat digestion.

• Pancreas: Secretes digestive enzymes and hormones (insulin, glucagon).

Insulin and Glucagon

These pancreatic hormones regulate blood glucose levels:

• Insulin: Lowers blood glucose by promoting cellular uptake and storage as glycogen.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown and glucose release from the liver.

Equation:

Absorptive Processes

Nutrients are absorbed in the small intestine via:

• Passive diffusion: Movement from high to low concentration without energy.

• Facilitated diffusion: Uses carrier proteins, no energy required.

• Active transport: Requires energy (ATP) to move nutrients against a gradient.

Role of Body Systems in Digestion and Absorption

• Circulatory System: Transports absorbed nutrients throughout the body.

• Lymphatic System: Absorbs and transports fats and fat-soluble vitamins.

• Nervous System: Regulates digestive tract motility and secretions.

• Endocrine System: Releases hormones that control digestion and metabolism.

Gut Microbiome, Probiotics, and Prebiotics

• Gut Microbiome: The community of microorganisms in the digestive tract, important for digestion, immunity, and health.

• Probiotics: Live beneficial bacteria found in foods (e.g., yogurt) or supplements.

• Prebiotics: Non-digestible food components (e.g., fiber) that promote growth of beneficial gut bacteria.

Digestive Disorders

• Heartburn/GERD: Acid reflux into the esophagus causing discomfort; chronic form is gastroesophageal reflux disease (GERD).

• Gastroenteritis: Inflammation of the stomach and intestines, often due to infection.

• Crohn’s Disease: Chronic inflammatory bowel disease affecting any part of the GI tract.

• IBS (Irritable Bowel Syndrome): Functional disorder causing abdominal pain, bloating, and altered bowel habits.

• Celiac Disease: Autoimmune reaction to gluten (a protein in wheat, barley, rye) damaging the small intestine.

• Gluten: Protein found in wheat and related grains; triggers immune response in celiac disease.

Nutritional Genomics

Nutritional genomics studies the interaction between genes and nutrients, aiming to personalize nutrition for optimal health.

• Includes nutrigenetics (how genes affect nutrient response) and nutrigenomics (how nutrients affect gene expression).

Summary Table: Digestive Enzymes and Their Functions

Additional info: Academic context and definitions have been expanded for clarity and completeness.