The Basics of Digestion: Structure, Function, and Disorders

Chapter 3: The Basics of Digestion

Learning Outcomes

This chapter covers the fundamental processes of digestion, the organs involved, the roles of enzymes, hormones, and bile, the absorption and transport of nutrients, the influence of body systems, and common digestive disorders.

• Define digestion and its processes.

• Describe digestive organs and their functions.

• Explain enzymes, hormones, and bile in digestion.

• Describe absorption mechanisms.

• Explain nutrient transport via circulatory and lymphatic systems.

• Describe regulatory systems (nervous, endocrine, genetic).

• Identify common digestive disorders and their symptoms.

What Is Digestion, and Why Is It Important?

Definition and Overview

Digestion is a multistep process that breaks down foods into absorbable components using both mechanical and chemical means within the gastrointestinal (GI) tract.

• The GI tract is approximately 30 feet long.

• Cells lining the GI tract function for a few days before shedding into the lumen (the interior space of the intestine).

• Main organs: Mouth, Esophagus, Stomach, Small and Large Intestines

• Accessory organs: Pancreas, Liver, Gallbladder

Main Roles of the GI Tract

• Break down food into smallest components

• Absorb nutrients

• Prevent entry of harmful microorganisms

• Maintain the gut microbiome

Digestion: Mechanical and Chemical Processes

Mechanical Digestion

Mechanical digestion involves physical breakdown and movement of food.

• Chewing and grinding in the mouth

• Segmentation: Sloshing motion that mixes chyme with secretions, increasing contact with intestinal walls

• Pendular movement: Constrictive wave that enhances nutrient absorption in the small intestine

• Peristalsis: Forward, rhythmic motion that propels the bolus (food mass) through the GI tract

Chemical Digestion

Chemical digestion uses digestive juices and enzymes to break down food into absorbable nutrients.

• Enzymes catalyze the breakdown of macromolecules

• Digestive juices contain acids and other chemicals

Key Terms

• Bolus: Mass of chewed food ready to be swallowed

• Chyme: Semi-liquid mixture of partially digested food and digestive juices

Contact Time

• Food remains in the small intestine for 3 to 6 hours, depending on type and amount

Example: Peristalsis

Peristalsis moves the bolus from the esophagus to the stomach by coordinated muscle contractions.

Organs of the GI Tract and Their Functions

Mouth

• Site of both mechanical and chemical digestion

• Saliva contains water, electrolytes, mucus, and enzymes

• Functions: Softens, lubricates, and dissolves food particles

Pharynx and Esophagus

• Bolus moves into the pharynx and is swallowed

• Epiglottis closes off the trachea during swallowing to prevent choking

• Peristalsis pushes bolus down the esophagus

• Gastroesophageal sphincter (LES) allows food into the stomach and prevents acid reflux

• Gastroesophageal reflux disease (GERD): Chronic acid reflux causing heartburn

Stomach

• Muscular organ that churns and mixes food with digestive juices

• Produces hydrochloric acid (HCl), mucus, digestive enzymes, and intrinsic factor (for vitamin B12 absorption)

• Gastrin: Hormone that stimulates digestive activities

• Chyme formation

• Pyloric sphincter: Controls release of chyme into the small intestine

Small Intestine

• Major site of digestion and absorption

• Three segments: Duodenum, Jejunum, Ileum

• Surface area increased by villi, microvilli, and circular folds

Large Intestine

• Sections: Cecum, Colon, Rectum

• Absorbs water and electrolytes

• No digestive enzymes; chemical digestion by bacteria

• Bacteria produce vitamin K and biotin

• Feces stored in rectum, expelled via anal sphincters

Accessory Organs: Liver, Gallbladder, Pancreas

Liver

• Largest internal organ

• Produces bile for fat digestion

• Metabolizes carbohydrates, fats, and proteins

• Stores vitamins (A, D, B12, E), copper, iron, and glycogen

• Detoxifies alcohol

Gallbladder

• Concentrates and stores bile

• Releases bile into GI tract when fat is ingested

Pancreas

• Produces hormones: insulin and glucagon (regulate blood glucose)

• Secretes digestive enzymes and bicarbonate (neutralizes acidic chyme)

Hormones, Enzymes, and Bile in Digestion

Hormones

• Released from endocrine glands in the stomach and small intestine

• Regulate digestive secretions and enzyme activity

• Gastrin: Stimulates stomach to release HCl and enzymes

• Ghrelin: Stimulates hunger

• Secretin: Causes pancreas to release bicarbonate

• Cholecystokinin (CCK): Stimulates pancreas to secrete enzymes, controls pace of digestion

Enzymes

• Catalyze chemical reactions to break down food

• Produced by salivary glands, stomach, pancreas, and small intestine

• Pancreatic enzymes digest most nutrients

• Amylase: Digests carbohydrates

• Lipase: Digests fats

• Trypsin, chymotrypsin, carboxypeptidase: Digest proteins

Bile

• Yellowish-green substance made in liver, stored in gallbladder

• Emulsifies large fat globules into smaller droplets

• Recycled via enterohepatic circulation

Table: Functions of Digestive Secretions

Secretion	Source	Main Function
Saliva	Mouth	Moistens food, begins carbohydrate digestion
Mucus	Stomach, intestines	Lubrication, protects GI tract lining
Hydrochloric acid (HCl)	Stomach	Activates enzymes, begins protein digestion
Bile	Liver (stored in gallbladder)	Emulsifies fats
Bicarbonate	Pancreas	Neutralizes stomach acid
Digestive enzymes	Salivary glands, stomach, pancreas, small intestine	Break down food into absorbable components
Hormones	Stomach, small intestine	Regulate digestive activity

Absorption of Nutrients

Process of Absorption

• Nutrients are absorbed through intestinal walls into the body's transport systems

• Transport systems: Circulatory system (blood) for water-soluble nutrients, Lymphatic system for fat-soluble nutrients

• Most nutrients are sent to the liver for processing

• GI tract is highly efficient: 92–97% of nutrients are absorbed

Methods of Absorption

• Passive diffusion: Nutrients move from high to low concentration

• Facilitated diffusion: Requires a carrier protein

• Active transport: Requires energy and a carrier; moves nutrients from low to high concentration

Transport and Storage of Nutrients

After Absorption

• Circulatory system distributes water-soluble nutrients

• Lymphatic system distributes fat-soluble nutrients

• Some nutrients are stored in the body for later use

• Excretory system removes waste products

Regulation of Digestion and Nutrient Use

Nervous and Endocrine Systems

• Nervous system stimulates appetite

• Ghrelin signals the brain to eat

• Endocrine system releases hormones (e.g., insulin, glucagon) to regulate nutrient use

Genetic Mechanisms

• Nutritional genomics: Studies how food components interact with gene expression

• Genes determine inherited traits and influence nutrient metabolism

Common Digestive Disorders

Mouth and Throat Disorders

• Gingivitis and periodontal disease: Inflammation of gums and supporting tissues

• Dysphagia: Difficulty swallowing

Esophagus and Stomach Disorders

• Heartburn: Caused by weak lower esophageal sphincter

• GERD: Chronic acid reflux

• Gastroenteritis: Stomach flu, viral or bacterial

• Peptic ulcers: Erosions caused by drugs, alcohol, or bacteria

Gallbladder Disorders

• Gallstones: Crystalline structures that may require surgery

Intestinal Disorders

• Flatulence: Release of intestinal gas

• Constipation: Often due to low fiber or fluid intake

• Diarrhea: Loss of fluids and electrolytes; can be serious if prolonged

• Hemorrhoids: Swelling of veins in rectum and anus

• Irritable bowel syndrome (IBS): Changes in colon rhythm

• Celiac disease: Autoimmune disorder related to gluten

• Inflammatory bowel disease: Includes several types

• Colon cancer: Leading form of cancer, curable if detected early

Table: Common Digestive Disorders

Disorder	Location	Symptoms	Causes
Gingivitis	Mouth	Inflamed gums	Poor oral hygiene
Dysphagia	Throat/Esophagus	Difficulty swallowing	Neurological or structural issues
GERD	Esophagus	Heartburn	Weak LES, certain foods
Peptic ulcers	Stomach	Pain, erosion	Drugs, alcohol, bacteria
Gallstones	Gallbladder	Pain, blockage	Cholesterol, bile imbalance
Constipation	Colon	Infrequent stools	Low fiber/fluid
Diarrhea	Colon	Loose stools	Infection, intolerance
IBS	Colon	Abdominal pain, irregularity	Unknown, functional
Celiac disease	Small intestine	Malabsorption	Autoimmune, gluten
Colon cancer	Colon	Varied	Genetic, lifestyle

Probiotics: Do You Need Them?

Points of View

• Some studies show benefits for diarrhea-related conditions and regularity

• Evidence varies by product; some have strong support, others limited efficacy

• Potential side effects: bloating, gas

• Lack of strict FDA regulation may affect product quality

Key Equations and Scientific Terms

• Enzyme action: $E + S \rightarrow ES \rightarrow E + P$ Where E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product

• Passive diffusion: $J = -D \frac{dC}{dx}$ Where J = flux, D = diffusion coefficient, C = concentration, x = position

Additional info: The chapter integrates basic anatomy, physiology, and clinical nutrition concepts to provide a comprehensive overview of digestion for nutrition students.