The Digestive System: Structure, Function, and Nutrition

Digestive System Overview

Major Groups of Digestive Organs

The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. It consists of two main groups of organs:

• Alimentary canal (gastrointestinal or GI tract): A continuous tube from mouth to anus, including the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

• Accessory digestive organs: These include the teeth, tongue, gallbladder, and digestive glands (salivary glands, liver, pancreas) that assist in digestion.

Gastrointestinal Tract Activities

Six Essential Activities

The digestive process involves six coordinated activities:

1. Ingestion: Taking food into the mouth.

2. Mechanical breakdown: Physical breakdown of food (chewing, churning, segmentation).

3. Propulsion: Movement of food through the GI tract (swallowing and peristalsis).

4. Digestion: Chemical breakdown of food into smaller molecules.

5. Absorption: Transport of digested nutrients into blood or lymph.

6. Defecation: Elimination of indigestible substances as feces.

Motility: Peristalsis and Segmentation

Movement of Food Through the GI Tract

• Peristalsis: Involves alternating contractions and relaxations of adjacent segments of the alimentary canal, propelling food forward.

• Segmentation: Nonadjacent segments contract and relax, mixing food and promoting absorption by moving it back and forth.

Structure of the GI Tract Wall

Four Main Layers

• Mucosa: Innermost layer; mucous membrane in contact with the lumen.

• Submucosa: Connective tissue containing blood vessels, lymphatics, and nerves.

• Muscularis: Layers of smooth muscle responsible for movement and motility.

• Serosa: Outermost protective layer.

Sphincters are thick muscular rings that regulate passage between sections of the GI tract.

The Mouth: Processing and Swallowing Food

Teeth and Tongue

• Teeth: Bite and chew food; types include incisors, canines, premolars, and molars. Adults have 32 permanent teeth.

• Tongue: Positions and tastes food, aids in speech, and is made of skeletal muscle with taste receptors.

Salivary Glands and Saliva

• Salivary glands: Parotid, submandibular, and sublingual glands secrete saliva.

• Saliva composition: Contains mucin, salivary amylase (begins starch digestion), bicarbonate (maintains pH), and lysozyme (inhibits bacteria).

Pharynx and Esophagus: Delivering Food to the Stomach

Swallowing Mechanism

• Voluntary phase: Tongue pushes bolus into pharynx.

• Involuntary phase: Swallowing reflex moves food through pharynx and esophagus via peristalsis.

• Epiglottis: Closes airway during swallowing to prevent food from entering the trachea.

Stomach: Storage, Digestion, and Regulation

Functions of the Stomach

• Food storage: Temporarily stores food before digestion.

• Protein digestion: Gastric juice (acid and enzymes) begins protein breakdown.

• Regulation: Controls delivery of chyme (partially digested food) to the small intestine via the pyloric sphincter.

Gastric Juice Components

• Hydrochloric acid (HCl): Lowers pH to about 2, denatures proteins, and activates enzymes.

• Intrinsic factor: Needed for vitamin B12 absorption.

• Mucus: Protects stomach lining from acid.

• Pepsinogen: Converted to pepsin, which begins protein digestion.

Peptic ulcers can form if the mucus barrier is damaged, often due to Helicobacter pylori infection or NSAID use.

Small Intestine: Digestion and Absorption

Structure and Function

• Digestion: Neutralizes stomach acid, adds digestive enzymes and bile, and breaks down macromolecules into absorbable units.

• Absorption: About 90% of nutrients are absorbed here.

The small intestine has three regions: duodenum (most digestion), jejunum and ileum (absorption). Adaptations such as folds, villi, and microvilli increase surface area for absorption.

Accessory Organs: Pancreas, Liver, Gallbladder

Digestive Functions

• Pancreas: Secretes digestive enzymes (proteases, amylase, lipase) and sodium bicarbonate to neutralize stomach acid.

• Liver: Produces bile to emulsify fats, processes and stores nutrients, detoxifies chemicals, and synthesizes plasma proteins.

• Gallbladder: Stores and concentrates bile, releasing it into the small intestine as needed.

Large Intestine: Absorption and Elimination

Structure and Function

• Absorbs: Water and some nutrients.

• Eliminates: Indigestible material as feces.

• Regions: Cecum, appendix, colon (ascending, transverse, descending, sigmoid), rectum, and anus.

Intestinal Microbiota

• Synthesize vitamin K and break down dietary fiber to short-chain fatty acids (SCFA).

• May reduce risk of metabolic and cardiovascular diseases.

• Help prevent colonization by pathogenic bacteria.

Digestion and Absorption of Nutrients

Proteins

• Digested by enzymes from the stomach (pepsin), pancreas (trypsin, chymotrypsin, carboxypeptidase), and small intestine.

• Amino acids are absorbed by active transport into epithelial cells, then by facilitated diffusion into capillaries.

Carbohydrates

• Polysaccharides are digested to monosaccharides by salivary amylase, pancreatic amylase, and intestinal enzymes.

• Monosaccharides are absorbed by active transport and facilitated diffusion into capillaries.

Lipids

• Broken down to monoglycerides and fatty acids by pancreatic and intestinal lipases.

• Absorbed, reassembled into triglycerides, and packaged as chylomicrons, which enter lymphatic vessels (lacteals) and then the bloodstream.

Water, Vitamins, and Minerals

• Water: Absorbed by osmosis in both small and large intestine.

• Vitamins: Fat-soluble vitamins are absorbed with lipids; water-soluble vitamins by active or facilitated transport.

• Minerals: Absorbed by active or facilitated transport.

Nutrition and Dietary Recommendations

General Guidelines

• Eat a variety of foods from all food groups.

• Maintain a healthy weight and limit sugar, salt, and alcohol intake.

• Prefer whole grains, fruits, and vegetables.

Macronutrients

• Carbohydrates: Main energy source; 45–65% of caloric intake.

• Lipids: Essential for cell structure and energy; unsaturated fats are healthier than saturated or trans fats.

• Proteins: Needed for tissue repair and enzymes; complete proteins contain all essential amino acids.

Micronutrients

• Vitamins: Organic compounds essential for normal function; classified as fat-soluble or water-soluble.

• Minerals: Inorganic elements required for various physiological processes; classified as major or trace minerals based on daily requirements.

Energy Balance and Body Weight

Basal Metabolic Rate (BMR)

• Energy required for essential body activities at rest.

• Influenced by gender, age, body composition, health, and genetics.

Energy Balance

• Maintaining weight requires balancing caloric intake with expenditure.

• Excess calories are stored as fat; losing fat requires a caloric deficit.

Eating Disorders

• Anorexia nervosa: Extreme dieting and weight loss, potentially fatal.

• Bulimia nervosa: Binge eating followed by purging.

• Binge eating disorder: Recurrent episodes of excessive eating.

Disorders of the Digestive Tract

Common Disorders

• Food poisoning and food-borne infections: Caused by bacterial toxins or microorganisms; symptoms include diarrhea and vomiting.

• Food allergies: Immune response to specific foods; may cause hives, anaphylaxis.

• Lactose intolerance: Inability to digest lactose due to lack of lactase enzyme; managed by avoiding dairy or using supplements.

• Peptic ulcers: Erosions in the stomach lining, often due to Helicobacter pylori infection or NSAID use; treated with antibiotics and antacids.

• Gluten intolerance and colon polyps: Other notable digestive disorders.