Topic 13

Functions of the Digestive System

Key Functions

  • Ingestion: Placing food in the mouth.

  • Propulsion: Movement of food through the digestive system.

  • Peristalsis: Waves of contraction and relaxation that push food along.

  • Mechanical Breakdown: Prepares food for chemical breakdown.

Example processes:

  • Mixing by the tongue in the mouth.

  • Churning in the stomach.

  • Segmentation in the small intestine.

Food Breakdown and Digestion

Digestion Process

  • Digestion: Enzymatic breakdown of large molecules into smaller units.

    • Carbohydrates ➔ Monosaccharides.

    • Proteins ➔ Amino acids.

    • Fats ➔ Fatty acids and glycerol.

  • Absorption: End products enter blood or lymph.

  • Defecation: Elimination of indigestible substances as feces.

Schematic Summary of Activities

Gastrointestinal Tract Activities

Processes illustrated:

  • Ingestion (Mouth)

  • Propulsion:

    • Swallowing (Oropharynx)

    • Peristalsis (Esophagus, Stomach, Small/Large Intestine)

  • Mechanical breakdown (Chewing, Churning, Segmentation)

  • Absorption (Blood vs Lymph vessels)

  • Defecation (Anus)

Digestive System Organs

Organ Groups

Two Main Groups:

  • Alimentary Canal (GI tract): Continuous, coiled tube.

  • Accessory Digestive Organs: Teeth, tongue, and large organs involved in digestion.

Organs of the Alimentary Canal


Composition:Continuous, coiled, hollow tube from stomach to anus:

  • Mouth

  • Pharynx

  • Esophagus

  • Stomach

  • Small Intestine

  • Large Intestine

  • Anus

Anatomy of the Mouth

Mouth Structure and Function


  • Mouth (Oral Cavity): Lined with mucous membrane.Components:

  • Lips (protects anterior opening)

  • Cheeks (forms lateral walls)

  • Hard/Soft Palate (roof structure)

  • Tongue (muscle involved in mastication and deglutition)

Functions:

  • Mastication of food.

  • Mixing with saliva to form bolus.

  • Taste sensation via taste buds.

The Pharynx and Esophagus

Food Passage

Food moves from the mouth to:

  • Oropharynx

  • Laryngopharynx (leads to esophagus)

Esophagus Characteristics:

  • 10 inches long, conducts food via peristalsis.

Tissue Layers in Alimentary Canal

Layers Overview

Four Layers from innermost to outermost:

  1. Mucosa

  2. Submucosa

  3. Muscularis externa:

    • Longitudinal

    • Circular

    • Oblique (only in stomach)

  4. Serosa

Stomach Functions

Stomach Role

Functions:

  • Temporary food storage

  • Site of food breakdown

  • Chemical breakdown of proteins begins

Can hold up to 4 liters when full; internal folds (rugae) present when empty.

Internal Regions of the Stomach

Stomach Anatomy

Regions:

  • Cardial region (near heart)

  • Fundus (lateral to cardiac region)

  • Body (mid-portion)

  • Pylorus (funnel-shaped end)

Stomach Mucosa Structure

Mucosal Composition


Simple columnar epithelium with gastric pits leading to gastric glands.Gastric juice components:

  • Mucous cells produce alkaline mucus

  • Chief cells (produce pepsinogens)

  • Parietal cells (produce hydrochloric acid)

  • Enteroendocrine cells (produce gastrin)

Alcohol and aspirin absorbable in the stomach.

The Small Intestine

Key Characteristics

  • Length: 2–4 m (7–13 feet).

  • Nutrient Absorption: Major site for nutrients entering the blood.

  • Subdivisions: Duodenum, jejunum, ileum.

Small Intestine Structural Features

Surface Area Enhancements


Microvilli, Villi, and Circular folds: Increase absorption surface.Digestion: Enzymes from intestinal cells and pancreas accessible.

Large Intestine Functions

Overview




Characteristics:1.5 m long; delivers waste from the body.Absorbs: Water, vitamin K, B.Subdivisions: Cecum, Colon (ascending, transverse, descending, sigmoid), Rectum, Anal Canal.

Accessory Digestive Organs

Important Organs

Key Players: Teeth, Tongue, Salivary glands, Pancreas, Liver, Gallbladder.

Teeth Overview

Masticatory Function

Humans have two sets of teeth:

  • Deciduous (20 teeth by age 2).


  • Permanent (32 teeth).Classification:

  • Incisors (cutting)

  • Canines (tearing)

  • Premolars (grinding)

  • Molars (grinding).

Salivary Glands Function

Gland Types

Three pairs:

  • Parotid

  • Submandibular

  • Sublingual.

Saliva Composition: Mucus, serous fluid, and salivary amylase for digestion.

Pancreas Overview

Digestive Functions

  • Location: Posterior to the peritoneum.

  • Produces: Digestive enzymes and hormones.

  • Neutralizes acidic chyme with alkaline secretion.

Liver and Gallbladder

Major Functions


Liver: Largest gland; produces bile, detoxifies substances.Gallbladder: Stores bile and releases it during digestion.

Tooth Structure

Tooth Anatomy

Major regions:

  • Crown (covered by enamel)

  • Root (connected to gums via cement).

  • Pulp Cavity: Contains connective tissue, blood vessels, nerves.

Digestion and Absorption Flowchart

Carbohydrate Breakdown

  • Enzymes involved: Salivary amylase and pancreatic amylase.

  • Absorption: Monosaccharides transported to the liver.

Protein Digestion and Absorption

Protein Enzymes Key Enzymes:

  • Pepsin (stomach)

  • Trypsin (pancreas).

  • Absorption: Amino acids enter capillary blood and reach the liver.

Fat Digestion and Absorption

Enzymatic Breakdown Key Enzyme: Pancreatic lipase.

  • Absorption: Fatty acids and monoglycerides enter lymphatic circulation.

Hormones in Digestion

Summary Table


Major Hormones: Gastrin, Secretin, Cholecystokinin, etc.Functions: Regulation of gastric secretions, enzyme activity, and digestion coordination.

Key Definitions

Digestive Terminology

  • Mastication: Chewing.

  • Deglutition: Swallowing.

  • Bolus: Food mass in esophagus.

  • Chyme: Food mixed with digestive enzymes.

Metabolism Overview

Metabolic Processes

  • Metabolism: All chemical reactions necessary for life.

  • Catabolism: Breakdown of substances.

  • Anabolism: Building larger molecules from smaller units.

Liver's Role in Metabolism

Functions Overview

Key Functions:


  • Bile production, drug detoxification, blood protein synthesis, and cholesterol production.Homeostatic Functions:

  • Glycogenesis, Glycogenolysis, and Gluconeogenesis to regulate blood glucose.

Blood Glucose Regulation

Homeostasis Flowchart

Mechanisms:

  • Glycogenesis, Gluconeogenesis, Glycogenolysis based on blood glucose levels.

Digestive Disorders

Common Conditions

  • Congenital defects: Cleft palate, cleft lip.

  • Inborn errors: Phenylketonuria (PKU), cystic fibrosis (CF).

Nutritional Information

Key Insights

  • Energy Source: Foods converted to ATP; measured in kilocalories (kcal).

  • Major Nutrients: Carbohydrates, Lipids, Proteins, Water; Minor Nutrients: Vitamins, Minerals.

Dietary Recommendations

  • Healthy Eating Pyramid and USDA MyPlate outlining food servings and balance.

Food Groups and Major Nutrients

Nutritional Composition

Food Groups:

  • Fruits

  • Vegetables

  • Grains

  • Milk Products

  • Meats.

Major Nutrients distributed within these groups.

Dietary Sources

Nutrient Origins

  • Carbohydrates (from plants)

  • Lipids (animal fats/oils)

  • Proteins (animal products, legumes)

  • Vitamins, and Minerals (various foods).

Conclusion

  • Lecture Presentation by [Instructor Name] for Chapter 14, focusing on the digestive system and metabolism.

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