Digestive System and Metabolism: Study Guide for Anatomy & Physiology

Chapter 24: Digestive System

Overview of the Digestive System

The digestive system is responsible for the breakdown of food, absorption of nutrients, and elimination of waste. It consists of the gastrointestinal (GI) tract and accessory organs.

• GI Tract: Includes mouth, esophagus, stomach, small intestine, and large intestine.

• Accessory Organs: Liver, gallbladder, pancreas.

• Functions: Ingestion, digestion, absorption, secretion, motility, and excretion.

• Histological Layers: Four main layers: mucosa, submucosa, muscularis externa, serosa/adventitia.

Functions of the Digestive System

• Digestion: Mechanical and chemical breakdown of food.

• Absorption: Movement of nutrients from the GI tract into the blood or lymph.

• Secretion: Release of digestive enzymes, acids, and hormones.

• Motility: Movement of food through the GI tract via peristalsis and segmentation.

Major Structures and Regions

• Oral Cavity: Includes teeth, tongue, salivary glands; functions in mechanical breakdown and mixing of food.

• Pharynx and Esophagus: Conduct food to the stomach; involved in swallowing.

• Stomach: Secretes acid and enzymes; churns food to produce chyme.

• Small Intestine: Main site of nutrient absorption; divided into duodenum, jejunum, ileum.

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

Histological Layers of the GI Tract

• Mucosa: Innermost layer; contains epithelium, lamina propria, muscularis mucosae.

• Submucosa: Connective tissue with blood vessels, nerves, and glands.

• Muscularis Externa: Smooth muscle responsible for peristalsis and segmentation.

• Serosa/Adventitia: Outermost layer; serosa is a serous membrane, adventitia is connective tissue.

Digestive Enzymes and Secretions

• Salivary Glands: Secrete saliva containing amylase for carbohydrate digestion.

• Stomach: Chief cells secrete pepsinogen; parietal cells secrete hydrochloric acid.

• Pancreas: Secretes digestive enzymes (amylase, lipase, proteases) and bicarbonate.

• Liver: Produces bile for fat emulsification.

Swallowing and Movement of Food

• Deglutition (Swallowing): Coordinated movement involving oral cavity, pharynx, and esophagus.

• Peristalsis: Wave-like contractions that move food through the GI tract.

• Segmentation: Mixing contractions in the small intestine.

Absorption and Elimination

• Absorption: Most nutrients absorbed in the small intestine; water and electrolytes in the large intestine.

• Elimination: Removal of indigestible substances via defecation.

Specialized Structures

• Sphincters: Muscular rings that control passage between regions (e.g., lower esophageal, pyloric, ileocecal).

• Villi and Microvilli: Increase surface area for absorption in the small intestine.

Example Table: Major Regions and Functions of the GI Tract

Region	Main Function
Mouth	Mechanical breakdown, mixing with saliva
Stomach	Chemical digestion, mixing, storage
Small Intestine	Digestion, absorption
Large Intestine	Water absorption, feces formation

Chapter 25: Metabolism

Overview of Metabolism

Metabolism refers to all chemical reactions in the body that maintain life, including catabolism (breakdown) and anabolism (synthesis).

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

Food Molecules and Digestion

• Carbohydrates: Broken down into glucose; primary energy source.

• Proteins: Broken down into amino acids; used for synthesis and energy.

• Lipids: Broken down into fatty acids and glycerol; energy storage.

Enzymes and Metabolic Pathways

• Enzymes: Biological catalysts that speed up chemical reactions.

• Active Site: Region on enzyme where substrate binds.

• Binding Site: Specific location for substrate interaction.

ATP Production and Cellular Respiration

• Aerobic Respiration: Occurs in mitochondria; requires oxygen; produces up to 36 ATP per glucose.

• Anaerobic Respiration: Occurs in cytoplasm; does not require oxygen; produces 2 ATP per glucose.

• Glycolysis: First step of glucose metabolism; occurs in cytoplasm.

• Krebs Cycle and Electron Transport Chain: Occur in mitochondria; produce most ATP.

Equation for Aerobic Respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + 36\text{ATP}$

Metabolic Rate and Energy Balance

• Basal Metabolic Rate (BMR): Energy expended at rest.

• Caloric Value: Amount of energy provided by nutrients.

• Essential Nutrients: Nutrients that must be obtained from the diet.

Vitamins and Minerals

• Vitamins: Organic compounds required in small amounts; classified as water-soluble (e.g., B, C) and fat-soluble (e.g., A, D, E, K).

• Minerals: Inorganic elements; classified as major (e.g., calcium, potassium) and trace (e.g., iron, zinc).

• Functions: Support enzyme function, bone structure, fluid balance.

Nutrition and Health

• Malnutrition: Lack of proper nutrition; includes undernutrition and overnutrition.

• Undernutrition: Insufficient intake of nutrients.

• Overnutrition: Excessive intake of nutrients.

• Nutrition Assessment: Evaluation of dietary intake and health status.

Example Table: Energy Yield of Nutrients

Nutrient	Energy Yield (kcal/g)
Carbohydrate	4
Protein	4
Lipid	9
Alcohol	7

Summary of Metabolism

• Metabolism includes all chemical reactions in the body.

• ATP is produced via aerobic and anaerobic pathways.

• Vitamins and minerals are essential for metabolic processes.

• Proper nutrition is vital for health and energy balance.

Essay Question Guidance

Major Layers of the GI Tract

• Mucosa: Epithelium, lamina propria, muscularis mucosae; varies by region (e.g., stomach has gastric pits, small intestine has villi).

• Submucosa: Connective tissue, blood vessels, nerves.

• Muscularis Externa: Two or three layers of smooth muscle; responsible for motility.

• Serosa/Adventitia: Outermost layer; serosa in abdominal cavity, adventitia in esophagus.

Structural and Functional Differences:

• Esophagus: Stratified squamous epithelium; adventitia.

• Stomach: Simple columnar epithelium; gastric glands; three muscle layers.

• Small Intestine: Villi and microvilli; specialized for absorption.

• Large Intestine: No villi; more goblet cells for mucus secretion.

Additional info: For essay responses, include both structural features and their functional significance for each region.