Digestive System and Metabolism: Study Guide (Chapters 22 & 23)

Chapter 22: Digestive System

Structure and Functions of the Digestive System

The digestive system is responsible for the breakdown and absorption of nutrients necessary for life. It consists of the gastrointestinal (GI) tract and accessory organs.

• Major Functions: Ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.

• GI Tract vs. Accessory Organs: The GI tract (alimentary canal) includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

• Mechanical vs. Chemical Digestion: Mechanical digestion involves physical breakdown (e.g., chewing, churning), while chemical digestion involves enzymatic breakdown of food.

• Peristalsis: Rhythmic contractions of smooth muscle that propel food through the GI tract.

• Pathway of Ingested Substances: Food passes from mouth → pharynx → esophagus → stomach → small intestine → large intestine → anus.

General Gross and Microscopic Anatomy of the GI Tract

The GI tract is composed of several layers and specialized structures that facilitate digestion and absorption.

• Major Organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.

• Histological Layers: Mucosa, submucosa, muscularis externa, serosa/adventitia.

• Functions: Each layer has specific roles in secretion, absorption, and motility.

Peritoneum and Mesenteries

The peritoneum is a serous membrane lining the abdominal cavity, while mesenteries are folds that support and stabilize the digestive organs.

• Visceral vs. Parietal Peritoneum: Visceral covers organs; parietal lines the cavity.

• Peritoneal Cavity: Space between the two peritoneal layers, containing lubricating fluid.

• Mesenteries: Double layers of peritoneum that anchor organs (e.g., lesser omentum, greater omentum, mesocolon).

• Intraperitoneal vs. Retroperitoneal: Intraperitoneal organs are surrounded by peritoneum; retroperitoneal organs are behind it.

Oral Cavity

The oral cavity initiates mechanical and chemical digestion.

• Boundaries: Lips, cheeks, palate, tongue, and oropharynx.

• Hard and Soft Palate: Separate oral and nasal cavities; soft palate closes off nasopharynx during swallowing.

• Tongue: Contains taste buds (papillae); aids in mixing food and swallowing.

• Salivary Glands: Parotid, submandibular, and sublingual glands secrete saliva containing enzymes (e.g., amylase).

Anatomy of the Pharynx

The pharynx is a muscular tube that serves as a passageway for food and air.

• Regions: Nasopharynx, oropharynx, laryngopharynx.

• Function: Directs food to the esophagus and air to the larynx.

Gross and Microscopic Anatomy of the Esophagus

The esophagus transports food from the pharynx to the stomach.

• Location: Posterior to the trachea, passes through the diaphragm.

• Histology: Mucosa (stratified squamous epithelium), submucosa, muscularis (skeletal and smooth muscle), adventitia.

• Function: Peristalsis moves food to the stomach.

Gross and Microscopic Anatomy of the Stomach

The stomach is a muscular organ that continues mechanical and chemical digestion.

• Regions: Cardia, fundus, body, pylorus.

• Gastric Glands: Secrete hydrochloric acid (HCl), pepsinogen, mucus.

• Specializations: Rugae (folds), gastric pits, parietal cells (HCl), chief cells (pepsinogen), mucous cells.

Gross and Microscopic Anatomy of the Small Intestine

The small intestine is the primary site for digestion and absorption.

• Regions: Duodenum, jejunum, ileum.

• Specializations: Circular folds (plicae circulares), villi, microvilli increase surface area.

• Histology: Mucosa with absorptive cells, goblet cells, intestinal glands (crypts of Lieberkühn), Peyer's patches.

Gross and Microscopic Anatomy of the Large Intestine, Rectum, and Anal Canal

The large intestine absorbs water and forms feces.

• Regions: Cecum, colon (ascending, transverse, descending, sigmoid), rectum, anal canal.

• Specializations: Teniae coli, haustra, abundant goblet cells.

• Anal Sphincters: Internal (smooth muscle, involuntary) and external (skeletal muscle, voluntary).

Liver, Gallbladder, and Pancreas

These accessory organs produce and store substances essential for digestion.

• Liver: Produces bile, processes nutrients, detoxifies blood. Histology: hepatic lobules, portal triads.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Produces digestive enzymes (acinar cells) and hormones (islets of Langerhans: insulin, glucagon).

Motility in the GI Tract

Motility refers to the movement of food through the digestive tract.

• Deglutition: Swallowing, involving coordinated muscle contractions.

• Types of Movement: Peristalsis (propulsion), segmentation (mixing), mass movement (large intestine).

• Defecation Reflex: Elimination of feces, regulated by voluntary and involuntary mechanisms.

Physiology of Digestion

Digestion involves enzymatic breakdown of macromolecules into absorbable units.

• Enzymes: Amylases (carbohydrates), proteases (proteins), lipases (fats).

• Hydrochloric Acid (HCl): Secreted by parietal cells, denatures proteins, activates pepsin.

• Bile: Emulsifies fats, aiding in digestion and absorption.

• Microbiome: Gut bacteria assist in digestion and synthesis of certain vitamins.

Processes of Absorption

Absorption is the movement of nutrients from the GI tract into the blood or lymph.

• Sites: Most absorption occurs in the small intestine.

• Mechanisms: Passive diffusion, facilitated diffusion, active transport, endocytosis.

• Absorbed Substances: Monosaccharides, amino acids, fatty acids, vitamins, minerals (e.g., calcium, iron, B12).

Hormonal and Neural Regulation of Digestive Processes

Digestive activities are regulated by the enteric nervous system (ENS), autonomic nervous system, and hormones.

• ENS: Local control of motility and secretion.

• Sympathetic vs. Parasympathetic: Sympathetic inhibits, parasympathetic stimulates digestive activity.

• GI Hormones: Gastrin, secretin, cholecystokinin (CCK), motilin, etc.

• Reflexes: Short (local) and long (central) reflexes coordinate digestive responses.

Predictions Related to Homeostatic Imbalance

Disruptions in digestive function can result from disease, injury, or surgical intervention.

• Examples: Removal of stomach (gastrectomy) affects digestion and absorption; diarrhea or constipation can result from altered motility or secretion.

Chapter 23: Metabolism & Nutrition

Nutrients

Nutrients are substances required for energy, growth, and maintenance of the body.

• Types: Macronutrients (carbohydrates, fats, proteins), micronutrients (vitamins, minerals).

• Essential vs. Non-Essential: Essential nutrients must be obtained from the diet; non-essential can be synthesized by the body.

• Vitamins: Water-soluble (e.g., B, C) and fat-soluble (e.g., A, D, E, K).

• Minerals: Major (e.g., calcium, sodium) and trace (e.g., iron, zinc).

• Proteins: Important for tissue repair, enzymes, hormones.

Introduction to Metabolism

Metabolism encompasses all chemical reactions in the body, including catabolism (breakdown) and anabolism (synthesis).

• Catabolic Reactions: Release energy by breaking down molecules.

• Anabolic Reactions: Use energy to build complex molecules.

• Enzymes: Biological catalysts that speed up metabolic reactions.

• Coenzymes: NAD (nicotinamide adenine dinucleotide), FAD (flavin adenine dinucleotide) are important in redox reactions.

Cellular Respiration and Metabolism of Carbohydrates, Fats, and Proteins

Cellular respiration is the process by which cells generate ATP from nutrients.

• Aerobic Respiration: Includes glycolysis, citric acid (Krebs) cycle, and electron transport chain.

• Anaerobic Respiration: Glycolysis in the absence of oxygen produces lactic acid.

• Metabolic Pathways: Glycogenesis (formation of glycogen), glycogenolysis (breakdown of glycogen), gluconeogenesis (formation of glucose from non-carbohydrates).

• Anabolic and Catabolic Processes: Lipolysis (breakdown of fats), lipogenesis (synthesis of fats), protein synthesis, and breakdown.

• Hormonal Regulation: Insulin (anabolic), glucagon (catabolic), cortisol, growth hormone, thyroid hormones.

Energy Balance and Thermoregulation

Energy balance is the relationship between energy intake and expenditure.

• Basal Metabolic Rate (BMR): The energy required to maintain basic physiological functions at rest.

• Factors Affecting BMR: Age, sex, body composition, hormones, temperature.

• Thermoregulation: Maintenance of body temperature through mechanisms such as sweating, shivering, and blood flow adjustments.

Application of Homeostatic Mechanisms

Metabolic processes help maintain homeostasis of blood glucose and body temperature.

• Blood Glucose Regulation: Insulin lowers, glucagon raises blood glucose levels.

• Temperature Homeostasis: Hypothalamus regulates responses to heat and cold.

Predictions Related to Homeostatic Imbalance

Disruptions in metabolism can lead to various disorders.

• Examples: Diabetes mellitus (impaired glucose regulation), hypothyroidism (reduced metabolic rate), hyperthermia/hypothermia (temperature imbalance).