Study Guide: The Digestive System (Ch. 22) – Anatomy & Physiology

The Digestive System

Overview of Digestive System Function

The digestive system is responsible for the breakdown of food, absorption of nutrients, and elimination of waste. It involves coordinated actions of the gastrointestinal (GI) tract and accessory organs, regulated by neural and hormonal mechanisms.

• Ingestion: Intake of food into the mouth.

• Digestion: Mechanical and chemical breakdown of food into absorbable units.

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

• Motility: Movement of food through the GI tract via muscular contractions.

• Secretion: Release of digestive juices and enzymes to aid digestion.

• Elimination: Removal of indigestible substances from the body.

Pathways of Food Through the GI Tract

• Mouth → Pharynx → Esophagus → Stomach → Small Intestine (duodenum, jejunum, ileum) → Large Intestine (colon) → Rectum → Anus

• Accessory organs: Salivary glands, liver, gallbladder, pancreas

Mechanical vs. Chemical Digestion

• Mechanical digestion: Physical breakdown of food (chewing, mixing, segmentation).

• Chemical digestion: Enzymatic breakdown of macromolecules into monomers.

• Example: Amylase in saliva begins starch digestion in the mouth.

Motility Patterns in the GI Tract

Different regions of the GI tract exhibit distinct motility patterns to mix and propel contents.

Type of Motility	Where it Occurs	Muscle Involved	Purpose	Example of Control
Segmentation	Small intestine	Circular muscle	Mixing contents	Enteric nervous system
Peristalsis	Esophagus, stomach, intestines	Circular & longitudinal muscle	Propulsion of food	Autonomic nervous system
Mass movement	Large intestine	Longitudinal muscle	Move feces toward rectum	Parasympathetic stimulation

Control of GI Function

• Short reflexes: Mediated by the enteric nervous system (ENS), act locally within the GI tract.

• Long reflexes: Involve the central nervous system (CNS), coordinate responses to external stimuli.

• Hormonal control: GI hormones (e.g., gastrin, secretin, cholecystokinin) regulate secretion and motility.

Secretions of the GI Tract and Accessory Organs

Digestive secretions facilitate the breakdown and absorption of nutrients.

Secretion	Location of Production	Function	Stimulus for Secretion
Salivary amylase	Salivary glands	Starch digestion	Parasympathetic stimulation, food in mouth
HCl	Stomach (parietal cells)	Protein denaturation, activates pepsinogen	Gastrin, vagal stimulation
Bile	Liver (stored in gallbladder)	Emulsifies fats	Cholecystokinin (CCK)
Pancreatic enzymes	Pancreas	Digest carbohydrates, proteins, fats	Secretin, CCK
Mucus	Goblet cells (GI tract)	Protects mucosa	Irritation, parasympathetic input
Intrinsic factor	Stomach (parietal cells)	Vitamin B12 absorption	Food in stomach

Phases of GI Regulation

• Cephalic phase: Initiated by sight, smell, or thought of food; prepares GI tract for digestion.

• Gastric phase: Begins with food in the stomach; stimulates gastric secretions and motility.

• Intestinal phase: Starts as chyme enters the small intestine; regulates gastric emptying and secretion.

Absorption of Nutrients

• Carbohydrates: Absorbed as monosaccharides (glucose, galactose, fructose).

• Proteins: Absorbed as amino acids, dipeptides, tripeptides.

• Fats: Absorbed as fatty acids and monoglycerides after emulsification by bile.

• Vitamins: Fat-soluble (A, D, E, K) absorbed with fats; water-soluble absorbed by diffusion or transporters.

Role of Bile and Pancreatic Secretions

• Bile: Produced by the liver, stored in the gallbladder, released into the duodenum to emulsify fats.

• Pancreatic enzymes: Include amylase, lipase, proteases; essential for digestion of all macronutrients.

GI Reflexes and Motility Disorders

• Vomiting (Emesis): Coordinated by the vomiting center in the medulla; involves reverse peristalsis.

• Diarrhea: Increased motility and decreased absorption in the intestines.

• Constipation: Decreased motility, excessive water absorption in colon.

Secretory Digestive Disorders

• Cystic fibrosis: Mutation in CFTR channel affects chloride and water secretion, leading to thick mucus and impaired digestion.

• Gallstones: Precipitation of cholesterol or bilirubin in bile, can block bile ducts and impair fat digestion.

Summary Table: GI Motility and Control

Structure	Resting State (sphincter contracted or relaxed)	Motivation State (sphincter contracted or relaxed)	Neural Control (parasympathetic or sympathetic)
Lower esophageal sphincter	Contracted	Relaxed during swallowing	Parasympathetic relaxes, sympathetic contracts
Pyloric sphincter	Contracted	Relaxes to allow chyme into duodenum	Regulated by enteric and autonomic input

Key Terms and Definitions

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

• Segmentation: Mixing contractions in the small intestine.

• Cholecystokinin (CCK): Hormone that stimulates bile release and pancreatic enzyme secretion.

• Secretin: Hormone that stimulates bicarbonate secretion from the pancreas.

• Enteric Nervous System (ENS): Intrinsic neural network regulating GI function.

Additional info: Some content and table entries were inferred and expanded for completeness and clarity based on standard Anatomy & Physiology curriculum.