Functions and Anatomy of the Digestive System

Study Notes

Ingestion: Taking in food and liquids.
Propulsion: Moving food through the digestive tract using peristalsis and mass movements.
Digestion:
- Mechanical Digestion: Physical breakdown of food (e.g., chewing).
- Chemical Digestion: Breakdown of macromolecules by enzymes (e.g., carbohydrates, proteins, lipids).
Secretion: Releasing digestive enzymes, acid, mucus, and bile.
Absorption: Nutrient molecules pass from the digestive tract into the blood or lymph.
Elimination: Removing indigestible substances as feces.

Oral Cavity: Includes teeth, tongue, and salivary glands. Mastication (chewing) begins mechanical digestion. Saliva contains amylase, starting carbohydrate digestion.
Pharynx and Esophagus: Food passageway. Swallowing (deglutition) involves voluntary and involuntary phases. The esophagus moves food using peristalsis.
Stomach: Temporary food storage. Gastric glands secrete hydrochloric acid (HCl) and pepsinogen (converted to pepsin in the acidic environment) for protein digestion. The stomach churns food into chyme.
Small Intestine: Primary site of digestion and absorption.
- Duodenum: Receives bile from the liver and gallbladder, and pancreatic enzymes for digestion.
- Jejunum and Ileum: Major regions for nutrient absorption. Villi and microvilli (forming the brush border) increase surface area for absorption.
Large Intestine: Absorbs water and electrolytes. Forms and stores feces. Contains bacteria that synthesize vitamins (e.g., vitamin K).
Accessory Organs:
- Liver: Produces bile (essential for fat emulsification) and processes nutrients.
- Gallbladder: Stores and concentrates bile.
- Pancreas: Secretes digestive enzymes (e.g., lipase, amylase, trypsin) and bicarbonate into the duodenum.

Nervous Regulation:
- Enteric Nervous System (ENS): Network of neurons regulating digestion independently of the central nervous system.
- Autonomic Nervous System: Parasympathetic (via the vagus nerve) stimulates digestion, while the sympathetic system inhibits it.
Hormonal Regulation:
- Gastrin: Released by the stomach to stimulate acid secretion and gastric motility.
- Cholecystokinin (CCK): Stimulates the release of bile and pancreatic enzymes in response to fats.
- Secretin: Stimulates bicarbonate release to neutralize stomach acid in the duodenum.

Carbohydrates:
- Salivary and pancreatic amylase break down starches into maltose and other disaccharides.
- Brush border enzymes (e.g., maltase, lactase) break down disaccharides into monosaccharides (e.g., glucose, fructose).
Proteins:
- Pepsin in the stomach initiates protein breakdown.
- Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) and brush border enzymes (e.g., aminopeptidase) continue digestion into amino acids.
Lipids:
- Bile salts emulsify fats in the small intestine.
- Pancreatic lipase breaks down triglycerides into fatty acids and monoglycerides.
Nucleic Acids:
- Pancreatic enzymes (ribonuclease, deoxyribonuclease) break down nucleic acids into nucleotides.

Monosaccharides and amino acids are absorbed through active transport into the blood.
Fatty acids and monoglycerides are absorbed by forming micelles and enter epithelial cells, where they are reformed into triglycerides and transported as chylomicrons into the lymphatic system.
Water and electrolytes are absorbed in the small and large intestines through osmosis and active transport mechanisms.

Peptic Ulcers: Erosion of the stomach or duodenal lining, often caused by Helicobacter pylori infection or NSAID use.
Gastroesophageal Reflux Disease (GERD): Backflow of stomach acid into the esophagus, causing heartburn and potential esophageal damage.
Liver Disease: Includes cirrhosis and hepatitis, impairing liver function, including detoxification and bile production.
Gallstones: Solid deposits of bile components that can block the bile ducts.
Inflammatory Bowel Diseases (IBD): Such as Crohn’s disease and ulcerative colitis, causing chronic inflammation of the digestive tract.