Overview of the Digestive System and Its Functions

Amylase

An enzyme that breaks down starch; secreted in saliva and by the pancreas.

Lipase

An enzyme that digests fats; secreted by the pancreas.

Pepsin

Active protein-digesting enzyme in the stomach, formed from pepsinogen.

Absorption

Uptake of nutrients from the GI tract into the blood or lymph.

Chyme

Semi-fluid mixture of food and digestive juices formed in the stomach.

Peristalsis

Wave-like muscular contractions that move food through the GI tract.

Microbiome

The community of bacteria in the GI tract, especially the large intestine, is crucial for digestion and immunity.

Ingestion

Intake of food.

Mechanical breakdown

Chewing, churning (stomach), segmentation (small intestine).

Propulsion

Swallowing and peristalsis.

Digestion

Chemical breakdown by enzymes.

Defecation

Elimination of indigestible substances.

Carbs

Broken into monosaccharides and absorbed into capillaries.

Proteins

Broken into amino acids and absorbed into capillaries.

Fats

Broken into fatty acids + monoglycerides and absorbed into lacteals (lymph).

Enteric Nervous System

Intrinsic nerve network controlling GI motility and secretions.

Cephalic phase

Triggered by smell/thought of food.

Gastric phase

Triggered by food in the stomach.

Intestinal phase

Begins when chyme enters the duodenum.

Stomach protection

Mucus coat, tight junctions, and epithelial renewal.

Ulcers

Often caused by Helicobacter pylori (H. pylori).

Small Intestine

Duodenum receives chyme, bile, pancreatic enzymes; jejunum is the main site of nutrient absorption; ileum has limited absorption and connects to large intestine.

Large Intestine

Absorbs water and vitamins; stores feces.

Disorders

IBS, colon cancer.

Microbiome

Helps synthesize vitamins, digest fiber, and regulate immunity.

LDL (Low-Density Lipoprotein)

Bad cholesterol; deposits in arteries.

HDL (High-Density Lipoprotein)

Good cholesterol; removes excess from blood.

Digestive System Organs

Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus; accessory organs: liver, pancreas, gallbladder.

Parietal Cells

Produce HCl (activates pepsinogen).

Chief Cells

Produce Pepsinogen (converted to pepsin to digest proteins).

Mucous Cells

Produce Mucus (protects stomach lining).

Enteric Nervous System

Controls digestive activity (motility, enzyme secretion, blood flow) independently of CNS.

Intestinal Phase

Triggers hormonal and neural responses (e.g., secretin, CCK) that inhibit gastric secretion and motility.

Low pH in Stomach

Activates pepsin, kills microbes, denatures proteins for easier digestion.

Liver Contribution to Digestion

Produces bile, which emulsifies fats to aid fat digestion.

Pancreas Contribution to Digestion

Produces digestive enzymes (amylase, lipase, proteases) and bicarbonate to neutralize acid.

Structures Increasing Absorptive Surface Area

Circular folds, villi, and microvilli.

Regions of the Small Intestine

Duodenum, jejunum, ileum.

Epithelial Lining Protection in Stomach

Mucus secretion, tight junctions between cells, and rapid epithelial regeneration.

Function of the Large Intestine

Absorbs water and vitamins; stores and eliminates feces.

Importance of Bacteria in Large Intestine

They synthesize vitamins, aid digestion, prevent harmful bacteria growth, and support immune function.

Cystic Fibrosis Effect on Digestion

Thick mucus blocks pancreatic ducts → enzymes can't reach intestines → poor digestion and absorption.

Difference between LDL and HDL

LDL: Delivers cholesterol to tissues—can clog arteries; HDL: Removes cholesterol from bloodstream—protects against heart disease.

Phases of Digestion in the Stomach

Cephalic, gastric, and intestinal phases.