Key Takeaways
- The GI tract runs from mouth to anus; accessory organs include liver, gallbladder, and pancreas
- Mechanical digestion is physical breakdown; chemical digestion uses enzymes
- The liver produces bile (stored in gallbladder); bile emulsifies fats
- Most nutrient absorption occurs in the small intestine via villi and microvilli
- The large intestine primarily absorbs water and houses beneficial bacteria
Last updated: January 2026
The Digestive System
The digestive system breaks down food into nutrients that can be absorbed and used by the body. This section covers the organs, processes, and enzymes involved in digestion.
Digestive Tract (GI Tract)
The alimentary canal is a continuous tube from mouth to anus:
| Organ | Function | Key Processes |
|---|---|---|
| Mouth | Mechanical digestion, starts chemical digestion | Chewing, salivary amylase breaks down starch |
| Pharynx | Swallowing | Moves food to esophagus |
| Esophagus | Transport | Peristalsis moves food to stomach |
| Stomach | Chemical digestion, storage | HCl, pepsin breaks down protein |
| Small intestine | Main digestion and absorption | Enzymes, nutrient absorption |
| Large intestine | Water absorption, waste formation | Absorbs water, forms feces |
| Rectum/Anus | Elimination | Stores and eliminates waste |
Accessory Organs
| Organ | Secretion | Function |
|---|---|---|
| Salivary glands | Saliva | Amylase (starch digestion), lubricates food |
| Liver | Bile | Emulsifies fats (breaks into smaller droplets) |
| Gallbladder | Stores bile | Releases bile when fats are detected |
| Pancreas | Enzymes + bicarbonate | Digests all nutrient types, neutralizes stomach acid |
Digestive Enzymes
| Enzyme | Source | Substrate | Product |
|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose |
| Pepsin | Stomach | Proteins | Peptides |
| Pancreatic amylase | Pancreas | Starch | Maltose |
| Trypsin | Pancreas | Proteins | Peptides |
| Lipase | Pancreas | Fats | Fatty acids + glycerol |
| Maltase | Small intestine | Maltose | Glucose |
| Lactase | Small intestine | Lactose | Glucose + galactose |
| Sucrase | Small intestine | Sucrose | Glucose + fructose |
Small Intestine Anatomy
The small intestine has three sections:
| Section | Length | Primary Function |
|---|---|---|
| Duodenum | ~10 inches | Receives chyme, bile, pancreatic juice |
| Jejunum | ~8 feet | Major absorption site |
| Ileum | ~12 feet | Absorbs remaining nutrients, B12 |
Absorption Structures
Villi: Finger-like projections that increase surface area Microvilli: Tiny projections on villi cells (brush border)
These structures dramatically increase the surface area for nutrient absorption.
Large Intestine Functions
| Function | Description |
|---|---|
| Water absorption | Absorbs remaining water from waste |
| Vitamin production | Bacteria produce vitamins K and B |
| Waste compaction | Forms feces |
| Storage | Stores waste until elimination |
Sections: Cecum → Ascending colon → Transverse colon → Descending colon → Sigmoid colon → Rectum
Chemical vs. Mechanical Digestion
| Type | Description | Examples |
|---|---|---|
| Mechanical | Physical breakdown | Chewing, churning, segmentation |
| Chemical | Enzyme breakdown | Amylase, pepsin, lipase |
Nutrient Absorption Sites
| Nutrient | Primary Absorption Site |
|---|---|
| Carbohydrates | Small intestine |
| Proteins (amino acids) | Small intestine |
| Fats | Small intestine |
| Water | Large intestine (mainly) |
| Vitamin B12 | Ileum |
| Most vitamins | Small intestine |
Hormones of Digestion
| Hormone | Source | Function |
|---|---|---|
| Gastrin | Stomach | Stimulates HCl secretion |
| Secretin | Small intestine | Stimulates bicarbonate release |
| Cholecystokinin (CCK) | Small intestine | Stimulates bile and enzyme release |
| Ghrelin | Stomach | Stimulates hunger |
| Leptin | Fat cells | Signals satiety |
Test Your Knowledge
Which organ is responsible for producing bile?
A
B
C
D
Test Your Knowledge
Where does most nutrient absorption occur?
A
B
C
D
Test Your Knowledge
Which enzyme begins protein digestion in the stomach?
A
B
C
D