The gastrointestinal (GI) system, also known as the digestive system, is a complex organ network responsible for breaking down food, absorbing nutrients, and eliminating waste. It combines mechanical digestion (chewing and churning) with chemical digestion (enzymes and hormones) to provide the body with essential nutrients.
This article explores the gastrointestinal system in detail—its functions, major organs, enzymes, and hormones that regulate digestion.
Functions of the Gastrointestinal System
The GI system ensures the body receives nutrients and energy by performing:
- Food Breakdown – Through chewing and enzyme action.
- Mechanical Digestion – Churning and mixing food in the stomach and intestines.
- Chemical Digestion – Enzymes and gastric juices breaking macronutrients into absorbable units.
- Absorption – Transfer of nutrients into blood and lymph vessels.
- Waste Elimination – Removal of undigested food and toxins through the rectum and anus.
- Detoxification – The liver neutralizes toxins and stores nutrients for future use.
Organs of the Gastrointestinal System
1. Mouth
- Digestion begins here with mastication (chewing).
- Food mixes with saliva containing salivary amylase and salivary lipase, starting carbohydrate and fat digestion.
- The tongue forms a bolus (chewed mass of food), which is swallowed.
- The epiglottis closes the airway, directing food into the esophagus.
Enzymes: Salivary amylase, salivary lipase
2. Esophagus
- A muscular tube transporting food from mouth to stomach.
- Peristalsis (wave-like muscle contractions) pushes food downward.
3. Stomach
Stores and churns food, mixing it with gastric juices.
Gastric juices include:
- Pepsinogen (chief cells) → becomes pepsin, breaking proteins.
- Hydrochloric acid (parietal cells) → creates acidic environment for enzymes.
- Mucus (goblet cells) → protects stomach lining.
Enzymes: Gastric lipase, pepsinogen, hydrochloric acid
Hormones: Gastrin, ghrelin
4. Small Intestine
The main site of digestion and absorption, divided into three regions:
1. Duodenum
- Receives chyme from stomach.
- Neutralized by pancreatic bicarbonate and bile.
- Produces enzymes: proteases, lactase, disaccharidases.
2. Jejunum
- Primary site of nutrient absorption.
3. Ileum
- Absorbs vitamin B12, bile salts, and remaining nutrients.
- Contains villi and microvilli for efficient absorption.
Enzymes: Proteases, lactase, disaccharidases
Hormones: Cholecystokinin, somatostatin, secretin, motilin
5. Liver
Produces bile, stored in the gallbladder and released into the duodenum.
Functions:
- Detoxification (ammonia to urea).
- Storage of glucose as glycogen.
- Production of plasma proteins.
- Metabolism of fats and carbohydrates.
Enzymes: Bile (stored in gallbladder)
6. Pancreas
A digestive and endocrine gland.
Produces pancreatic juices containing enzymes:
- Lipase → breaks down fats.
- Amylase → breaks down starch.
- Trypsinogen → digests proteins.
Enzymes: Pancreatic lipase, trypsinogen, amylase
Hormones: Insulin, glucagon
7. Large Intestine (Colon)
- Absorbs water, salts, and vitamins (e.g., vitamin K).
- Stores undigested material until elimination.
- Bacterial fermentation breaks down fiber.
- Waste accumulates in rectum and exits through anus.
Enzymes: None
Hormones: None
Enzymes of the Gastrointestinal System
| Enzyme | Function |
|---|---|
| Lipase | Breaks down fats into fatty acids and glycerol. |
| Amylase | Breaks down starch into sugars. |
| Trypsin | Digests proteins into amino acids. |
| Pepsin | Breaks down proteins in the stomach. |
| Lactase | Breaks down lactose into glucose and galactose. |
Hormones of the Gastrointestinal System
| Hormone | Function |
|---|---|
| Gastrin | Stimulates gastric acid secretion. |
| Ghrelin | Signals hunger to the brain. |
| Secretin | Stimulates pancreatic bicarbonate release. |
| Cholecystokinin | Stimulates gallbladder to release bile. |
| Somatostatin | Inhibits gastric and pancreatic secretions. |
| Motilin | Regulates intestinal contractions. |
Clinical Relevance of the Gastrointestinal System
Digestive system disorders are among the most common worldwide. These include:
- Gastritis and Peptic Ulcer Disease – Due to excessive gastric acid or infection (H. pylori).
- Irritable Bowel Syndrome (IBS) – Chronic functional disorder with abdominal pain and altered bowel habits.
- Inflammatory Bowel Disease (IBD) – Includes Crohn’s disease and ulcerative colitis.
- Gallstones – Hardened deposits of bile salts blocking ducts.
- Liver Disease – Hepatitis, cirrhosis, or fatty liver affecting metabolism.
- Pancreatitis – Inflammation of the pancreas disrupting enzyme release.
FAQ
1. What is the main function of the gastrointestinal system?
It breaks down food, absorbs nutrients, and eliminates waste.
2. Where does most nutrient absorption occur?
In the small intestine, particularly the jejunum and ileum.
3. What role does the liver play in digestion?
It produces bile for fat digestion and detoxifies harmful substances.
4. Which enzyme digests proteins in the stomach?
Pepsin, derived from pepsinogen, breaks down proteins.
5. What hormone triggers hunger?
Ghrelin, released mainly by the stomach.
6. What is the difference between the small and large intestine?
The small intestine absorbs nutrients, while the large intestine absorbs water and prepares waste for elimination.
