GI Hormones: Importance, Functions, Regulation & Clinical Significance (Complete Guide)

Learn about GI hormones, their types, sources, functions and regulation. Easy, exam-ready notes with diagrams for students of zoology and physiology.

The gastrointestinal (GI) system performs vital tasks such as digestion, absorption, motility and enzyme secretion. These functions are precisely controlled by a special group of chemical messengers called GI hormones. Together, these hormones ensure that the digestive process runs smoothly, efficiently, and in coordination with the needs of the body. In this article, we discuss the definition, classification, sources, functions, mechanisms, regulation, and clinical significance of GI hormones in a student-friendly manner.

This comprehensive, plagiarism-free guide is ideal for college students, competitive exam aspirants, and blog readers looking for clear yet detailed physiology notes.

What Are GI Hormones? – Simple Definition

GI hormones are chemical messengers produced by specialized endocrine cells located in the wall of the stomach, intestine and pancreas. After secretion, they enter the bloodstream and regulate digestive activities such as:

• gastric acid secretion
• enzyme release from pancreas
• bile flow from liver and gallbladder
• gut motility
• hunger and satiety
• glucose metabolism

These hormones act locally or systemically and maintain coordination between different parts of the digestive tract.

Classification of GI Hormones

GI hormones can be classified in several ways. The most commonly used classifications are:

Based on Chemical Structure

• Gastrin–Cholecystokinin Family: Gastrin, CCK
• Secretin Family: Secretin, GIP, Glucagon, VIP
• Motilin Family: Motilin
• Somatostatin Family: Somatostatin
• Others: Ghrelin, Serotonin, Substance P

2. Based on Site of Secretion

• Stomach: Gastrin, Ghrelin
• Duodenum: Secretin, CCK, GIP, Motilin
• Pancreas: Somatostatin, Pancreatic polypeptide
• Intestine: Serotonin, VIP

Major GI Hormones and Their Functions

Below is a clear and exam-ready description of all important GI hormones.

1. Gastrin

Source: G-cells of stomach (antrum)
Stimulus: Proteins, peptides, stomach distension, vagus nerve
Functions:

• Increases gastric acid (HCl) secretion
• Stimulates growth of gastric mucosa
• Increases gastric motility

2. Cholecystokinin (CCK)

Source: I-cells of duodenum
Stimulus: Fatty acids and amino acids
Functions:

• Releases pancreatic digestive enzymes
• Causes gallbladder contraction → bile release
• Slows gastric emptying
• Produces satiety

3. Secretin

Source: S-cells of duodenum
Stimulus: Acidic chyme (pH < 4.5)
Functions:

• Releases bicarbonate from pancreas
• Neutralizes stomach acid in duodenum
• Inhibits gastric acid secretion

4. GIP (Gastric Inhibitory Peptide) / GLP-1

Source: K-cells of small intestine
Stimulus: Glucose, fats, amino acids
Functions:

• Stimulates insulin release (incretin effect)
• Inhibits gastric acid secretion

5. Motilin

Source: M-cells of small intestine
Stimulus: Fasting state
Functions:

• Increases gut motility
• Initiates migrating motor complex

6. Somatostatin

Source: D-cells of stomach and pancreas
Functions:

• Inhibits secretion of almost all GI hormones
• Reduces gastric acid and pepsin
• Slows intestinal motility

7. Ghrelin

Source: Stomach (fundus)
Functions:

• Stimulates hunger
• Increases gastric motility
• Promotes growth hormone release

8. VIP (Vasoactive Intestinal Peptide)

Source: Enteric nervous system
Functions:

• Relaxes GI smooth muscle
• Increases intestinal secretion
• Dilates intestinal blood vessels

Regulation of GI Hormones

The secretion of GI hormones depends on three major factors:

1. Neural Regulation

• Vagus nerve stimulates gastrin and CCK.
• Enteric nervous system controls local hormone release.

2. Chemical Regulation

• Proteins → gastrin
• Fatty acids → CCK
• Acidic chyme → secretin
• Glucose → GIP

3. Feedback Mechanisms

• Secretin inhibits gastrin.
• Somatostatin inhibits almost all hormones.

Easy Diagram of GI Hormones (Text Diagram)

[ STOMACH ]
|– Gastrin → ↑ Acid secretion
|– Ghrelin → ↑ Hunger

[ DUODENUM / JEJUNUM ]
|– Secretin → ↑ Bicarbonate
|– CCK → ↑ Enzymes & bile
|– GIP → ↑ Insulin
|– Motilin → ↑ Motility

[ PANCREAS ]
|– Somatostatin → ↓ All GI secretions
|– Pancreatic Polypeptide → ↓ Pancreatic secretion

Clinical Importance of GI Hormones

1. Zollinger–Ellison Syndrome

Caused by a gastrin-secreting tumour → excessive acid → ulcers.

2. Diabetes and GIP

GIP and GLP-1 are reduced in type-2 diabetes.

3. Obesity and Ghrelin

High ghrelin → excessive hunger → weight gain.

4. Motility Disorders

Low motilin → slow gastric emptying.

Summary of GI Hormones (Table)

Conclusion

GI hormones play a crucial role in the coordinated regulation of digestion, metabolism and intestinal motility. They ensure that digestive organs act in harmony and respond to the type of food we eat. Understanding their sources and functions is essential for students of physiology, zoology, medicine, and allied fields.

Suggested Readings

• NCBI – Gastrointestinal Hormones
• Harvard Medical School – How Digestion Works
• Mayo Clinic – Digestive System Overview
• Cleveland Clinic – Gut Hormones
• Endocrine Society – Digestive Hormones
• NIH – Ghrelin and Appetite Regulation
• Nature – Incretins (GIP & GLP-1) Research

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