DIGESTIVE SYSTEM ANATOMY & PHYSIOLOGY
How the human digestive system is organised
Digestive system anatomy and physiology describe the structure of the gastrointestinal tract and the processes that move, break down, absorb, and ultimately eliminate food from the body.
The digestive system includes the gastrointestinal (GI) tract and several accessory digestive organs. The GI tract is a continuous muscular passage running from the mouth to the anus. Along this route, the digestive system mechanically and chemically processes food so that nutrients can be absorbed and used by the body.
The liver, pancreas and gallbladder are not part of the hollow GI tract itself, but they make important contributions to digestion by producing or storing substances that enter the digestive tract.
Digestive system diagram: the path of food
A digestive system diagram usually shows the principal organs in the order through which food travels:
Mouth → Pharynx → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus
Food moves through this pathway by coordinated muscular activity. Different parts of the system specialise in transport, mechanical processing, chemical digestion, absorption, storage or elimination.
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Digestive system anatomy: major organs of the gastrointestinal tract
The anatomy of the digestive system can be understood by following the gastrointestinal tract from its beginning to its end.
Mouth and pharynx
Digestion begins in the mouth. Teeth mechanically break food into smaller pieces while the tongue mixes it with saliva and helps form a bolus for swallowing. Saliva moistens food and contains enzymes that begin the digestion of some carbohydrates.
Oesophagus
The oesophagus is a muscular tube connecting the pharynx with the stomach. Its main digestive role is transport rather than nutrient absorption. Coordinated muscular contractions move swallowed material towards the stomach.
Stomach
The stomach temporarily stores food and mixes it with gastric secretions. Acid and digestive enzymes contribute to chemical digestion, while muscular contractions churn the contents into a semi-fluid mixture known as chyme.
Small intestine
The small intestine consists of the duodenum, jejunum and ileum. It is the principal site where digestion is completed and where most nutrients are absorbed.
Large intestine
The large intestine includes the caecum, colon and rectum. Water and electrolytes continue to be absorbed, while intestinal contents become progressively more solid and eventually form stool.
Rectum and anus
The rectum temporarily stores stool before a bowel movement. The anal canal and its sphincters help control the final stage of defecation.
Accessory organs of the digestive system
Several organs contribute to digestion even though food does not pass directly through them.
Liver
The liver produces bile, which assists with the digestion and absorption of fats. It also processes many substances absorbed from the digestive tract.
Gallbladder
The gallbladder stores and concentrates bile produced by the liver and releases it into the small intestine when needed during digestion.
Pancreas
The pancreas releases bicarbonate-rich digestive fluid and enzymes into the duodenum. These enzymes help digest carbohydrates, proteins and fats.
Digestive system physiology: how digestion works
Digestive system physiology concerns what the digestive organs do and how their activities are coordinated. The main processes include ingestion, movement of food, mechanical digestion, chemical digestion, absorption and elimination.
| Process | What happens |
|---|---|
| Ingestion | Food and drink enter the digestive tract through the mouth. |
| Motility | Muscular activity mixes digestive contents and moves them through the GI tract. |
| Mechanical digestion | Chewing, churning and mixing physically reduce food and increase its contact with digestive secretions. |
| Chemical digestion | Acid, enzymes and other digestive substances break larger food molecules into smaller components. |
| Absorption | Digested nutrients, water and other substances cross the intestinal lining into blood or lymph. |
| Defecation | Material that remains after digestion and absorption is eventually eliminated as stool. |
Chemical and mechanical digestion
Mechanical digestion
Mechanical digestion changes the physical form of food without fundamentally changing its chemical composition. It begins with chewing and continues through muscular mixing and churning in the stomach and intestines.
Chemical digestion
Chemical digestion uses enzymes and digestive secretions to break large nutrient molecules into smaller components that the body can absorb. Carbohydrates are ultimately reduced to simple sugars, proteins to amino acids and small peptides, and dietary fats to absorbable lipid components.
How does food move through the digestive tract?
The walls of much of the gastrointestinal tract contain smooth muscle. Coordinated contractions known as peristalsis help move food, liquid and digestive contents forwards.
Other patterns of muscular activity mix the contents with digestive secretions and increase contact with the intestinal lining. The enteric nervous system, autonomic nerves, hormones, and local signals within the digestive tract control movement.
Sphincters at important junctions help regulate the passage of digestive contents from one region to another and reduce inappropriate backward movement.
Where does absorption take place?
Although digestion begins before food reaches the small intestine, most nutrient absorption takes place in the small intestine. Its inner surface provides a very large absorptive area through folds, villi and microscopic microvilli.
Blood vessels transport many absorbed nutrients first to the liver through the hepatic portal circulation. Much of the absorbed dietary fat follows a different route and initially enters lymphatic vessels within the intestinal villi.
The large intestine also performs important absorptive functions, particularly the recovery of water and electrolytes from the remaining intestinal contents.
The basic layers of the digestive tract wall
From the oesophagus through much of the gastrointestinal tract, the wall follows a common general organisation with four principal layers.
- Mucosa — the innermost layer facing the contents of the digestive tract. It is involved in secretion, protection and absorption, depending on the organ.
- Submucosa — connective tissue containing larger blood vessels, lymphatic vessels and nerves.
- Muscularis externa — smooth muscle responsible for much of the mixing and propulsion of digestive contents.
- Serosa or adventitia — the outer covering, depending on the location of the organ.
Digestive system anatomy and physiology: frequently asked questions
What is the anatomy and physiology of the digestive system?
Digestive anatomy describes the structures and organs of the gastrointestinal tract and its accessory organs. Digestive physiology describes how those structures move food, perform mechanical and chemical digestion, absorb nutrients and water, and eliminate waste.
What organs make up the gastrointestinal tract?
The gastrointestinal tract runs from the mouth to the anus and includes the mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anal canal.
What are the accessory organs of the digestive system?
Important accessory digestive structures include the teeth, tongue and salivary glands as well as the liver, gallbladder and pancreas. They assist digestion even though food does not travel through most of these organs.
What is the correct order of the digestive tract?
The path of food is mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus.
Where does most digestion and absorption occur?
A substantial part of chemical digestion and most nutrient absorption occur in the small intestine. Digestive secretions from the pancreas, liver and intestinal wall contribute to this process.
What is peristalsis?
Peristalsis is a coordinated, wave-like contraction of smooth muscle that helps move food and other contents through the gastrointestinal tract.
What is the difference between digestion and absorption?
Digestion breaks food into smaller chemical components. Absorption is the movement of substances that are ready for use across the digestive lining and into blood or lymphatic vessels.
Digestive anatomy and bowel wellbeing
Colon hydrotherapy works specifically with the contents of the large bowel. It does not cleanse the stomach, small intestine, liver or bloodstream. At Parkland Natural Health, we provide colon hydrotherapy at our Holborn clinic after consultation and suitability screening.
Sources and further reading
- NIDDK: Your Digestive System & How It Works
- OpenStax Anatomy and Physiology: Overview of the Digestive System
- OpenStax Anatomy and Physiology: Chemical Digestion and Absorption
This article provides general educational information about normal digestive anatomy and physiology. It is not intended to diagnose digestive symptoms or replace individual medical advice.
