The human alimentary canal, also known as the digestive tract, is a continuous tube that runs from the mouth to the anus. It is responsible for the ingestion, digestion, absorption, and elimination of food. By breaking down complex molecules into simpler nutrients, it ensures that the body receives the energy and building blocks necessary for survival, growth, and repair.
The alimentary canal includes specialized organs like the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. Supporting organs such as the liver, pancreas, and gallbladder also play critical roles in digestion.
Structure of the Human Alimentary Canal
Mouth (Buccal Cavity)
Digestion begins in the mouth, where food is broken down mechanically by chewing and chemically by saliva.
- Saliva contains the enzyme salivary amylase, which breaks down starch into simple sugars.
- The tongue helps in mixing food with saliva and aids swallowing.
Oesophagus (Food Pipe)
The chewed food (bolus) passes through the esophagus by rhythmic muscular contractions known as peristalsis, which push food into the stomach.
Stomach
The stomach acts as a muscular sac that stores and digests food.
- Gastric glands secrete hydrochloric acid (HCl), pepsin (a protein-digesting enzyme), and mucus.
- HCl provides an acidic environment, essential for pepsin to act.
- Food is churned into a semi-liquid form called chyme.
Small Intestine
The small intestine is the longest part of the alimentary canal, measuring about 6–7 meters (20–23 feet).
- It is divided into three parts: duodenum, jejunum, and ileum.
- Responsible for the complete digestion of carbohydrates, proteins, and fats.
- The liver secretes bile, which emulsifies fats.
- The pancreas secretes pancreatic juice containing enzymes like trypsin (for proteins) and lipase (for fats).
- The inner lining has villi, finger-like projections that increase surface area for nutrient absorption.
Large Intestine (Colon)
The large intestine is about 1.5 meters (5 feet) long. It is wider than the small intestine and consists of the cecum, colon, and rectum.
- Its primary function is the absorption of water, salts, and some minerals.
- Beneficial bacteria in the large intestine help ferment undigested food.
- The final waste is expelled through the anus, regulated by the anal sphincter.
Sphincter Muscle and Its Role
The sphincter muscle regulates the movement of food from the stomach into the small intestine.
- It ensures that food passes gradually, allowing sufficient time for digestion.
- At the other end, the anal sphincter controls the exit of undigested waste from the large intestine.
Functions of the Small Intestine
1. Digestion:
- Pancreatic enzymes and bile help break down carbohydrates, proteins, and fats.
- Intestinal glands secrete digestive juices for final breakdown.
2. Absorption:
- Villi and microvilli provide a large surface area.
- Nutrients like glucose, amino acids, fatty acids, and glycerol are absorbed into the bloodstream and lymph.
- Herbivores (e.g., cows) have longer intestines to digest cellulose.
- Carnivores (e.g., tigers) have shorter intestines since meat is easier to digest.
Summary of Absorption in Different Parts of the Digestive System
| Organ | Absorption |
|---|---|
| Mouth | Some drugs absorbed through mucosa into blood capillaries. |
| Stomach | Absorbs water, alcohol, and simple sugars. |
| Small Intestine | Principal organ of absorption; absorbs glucose, amino acids, fatty acids, glycerol, vitamins, and minerals. |
| Large Intestine | Absorbs water, minerals, and certain drugs. |
Importance of Accessory Organs
- Liver: Produces bile for fat digestion and detoxifies blood.
- Gallbladder: Stores bile and releases it into the small intestine.
- Pancreas: Secretes digestive enzymes and hormones (insulin, glucagon).
Key Highlights of the Digestive Process
- Mouth: Mechanical and enzymatic digestion begins.
- Stomach: Acidic digestion of proteins by pepsin.
- Small Intestine: Main site of digestion and absorption.
- Large Intestine: Water absorption and waste elimination.
FAQs on the Human Alimentary Canal
Q1. What is the main function of the human alimentary canal?
Its primary function is to digest food, absorb nutrients, and eliminate undigested waste.
Q2. Why is the small intestine longer in herbivores?
Because cellulose from plant material requires more time for digestion.
Q3. What role does bile play in digestion?
Bile emulsifies fats, breaking them into smaller droplets for efficient enzyme action.
Q4. How is nutrient absorption maximized in the small intestine?
The presence of villi and microvilli increases surface area for absorption.
Q5. What is the difference between small and large intestines?
The small intestine is longer and narrower, focusing on nutrient absorption, while the large intestine is shorter and wider, specializing in water absorption.
