1.2 Human Alimentary Canal

Introduction

The human digestive system is a complex arrangement of organs that work together to break down food, absorb nutrients, and remove waste. The main part of this system is a long continuous tube called the alimentary canal, supported by several associated glands. This topic introduces the overall structure of the human alimentary canal and the organs associated with it.

Definition

The alimentary canal (also called the gut, digestive tract, or gastrointestinal tract) is a continuous muscular tube, about 9 meters long in an adult, that begins at the mouth and ends at the anus. It is specialized at various points along its length, with each region designed to carry out a different function in the overall process of digestion, absorption, and egestion.

The digestive system consists of the alimentary canal and the associated digestive glands.

Main Idea

The human digestive system consists of two sets of organs:

• The alimentary canal — concerned with ingestion, digestion, absorption, and egestion of undigested food.
• The digestive glands — associated with the alimentary canal and help in digestion.

The alimentary canal is lined internally by a mucous membrane.

Core Concept

Parts of the Alimentary Canal

The alimentary canal consists of the following parts in sequence:

• Mouth
• Oral cavity
• Pharynx
• Oesophagus (esophagus)
• Stomach
• Small intestine
• Large intestine

Digestive Glands (Associated Organs)

The digestive glands associated with the alimentary canal are:

• Salivary glands
• Gastric glands
• Intestinal glands
• Liver
• Pancreas

Detailed Explanation

1. Mouth

What it is: The external opening that leads into the oral cavity or mouth cavity.

Function: Entry point for food into the digestive system.

2. Oral Cavity

What it is: The cavity inside the mouth.

• Around the sides and front of the oral cavity are the upper and lower jaws.
• The upper jaw is fixed while the lower jaw is movable.
• The oral cavity contains teeth and tongue.

Function: Mechanical digestion begins here (chewing), and chemical digestion of starch starts (salivary amylase).

3. Pharynx

What it is: A muscular tube that leads from the mouth and nasal cavity to the oesophagus and to the trachea.

Function: The pharynx is a common passage for food and air.

Important point: Because it is a shared pathway, the epiglottis prevents food from entering the trachea during swallowing.

4. Oesophagus (Esophagus)

What it is: A muscular tube, about 25 cm long.

Function: It leads from the pharynx to the stomach.

Important point: In the oesophagus, no digestion takes place. Food moves through it by peristalsis.

5. Stomach

What it is: The stomach is the dilated (enlarged) part of the digestive tube. It is roughly J-shaped and somewhat transversely placed on the left of the abdomen just beneath the diaphragm.

• It has two openings, which are guarded by valves.
• The opening between the oesophagus and stomach is called the cardiac opening, which is guarded by the cardiac sphincter.
• The region of the stomach around the cardiac opening is the cardiac end.
• The opening of the stomach into the duodenum is called the pyloric opening, which is guarded by muscles called the pyloric sphincter.

Function: Mechanical digestion (churning), chemical digestion of proteins (by pepsin), and limited absorption.

6. Small Intestine

What it is: The small intestine is small in diameter but long in length. It is divided into three parts:

Function: Most chemical digestion and absorption occur here.

Important point: Bile duct from gall bladder, hepatic duct from liver, and pancreatic duct from pancreas empty their secretions into the duodenum.

7. Large Intestine

What it is: A tube leading from the small intestine to the anus.

Length: About 1.5 meters.

Diameter: Larger than the small intestine.

• Caecum — first part of the colon. The ileum enters it from the side. It has a pouch-like end to its base called the appendix (about 10 cm long).
• Colon — divided into:
• Ascending colon (right)
• Transverse colon (across)
• Descending colon (left)
• Sigmoid colon (connects to the rectum)
• Rectum — a straight, 8-inch chamber that connects the large intestine to the anus. It temporarily stores faeces.
• Anus — the last part of the digestive tract. It is a 2-inch long canal surrounded by sphincter muscles that are important in allowing control of stool.

Function: Absorption of water and some vitamins; formation and storage of faeces; egestion.

Intermediate Level

The alimentary canal is a continuous tube with specialized regions. Each region has a specific role:

The associated glands (salivary glands, gastric glands, intestinal glands, liver, pancreas) secrete enzymes and other substances that help in digestion. The liver produces bile, which emulsifies fats. The pancreas produces pancreatic juice containing enzymes for starch, protein, and fat digestion.

Important Points

• - The alimentary canal is about 9 meters long in an adult.
• - It begins at the mouth and ends at the anus.
• - It is lined internally by mucous membrane.
• - The main parts are: mouth, oral cavity, pharynx, oesophagus, stomach, small intestine, and large intestine.
• - The digestive glands are: salivary glands, gastric glands, intestinal glands, liver, and pancreas.
• - No digestion takes place in the oesophagus.
• - The stomach has two sphincters: cardiac and pyloric.
• - The small intestine has three parts: duodenum, jejunum, and ileum.
• - The large intestine has: caecum (with appendix), colon (ascending, transverse, descending, sigmoid), rectum, and anus.
• - The pharynx is a common passage for food and air.

Remember

• - Alimentary canal = mouth → oral cavity → pharynx → oesophagus → stomach → small intestine → large intestine → anus.
• - Digestive glands = salivary glands, gastric glands, intestinal glands, liver, pancreas.
• - Oesophagus = no digestion, only transport.
• - Stomach = J-shaped, cardiac and pyloric sphincters.
• - Small intestine = duodenum, jejunum, ileum.
• - Large intestine = caecum, appendix, colon, rectum, anus.
• - Pharynx = common passage for food and air.

Common Mistakes

• 1. Thinking digestion occurs in the oesophagus.
• - No digestion takes place in the oesophagus. Food only moves through it by peristalsis.
• 2. Confusing the small and large intestine.
• - The small intestine is longer (about 6–7 meters) but smaller in diameter.
• - The large intestine is shorter (about 1.5 meters) but larger in diameter.
• 3. Forgetting the appendix is part of the large intestine.
• - The appendix is attached to the caecum, which is the first part of the colon.
• 4. Mixing up the cardiac and pyloric sphincters.
• - Cardiac sphincter = between oesophagus and stomach.
• - Pyloric sphincter = between stomach and duodenum.
• 5. Thinking the stomach is the main site of absorption.
• - Most absorption occurs in the small intestine, not the stomach.
• 6. Believing the pharynx is only for food.
• - The pharynx is a common passage for both food and air.

Quick Revision

60-Second Revision

• Alimentary canal = mouth → oral cavity → pharynx → oesophagus → stomach → small intestine → large intestine → anus
• Length = about 9 meters
• Digestive glands = salivary glands, gastric glands, intestinal glands, liver, pancreas
• Oesophagus = no digestion, only transport
• Stomach = cardiac sphincter (top), pyloric sphincter (bottom)
• Small intestine = duodenum + jejunum + ileum (main site of digestion and absorption)
• Large intestine = caecum + appendix + colon + rectum + anus (water absorption, egestion)
• Pharynx = common passage for food and air
• Warning: Do not confuse cardiac sphincter with pyloric sphincter.