1.0 Introduction to Digestion
The food we eat, like bread, fruits, or pulses, consists of complex substances. Our body cannot use these directly. To get energy, our body must break down these complex "bricks" into simple, soluble "sand" that can be absorbed by the blood. This process is called Digestion.
Digestion: The process of breaking down complex, insoluble food components into simple, soluble substances that the body can absorb.
Main Components of the Digestive System:
The human digestive system consists of two main parts:
- Alimentary Canal: A long, muscular tube (about 9 meters) starting from the mouth and ending at the anus.
- Digestive Glands: Organs like the liver and pancreas that secrete juices to help in chemical digestion.
Steps of Nutrition:
Digestion is a 5-step journey:
- Ingestion: Taking food into the mouth.
- Digestion: Breaking down of food (Mechanical & Chemical).
- Absorption: Passing of digested food into the blood.
- Assimilation: Use of absorbed food by cells for energy and growth.
- Egestion: Removal of undigested waste from the body.
Do not confuse Excretion with Egestion. Egestion is the removal of undigested food, while excretion is the removal of chemical wastes produced inside the body's cells.
The alimentary canal is about 30 feet long in an adult. That’s about as long as a school bus!
2.0 The Mouth: Where the Journey Begins
Digestion starts the moment you take a bite of food. The mouth (or Buccal Cavity) performs both mechanical and chemical digestion using three main tools: Teeth, Tongue, and Saliva.
1. The Teeth (Mechanical Digestion)
Humans have two sets of teeth in their lifetime: Milk teeth (20) and Permanent teeth (32). There are four types of teeth, each with a specific job:
- Incisors (8): Chisel-shaped for biting and cutting.
- Canines (4): Pointed and sharp for tearing food.
- Premolars (8): Broad surfaces for crushing and grinding.
- Molars (12): Larger surfaces for fine grinding.
Enzymes: These are biological catalysts that speed up chemical reactions (like breaking down food) without being used up themselves.
Chemical digestion of Starch begins in the mouth! Salivary Amylase breaks down starch into a simpler sugar called Maltose. This is why bread tastes sweet if you chew it for a long time.
Enamel, the white outer covering of your teeth, is the hardest substance in the entire human body—even harder than bone!
3.0 The Oesophagus and The Stomach
After food is chewed and mixed with saliva, it forms a soft ball called a bolus. This bolus now travels down a pipe to reach the main "churning machine" of our body—the stomach.
1. The Oesophagus (Food Pipe)
It is a 25 cm long muscular tube. No digestion happens here; it simply acts as a passage.
Peristalsis: Food doesn't just "fall" down the pipe due to gravity. The muscles of the oesophagus contract and relax in a wave-like motion to push the food forward. This is why you can even swallow food while hanging upside down!
2. The Stomach
A J-shaped muscular bag that can store food for about 3 to 4 hours. It secretes Gastric Juice, which contains three vital components:
- Hydrochloric Acid (HCl): Kills harmful bacteria that enter with food and makes the medium acidic for enzymes to work.
- Pepsin: An enzyme that starts the digestion of Proteins into smaller peptides.
- Mucus: Protects the inner lining of the stomach from being damaged by its own acid.
By the time food leaves the stomach, it has been churned into a thick, creamy soup-like consistency. This partially digested food is now called Chyme.
The Hydrochloric Acid in your stomach is strong enough to dissolve a metal razor blade! The only reason it doesn't burn your stomach is because of the thick layer of mucus.
4.0 The Small Intestine & Associated Glands
The small intestine is the longest part of the alimentary canal (about 7 meters!) and the site where the "final battle" of digestion takes place. It receives help from two very important glands: the Liver and the Pancreas.
The Supporting Glands:
1. The Liver: The largest gland in the body. It secretes Bile Juice, which is stored in the Gallbladder.
Function: Bile breaks down large fat droplets into tiny ones (Emulsification).
2. The Pancreas: A leaf-shaped gland located below the stomach. It secretes Pancreatic Juice.
Function: Contains enzymes like Trypsin (for proteins), Amylase (for starch), and Lipase (for fats).
Final Digestion in the Small Intestine:
By the time food travels through the small intestine, it is completely broken down into its simplest forms:
- Carbohydrates → Glucose
- Proteins → Amino Acids
- Fats → Fatty acids and Glycerol
The Small Intestine is the primary site for Absorption. It has millions of finger-like outgrowths called Villi which increase the surface area for rapid absorption of digested food into the blood.
The small intestine is called "small" because of its diameter, not its length. In terms of length, it is much longer than the large intestine!
5.0 The Large Intestine and Egestion
By the time food reaches the end of the small intestine, all useful nutrients have been absorbed. What remains is a watery mixture of undigested food fiber and waste. This enters the final section of the alimentary canal: the Large Intestine.
1. The Large Intestine
It is about 1.5 meters long and wider than the small intestine. It consists of three parts: Caecum, Colon, and Rectum.
- Function: Its main job is to absorb water and some salts from the undigested food material.
- Storage: The remaining semi-solid waste (feces) is stored in the Rectum temporarily.
Egestion: The process of eliminating undigested food waste from the body through the Anus.
Summary of the Digestive Path:
Mouth → Oesophagus → Stomach → Small Intestine → Large Intestine → Anus
Don't forget the step Assimilation! After absorption, the blood carries glucose to cells to produce energy, amino acids to build muscles, and fatty acids to be stored as energy reserves.
The Appendix is a small finger-like projection at the junction of the small and large intestines. In humans, it is a vestigial organ (has no function), but in grass-eating animals, it helps digest tough cellulose!