The human digestive system is a complex and efficient process that breaks down food into nutrients, which the body uses for energy, growth, and cell repair. From the moment food enters the mouth to its eventual excretion, digestion involves multiple organs working seamlessly together. Here’s a step-by-step look at how it works.
1. Ingestion: The Mouth
Digestion begins in the mouth, where mechanical and chemical breakdown starts.
- Mechanical digestion: Teeth chew food into smaller pieces, increasing its surface area.
- Chemical digestion: Saliva, produced by the salivary glands, contains amylase, an enzyme that breaks down carbohydrates into simpler sugars.
Once food is sufficiently chewed and mixed with saliva, it forms a soft mass called a bolus, which is then swallowed and moves into the oesophagus.
2. The Oesophagus: Transport to the Stomach
The oesophagus is a muscular tube that connects the mouth to the stomach.
- Peristalsis (wave-like muscle contractions) pushes the bolus downward.
- A ring-like muscle called the lower oesophageal sphincter opens to let food into the stomach and then closes to prevent acid reflux.
3. The Stomach: Acid and Enzyme Breakdown
The stomach acts as a mixer and churner, using gastric juices to further digest food.
- Gastric glands secrete hydrochloric acid (HCl) and pepsin, which break down proteins.
- The stomach’s muscular walls contract to mix food into a semi-liquid substance called chyme.
- After 2–6 hours, the chyme is gradually released into the small intestine via the pyloric sphincter.
4. The Small Intestine: Nutrient Absorption
Most digestion and nutrient absorption occur in the small intestine, which is divided into three parts:
- Duodenum: Bile (from the liver) and pancreatic enzymes (from the pancreas) neutralise stomach acid and break down fats, proteins, and carbohydrates.
- Jejunum: Absorbs sugars, amino acids, and fatty acids.
- Ileum: Absorbs remaining nutrients, including vitamin B12 and bile salts.
The inner walls of the small intestine are lined with villi—tiny, finger-like projections that increase surface area for maximum nutrient absorption into the bloodstream.
5. The Large Intestine: Water Absorption and Waste Formation
What remains after nutrient absorption passes into the large intestine (colon).
- Water and electrolytes are absorbed, turning liquid waste into faeces.
- Gut bacteria (microbiota) ferment undigested fibres, producing vitamins (like vitamin K) and gases.
- Faeces are stored in the rectum until expelled through the anus during defecation.
6. Accessory Organs: Liver, Pancreas, and Gallbladder
Several organs assist digestion without food passing through them:
- Liver: Produces bile, which emulsifies fats for easier digestion.
- Gallbladder: Stores and releases bile into the small intestine.
- Pancreas: Secretes digestive enzymes (lipase, protease, amylase) and bicarbonate to neutralise stomach acid.
In summary, digestion is a finely coordinated process involving mechanical and chemical breakdown, nutrient absorption, and waste elimination. Each organ plays a crucial role, ensuring the body receives the energy and nutrients it needs to function. Maintaining a healthy diet supports efficient digestion and overall well-being.
