The Digestive System

1. Mouth

• Food is physically broken down by the teeth.
• The tongue mixes food with saliva from the salivary glands.
• Saliva contains the enzyme amylase, which breaks down starch into shorter chains.

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2. Oesophagus

• Food is swallowed and enters the oesophagus, which carries food from the mouth to the stomach.
• The walls of the oesophagus squeeze in and out to move food along. This muscular action is called peristalsis.
• The epiglottis, a flap of tissue, covers the trachea (windpipe) to prevent food from going down the wrong way.

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3. Stomach

• Food is churned in the stomach and mixed with gastric juice, forming a mixture called chyme.
• The enzyme pepsin begins breaking down proteins.
• Hydrochloric acid in the stomach kills most bacteria that may be present in food.
• Food remains in the stomach for up to four hours before moving to the small intestine.

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4. Small Intestine

• Chemical digestion occurs in the small intestine.
• The first part of the small intestine is the duodenum:
  • Chyme from the stomach mixes with digestive juices from the pancreas, gallbladder, and the small intestine.
  • Gallbladder releases bile, which breaks fats into droplets.
• Digestive enzymes break down:
  • Proteins → Amino acids
  • Fats → Fatty acids and glycerol
  • Carbohydrates → Simple sugars like glucose.
• The walls of the small intestine are covered in tiny finger-like structures called villi, which absorb nutrients into the bloodstream.

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5. Large Intestine

• Waste not absorbed through the walls of the small intestine passes into the large intestine (also called the bowel or colon).
• Water is absorbed into the bloodstream, causing waste to become more solid (now called faeces).
• Faeces are pushed along the large intestine by peristalsis and leave the body through the rectum and anus.