Digestive system organs and roles - Biology IGCSE Study Notes

The human digestive system is a specialized tube (approximately 9 meters long) that breaks down food into small, soluble molecules for absorption into the bloodstream. This process is called digestion.

Key organs and their roles:

Mouth: Mechanical digestion begins through mastication (chewing), breaking food into smaller pieces. Salivary glands secrete saliva containing the enzyme amylase, which begins chemical digestion of starch into maltose. The tongue shapes food into a bolus for swallowing.

Oesophagus: A muscular tube connecting mouth to stomach. Peristalsis (rhythmic muscle contractions) pushes the bolus downward, independent of gravity.

Stomach: A muscular bag with several functions: (1) mechanical digestion through churning movements, (2) chemical digestion via gastric juice containing pepsin (protease enzyme) and hydrochloric acid (pH 2), (3) kills pathogens, (4) stores food for 2-4 hours. The acid denatures proteins and provides optimal pH for pepsin.

Small intestine (duodenum and ileum): Primary site of digestion and absorption. The duodenum receives bile from the liver/gallbladder (emulsifies fats, neutralizes acid) and pancreatic juice containing amylase, protease, and lipase enzymes. The ileum completes digestion and absorbs nutrients through its extensively folded inner surface with millions of villi and microvilli.

Large intestine (colon): Absorbs water and minerals from undigested material, forming semi-solid faeces.

Rectum and anus: Store and eliminate faeces through egestion.

Cambridge Key Term: Egestion (removal of undigested waste) differs from excretion (removal of metabolic waste).

Think of the digestive system as a food processing factory assembly line, where raw materials undergo progressive transformation.

The Mouth: The Shredder Station Like a paper shredder increases surface area for recycling, your 32 teeth mechanically break food into smaller pieces, increasing surface area for enzyme action. Your salivary glands produce 1-1.5 liters of saliva daily—enough to fill a large water bottle! The enzyme amylase works so efficiently that if you chew a piece of bread long enough, you'll notice it tastes sweeter as starch converts to maltose sugar.

The Stomach: The Industrial Mixer Your stomach functions like a cement mixer, churning food with acidic gastric juice for 2-4 hours. The hydrochloric acid is so strong (pH 2) it could dissolve a razor blade given time! Yet your stomach protects itself with a thick mucus layer—nature's non-stick coating. This acid sterilizes your food, killing 99.9% of bacteria (similar to hand sanitizer), explaining why you're less likely to get food poisoning from well-chewed food.

The Small Intestine: The Absorption Warehouse Stretched out, your small intestine would cover a tennis court due to villi and microvilli—a biological solution to maximize absorption area in limited space. Like Amazon warehouses optimize delivery routes, your small intestine's design ensures nutrients reach your bloodstream efficiently.

Real-world connection: Understanding digestion helps explain lactose intolerance (lack of lactase enzyme in small intestine), celiac disease (damage to villi), and why elderly people need smaller, frequent meals (reduced stomach acid and enzyme production with age).