The human digestive system

Define digestion and explain why food needs to be broken down into smaller molecules.
Identify the main organs of the digestive system and describe their primary roles in digestion and absorption.
Describe the structure of the villi in the small intestine and explain how they are adapted for the efficient absorption of digested nutrients.

Your body needs to break down the large molecules in food (like starches and proteins) into much smaller molecules (like sugars and amino acids) that can be absorbed into your blood and used by your cells.
This whole process is called digestion, and it happens in your digestive system.
The digestive system is a long tube running through your body, along with some helper organs.
It works to break down the food and then absorb the small pieces.

Mouth: Digestion starts here! Your teeth chew the food (mechanical breakdown), mixing it with saliva, which starts chemically breaking down some starches.
Oesophagus: A muscular tube that pushes the swallowed food down towards your stomach.
Stomach: A muscular bag that churns the food, mixing it with strong acid and digestive juices (enzymes) to break down proteins and kill germs.
Small Intestine: A very long, coiled tube where most chemical digestion happens. Digestive juices from the pancreas and liver are added here, along with enzymes from the intestine wall, to break down fats, proteins, and carbohydrates into small molecules. This is also where these small nutrient molecules get absorbed into your bloodstream.
Large Intestine: Undigested food passes into this wider tube. Its main job is to absorb water back into the body, leaving behind a more solid waste material.
Rectum: Stores the waste material (faeces) at the end of the large intestine.
Anus: A ring of muscle through which faeces leave the body.

The main goal of digestion is to get useful nutrient molecules small enough to pass into your blood.
This happens mostly in the small intestine.
1. The inside wall of the small intestine isn't smooth; it's covered in millions of tiny, finger-like projections called villi.
2. These villi massively increase the surface area available for absorption.
3. They have thin walls (only one cell thick) and a good blood supply (lots of tiny blood vessels called capillaries) so the small, digested nutrient molecules can pass through quickly and easily into the bloodstream.
Once in the blood, these nutrients are transported all around your body to be used by your cells for energy, growth, and repair.

Digestion: The process of breaking down large food molecules into smaller molecules that can be absorbed.
Digestive system: The organs that work together to digest food and absorb nutrients.
Nutrients: Substances from digested food that are absorbed and used by the body for energy, growth, and repair.
Absorption: The process by which small nutrient molecules pass from the digestive system into the bloodstream.
Oesophagus: A muscular tube that pushes the swallowed food down towards your stomach.
Stomach: A muscular bag that churns the food, mixing it with strong acid and digestive juices (enzymes) to break down proteins and kill germs.
Small Intestine: The main organ where chemical digestion is completed and most nutrient absorption occurs.
Enzymes: Biological catalysts (chemicals) produced by the body that help speed up the chemical breakdown of food.
Villi: Tiny, finger-like projections lining the small intestine wall that increase the surface area for absorption.
Large Intestine: The part of the digestive system where water is absorbed from undigested food.

Research the roles of the accessory digestive organs (liver, gallbladder, and pancreas) and the substances they produce to help with digestion.
Investigate the cause and effects of common digestive disorders, such as ulcers, coeliac disease, or irritable bowel syndrome.