The entire digestive system, from the mouth to the anus, is about nine metres long. Looped and coiled into the lower abdomen, the small intestine makes up two-thirds of this length. Digestive juices from the small intestine’s lining are added to the food to complete its chemical breakdown. The resulting nutrients are so small that they can pass through the lining into the blood, to be carried away to the liver. The liver acts as a kind of food processor, making new chemicals from the nutrients it receives and storing them until they are required. Those substances the body does not need, including impurities in the blood, it sends on to the kidneys.
THE KIDNEYS
The two kidneys receive a very large flow of blood—more than one litre per minute. It passes through about one million microscopic filtering units, called nephrons, packed into the outer layer of each kidney. The nephrons remove waste substances and excess water from the blood. These flow through the kidney’s inner layer, where some water is taken back into the blood according to the body’s needs. The resulting liquid waste is called urine. It dribbles down a tube, the ureter, to a stretchy bag in the lower abdomen, the bladder. It is stored here until it can be passed to the outside.
NUTRIENTS
The body needs a wide range of nutrients to stay healthy. There are six main groups of nutrients—proteins, fats, carbohydrates, vitamins, minerals and fibre. Different kinds of foods are rich in different groups. Proteins are found in meat, poultry, fish, milk, beans and green vegetables. They help to build and maintain muscles and other body parts, so they are important for growth. Fats (lipids) are found in meat, dairy products, pies, and some oily fruits and vegetables like avocado, olive and sunflower seeds. Small amounts are needed to build the walls around the body’s microscopic cells, and also for healthy nerves. Carbohydrates such as starches and sugars are found in bread, pasta, rice and other grains, and potatoes. They are the main source of energy for movement, digestion and other life processes.
VITAMINS AND MINERALS
Many vitamins and minerals are needed for good health, but usually in small amounts. Vitamins have letters such as A, B and C. Lack of a vitamin may cause illness. For example, lack of vitamin A from tomatoes, carrots, cheese, fish and liver may result in poor eyesight. Minerals include calcium and iron. Iron is found in meat, green vegetables and nuts. It is needed for healthy blood. Its lack causes a type of anaemia.
FIBRE
Fibre is found only in plant foods, chiefly in breads, pastas and other products made from whole meal grains or cereals, and also in many fresh fruits and vegetables. Fibre is not actually digested and absorbed by the body, but it helps the digestive system to work effectively and stay healthy. It adds bulk to the food so that the stomach and intestines can grip and squeeze the food along.
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