Small intestine
Enzymes
To prevent the sensitive intestinal villi, located in the small intestine, from becoming overburdened, the food will not get past the sphincter that separates the stomach from the duodenum until the small intestine receives a signal that the food is not too fatty, acidic or rich in protein. With the help of the enteric nervous system in the gastrointestinal tract and certain cells in the intestinal wall, substances (such as hormones) are formed to enable the body to sense how much and what kind of food we have eaten. This ensures that substances produced that are able break down the food into smaller components.
On the intestinal surface you find the intestinal villi – small outgrowths that are also able to absorb nutrients from the food. If one were able to smooth out all the creases and villi of the small intestine, its surface would amount to approximately 250 square metres – the size of a tennis court! The villi can be damaged, for instance, when people who are gluten intolerant continue to ingest gluten. This makes it difficult for the body to absorb nutrients from the food, and therefore makes you sick.
Absorption of carbohydrates, proteins and fat
Once the food has been passed from the stomach and entered the 3–5 metre-long small intestine, the nutrients are sucked out of the food and transferred into the body through the bloodstream. These nutrients consist of energy in the form of carbohydrates and fats, proteins that, among other things, serve as building materials in our cells, and vitamins that help the cells function as they should. Also salts, such as calcium, potassium, magnesium and iron are absorbed from the food in the small intestine. This is also where most of the liquid from the food is reabsorbed. The inside of the small intestine is creased, in order to be as large as possible and enable it to get as much nutrition as it possibly can from the food.
Immune system highway
Tiny blood vessels (capillaries) are woven into the intestinal walls, among other places. The walls also contain lymphatic vessels, much like a highway grid of the immune system. If we need to quickly fight off a potential danger, such as bacteria or viruses, the immune cells are quick to respond! In fact, almost 80 per cent of our immune system is located within the intestinal tract, particularly in the small intestine! This is also where the intestinal bacteria and immune cells send signals to each other. This way, the body learns to distinguish harmful from harmless.
Kidneys
Eventually, the blood passes through the kidneys where it is purified from unwanted substances and form urine. More than 1400 litres of blood is filtered through the kidneys every day!
The urine is stored in the bladder and, when it is full, the body signals that it is time to pee. You then need to listen to your body and not to hold it in too long – the kidneys don’t like that! Urine contains water, salts and urine substances in the form of residues from proteins, etc. Make sure you drink enough water, as this makes it easier for the kidneys to remove toxins and other potentially harmful substances from the body. The yellow colour of urine comes from a substance called bilirubin – the same substance that also gives stool its dark colour. Urine tends to be darker and more concentrated in the morning, because you have not been drinking all night.
Carbohydrates
The carbohydrates we eat eventually become glucose, a type of sugar, which provides energy to all cells of the body – the muscles, brain, etc. We have energy reserves stored all around the body, for example in the form of fat in different parts of the body and as a layer of glycogen in the liver and muscles.
It is common to distinguish between fast and slow carbohydrates. Fast carbohydrates can quickly be absorbed by the body and be sent out into the bloodstream as energy for the cells. It takes somewhat longer for the body to divide slow carbohydrates into small components that can be absorbed into the blood, and therefore makes you feel full for a longer period of time.
Tip: if you have a headache and feel nauseous, it may simply be because you need to eat or drink water. When you grow and exercise a lot, it is also important to regularly replenish your energy reserves. It’s good to have a snack shortly before exercising. You also need to replenish your energy after you have completed your exercise.
Did you know that:
- Most adults are not able to break down lactose, resulting in the forming of gas inside the intestines. A mutation among people in the Nordic region allows us to tolerate milk (lactose).
- Bacteria in the intestinal tract can help stimulate our immune system to make us more resistant to cold viruses.
- 80% of our immune system is located in the small intestine.
- Our intestines contain 1–2 kilos of bacteria that help break down food.
- The stomach and intestines are surrounded by a network of neurons – the enteric nervous system – which has direct contact with the brain.