The digestive system: Discover the journey of your last meal

The digestive tract is a long, muscular tube that runs the entire length of your body. It is separated into five distinct sections, each with its own particular and specialised function.
Digestion begins in the mouth. As you chew your food, saliva is released, providing a slippery lubricant and kick-starting the break down of carbohydrates with an enzyme known as amylase. Touch receptors in your mouth tell you when it is time to swallow, and as your tongue comes upward, the food is pushed to the back of your throat.

As you swallow, you pass control of digestion over to your automatic motor functions. A flap of skin called the epiglottis folds down to cover the voice box, and the entrance to the lungs, and then a wave pushes the mouthful all the way down the oesophagus. When the food reaches your stomach, it passes through a ring of muscle known as the cardiac sphincter, which prevents it from coming back out the way it came in.

The inside of the stomach is a hostile environment, where the cells lining the walls pump out hydrochloric acid and protein-digesting enzymes. The presence of food triggers stretch receptors in the stomach lining, which in turn trigger a series of rhythmic contractions. These churn the stomach contents, mixing in the acid and enzymes, grinding down the food.

At the bottom of the stomach there is a second ring of muscle called the pyloric sphincter, which acts as a gatekeeper to the small intestine. The sphincter prevents anything larger than about two centimetres (0.8 inches) in diameter passing through, returning it to the body of the stomach until it has been ground down further. This ensures that by the time it reaches the small intestine, your food is a runny, slightly lumpy paste, and is ready for the next stage of digestion.

The small intestine is the site of chemical digestion. Here, the pancreas adds digestive enzymes, and the liver adds a generous squirt of alkaline bile, delivered via the gall bladder. This bile not only neutralises the burning stomach acid, it also acts a little like washing-up liquid on dirty dinner dishes, helping to separate the food particles and forcing fats to disperse into tiny bubbles.

Muscles in the small intestine continue to squeeze and mix the contents together, allowing the enzymes to get to work inside the paste. As the nutrients are released, they are then absorbed over the walls of the intestine and into the bloodstream.

To ensure that everything keeps moving through the system, every five to ten minutes a wave of muscle contractions begins at the stomach and travels all the way down the intestines. Known as the migrating motor complex (MMC), this wave squeezes the digestive system like a tube of toothpaste, urging its contents further toward the colon.

As the food progresses through the small intestine, more and more of the nutrients are released by enzyme activity, and by the time it gets to the large intestine, most of the useful material has been absorbed into the bloodstream. However, the digestive process is not over, and here, bacteria help to break down even more of the undigested food.

The large intestine also absorbs most of the remaining water, leaving behind a combination of undigested material, dead cells and bacteria. When the waste has completed its journey through the large intestine it goes to the rectum for storage until there is a convenient time to get rid of it.

Journey of food

Journey of your food

Discover more amazing science in the latest issue of How It Works magazine. It’s available from all good retailers, or you can order it online from the ImagineShop. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, make sure you subscribe today!

Plus, take a look at:

Top 5 facts: Digestion

Why do we feel sleepy after eating?

The biology of hunger