Human digestive system
The human digestive system is a complex series of organs and glands that processes food. In order to use the food we eat as energy, our body has to break the food down into smaller molecules that it can process; it also has to excrete (or get rid of) waste.
Most of the digestive organs (like the stomach and intestines) are tube-like and contain the food as it makes its way through the body. The digestive system is essentially a long, twisting tube that runs from the mouth to the anus, plus a few other organs (like the liver and pancreas) that produce or store digestive chemicals. Without the digestive system, our bodies would not be able to get nutrients from the food we eat or get rid of the waste products that food makes and we would soon become ill!
Top 10 facts
- The small intestine is about 7 metres long, and about 2.5 centimetres in diameter. The surface area is around 250 square metres, or about the size of a tennis court!
- Some animals have stomachs with multiple compartments. (They're often mistakenly said to have multiple stomachs.) Cows, giraffes, deer and cattle have four-chambered stomachs, which help them digest their plant-based food.
- Some animals – including seahorses, lungfishes and platypuses – have no stomach. Their food goes from the oesophagus straight to the intestines.
- We make 1 to 3 pints of saliva a day.
- It takes your mouth, oesophagus, stomach, small intestine, large intestine, gallbladder, pancreas and liver just to digest a glass of milk.
- An adult’s stomach can hold approximately 1.5 litres of material.
- Food stays in your stomach for 3 to 4 hours.
- Cells along the inner wall of the stomach secrete roughly 2 litres of hydrochloric acid (the powerful chemical commonly found in some cleaning supplies, including toilet-bowl cleaners!) each day, which helps kill bacteria and aids in digestion. To protect itself from the corrosive acid, the stomach lining has a thick coating of mucus. But this mucus can't buffer the digestive juices indefinitely, so the stomach produces a new coat of mucus every two weeks.
- When your tummy rumbles, it’s the normal movements in the stomach and small intestines as food, fluid and gases pass through your gastrointestinal tract. When the tract is empty, however, the noises are louder because there's nothing in there to muffle the sound.
- Within the colon, a typical person harbours more than 400 distinct species of bacteria.
Did you know?
- When you eat something, the food doesn't simply fall through your oesophagus and into your stomach. The muscles in your oesophagus constrict and relax in a wavelike manner (called peristalsis). This motion pushes the food down through the small canal and into the stomach. Because of peristalsis, even if you were to eat while hanging upside down, the food would still be able to get to your stomach!
- The detergents used to wash clothes often contain several different classes of enzymes, which are also found in the human digestive system. The digestive system uses proteases to break down proteins like meat, amylases break down carbohydrates like bread and lipases break down fats like cheese. For example, your saliva contains both amylases and lipases, and your stomach and small intestine use proteases.
- Most people think that the stomach is the centre of digestion, and it does play a large role in digestion by churning food, mixing it with gastric juices, physically breaking up food bits and turning them into a thick paste called chyme. However, the stomach is actually involved in very little chemical digestion, the process that reduces food to the size of molecules, which is necessary for nutrients to be taken up into the bloodstream. Instead, the small intestine, which makes up about two-thirds of the length of the digestive tract, is where most of the digestion and absorption of nutrients takes place. After further breaking down the chyme with powerful enzymes, the small intestine absorbs the nutrients and passes them into the bloodstream.
Look through the gallery and see if you can spot the following:
- The digestive system
- Your internal organs!
- The mouth and teeth
Our body needs food to provide it with energy, vitamins, and minerals. However, in order to use food, we must first break it down into substances that the various organs and cells in our body can use. This is the job of our digestive system. The digestive system acts in stages to digest our food. Each stage is important and prepares the food for the next stage. The entire length of our digestive system is around 20 to 30 feet!
Chewing is the very first stage of the digestive system. When you chew your food it breaks up big pieces into little pieces that are easier to digest and swallow. Also, your saliva is more than just water. It has special enzymes in it that start to break down starchy food (potatoes, bread) while you chew.
Swallowing is the next step in the process. You might think it happens all by itself, but food doesn't just fall down our throats into our stomach. Firstly, our tongue helps to push the food into the back of our throat. Then there are special throat muscles that force the food down into a long tube that leads to our stomach, called the oesophagus. The food doesn't just fall down the pipe, muscles push the food along until it gets to our stomach. At the same time all this is going on, a flap blocks off our windpipe making sure food doesn't go the wrong way. (We call this "going down the wrong pipe" and it can make us choke.) This flap is called the epiglottis and, fortunately for us, it works automatically.
The next stage is the stomach. Food stays in the stomach for about four hours. While the food sits there, more enzymes go to work on it, breaking down things like proteins that our bodies can use. The stomach kills a lot of bad bacteria as well, so we don't get sick. However, if any of these bad bacteria get through, that’s when you might experience a tummy ache or sick bug.
The next step in the process is the small intestine. The first part of the small intestine works with juices from the liver and pancreas to continue to break down our food. The second part is where the food gets absorbed from the intestine and into our body through the blood.
The last stage is the large intestine. Any food that the body doesn't need or can't use is sent to the large intestine and later leaves the body as waste.
The liver and pancreas do a lot to help the digestive system along. Both work with the small intestine. The liver provides a substance called bile (and this is stored in the gall bladder). Bile helps break up fat into smaller parts. The pancreas provides more enzymes to help digest all sorts of food. The liver also processes all the digested food from your blood before it gets sent to hundreds of places in your body to be used.
Words to know:
absorb - to soak up a liquid or take in nutrients or chemicals gradually
bacteria - a single-celled microorganism. Various species are responsible for decay and many plant and animal diseases
chemicals - produced by or involved in the processes of chemistry
corrosive - able to destroy something progressively by chemical action
digestive - relating to or aiding in the digestion of food
energy - power from the physical or chemical resources, especially to provide light and heat or to work machines or even the body
enzyme - a complex chemical produced by living cells
excrete - to isolate and discharge (get rid of) waste matter generated during metabolism
glands - a cell or group of cells that secretes a specific substance
mucus - the clear slimy lubricating substance consisting mostly of mucins and water that coats and protects mucous membranes
nutrient - a substance that provides nourishment
organ - a complete and independent part of a plant or animal that has a specific function
process - a series of natural occurrences that produce change or development
saliva - the clear liquid secreted into the mouth by the salivary glands, consisting of water, mucin, protein, and enzymes. It moistens food and starts the breakdown of starches.
secrete - to produce and discharge a substance
Just for fun...
- A fun digestion game
- Can you build a digestive system (KS2)?
- Guide food through the digestive system
- Build a human being!
- Play Chewed-up Food Race
- Understand more about digestive systems by looking into a cow's stomach
- Play Centre of the Cell's It takes guts game and carry out virtual surgery to treat obesity or try your hand at Centre of the Cell's Poo Racer: collect bacteria points and fuel up with food and drink pit stops and help your poo vehicle travel through the gut!
Find out more
- Watch an interactive, animated guide to the digestive system
- A digestive system quiz to see how much you’ve learnt
- Print out and label the digestive system
- A digestive system word search
- Digestion is the breakdown of larger molecules into smaller molecules using enzymes; find out more about digestion in a step-by-step guide to the process for older children
- When you feel sick your body gets rid of food; find out more in What is puke?