Digestive system – how does it work?




Most of the digestion occurs in the first part of the small intestine, but the absorption of broken down nutrients, water, vitamins and minerals occurs in the rest of it. So it’s so long for a reason, not just to make it all cozy and cooped up in your stomach. A tube connects pancreas with duodenum where all the enzymes travel when the food is present. A separate tube connects the gallbladder and liver to the duodenum, which pushes through bile.

What happens after the absorption is the most exciting part and the point of our main interest. The amino acids and simply sugars travel via the portal vein to the liver for immediate processing. BUT the fatty acids are transported to the liver by a different route (the lymphatic system), this is a very important difference, make a mental note of it. 2. Once liver processed everything it needs, the rest appears in general circulation.

Raising levels of any of the three above-mentioned classes of nutrients reach the pancreas, where beta cells release the hormone that everyone heard about, yet hardly anyone knows anything about it – insulin. 

“Wait a second,” I hear, “we know what is insulin”! Diabetics needs it and they usually take it extra, because they don’t have enough of it and it does something with your sugar, like brings it down. Fine, but what does it actually do? Hmmm?

If you are someone who is just an ordinary mortal, with no diabetes, you wouldn’t know. Why would you? But I will tell you, because when I first found out, I was genuinely surprised that I lived all these years now knowing something this exciting!


Insulin – is a energy storage hormone. Insulin is a fat storage hormone. You heard me right! Insulin kindly delivers excess fatty acids or blood lipids to the fat cells. It’s the key to unlock your fat cells! Knock, knock, fat cells! “I brought you something darling,” says Mr Insulin, “here you go guys, make yourself comfortable, get inside here, and kindly transform yourself into a big fat FAT!”. The more insulin, the more fat. It’s going to sit there, all nice and comfortable, all getting cozy, as long as the insulin is around.

When the insulin drops, the process goes in reverse – triglycerides (those guys who were getting all comfortable in the fat cell, which looks like the white/yellowish fat around your steak, yam!) get broken down, causing the fat cells to shrink and the fatty acids re-enter the bloodstream and travel back to the liver, where they get burned by it or other organs and it’s gone! We repeat this insulin cycle every day and by playing it up and down, we burn the energy when we need and store the rest. But some people, obviously, just keep having a non-stop fat making party and eventually you become obese and have so much fat that it gets all around your organs and then you are in trouble.3

But back to the digestive system. As has been mentioned, gallbladder and liver sends bile to help the digesting process.  Bile is the primary way the body gets rid of excess cholesterol and heavy metals such as copper, but obviously to a limit, because high cholesterol is obviously a big problem these days. Cholesterol is type of lipid, and lipids are fat around your body. One might think that we are just made of pure fat, but we, of course are not. Not all of us.

Then once small intestine is done and the food, or what’s left form it, has made all the journey through your 6 meters or so of your guts, it gets to large intestine. Large intestine, also called the colon, is not responsible for digestion (obviously, because it’s all been already done by a 6 meter long factory). Instead, its purpose is to complete water and electrolyte (minerals found naturally in the body, such as potassium, calcium, sodium, and magnesium) absorption which begun by the small intestine.

Those components of food that are not needed or cannot be absorbed (like fibre, for example; or that plastic part you’ve accidentally eaten) are excreted from the colon in stool. Yes, you just poop it all out! What about  the colour of the stuff you produce? Well, the colour of the stool comes from the tiny amount of bile released from the liver each day that is not reabsorbed. 4

The process is obviously rather complex and requires precise coordination between all the different organs, use of hormones and nerves. We are rather complex creatures, yet obviously very clever ones, as we manage to do all that  while enjoying after food without even knowing what happens after you swallow 😉 The actual reason why we can do it is because gastrointestinal tract has its own nervous system, so we are very clever, we just don’t always know about it. 5


  1. One of the biggest pleasures in life is food. We eat various food for several reasons, e.g. to build muscle, for bone growth and to burn for energy. Whilst everyone knows at least something about food and some vague idea about nutrition, hardly anyone knows what happens inside you after you have eaten. I have to say that the inner world inside you, and I mean the physical part of it, is a very busy place indeed. Let’s see what is going inside you. Lots of germs!!

    Once the food arrives in the stomach, which is, by the way, the size of a your fist and it can stretch 10 times the size (but you will usually be satisfied once you have topped out 1 – 1.5 litres), hydrochloric acid will be released to start digesting the food into smaller components. Then the food enters the next part of the digestive tract, which is small intestine. Small intestine is about 250 cm (8 feet) for a baby, then it grows between to 350cm to 660cm (12 to 22 feet). Just think for a second, look at your stomach, you can have up to 6.5 meters long intestine, your organs are quite cooped up there!

    Once the food is in your small intestine, the party begins! A bunch of different enzymes (proteins) produced by pancreas digest various components of our food into even smaller components such as:

    Dietary fats into fatty acids

    Dietary protein is sliced into amino acids (so there is nothing mysterious about amino acids that everyone heard of, but no-one actually knows what it is; it’s just sliced protein)

    Carbohydrates into simple sugars (mostly glucose with some fructose molecules)

    Dietary fiber is turned into… nothing! It stays intact, as we can’t digest it. But it does magic for us, like slows the absorption of other nutrients and pushes things quicker through intestine. [note] I will dedicate an entire post for dietary fiber, because it totally needs and deserves that much attention

  2. It’s important because, as you will see later, when you over-indulge in sweet fats, for example, it becomes too much for the liver, as it has to deal with an overwhelming about nutrients at the same time, which are attacking it from different sides. I will talk about it later at length
  3. But this is an entire topic on its own
  4. This detailed info taken from here )) That act finishes your digesting cycle and then you can start it all again!
  5. This post has been written by occasionally referencing to Dr Lustig, “Fat Chance”, 2014, book

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