OK, digestion. Not an easy one to explain actually. I LOVED your Youtube video of the GI tract model! (Note from Blast Science - Alice - watch video above!)
Like all things, food is made of lots of little things stuck together to make big things. Digestion is the process where the body breaks big things, like potatoes and sausages, into things small enough to be absorbed by, or in between, the cells lining the GIT (GastroIntestinal Tract . Basically everything between your teeth and your ring piece!). The GIT is approximately 9 meters long. That the width of a full sized tennis court! Nearly all of it is between the bottom of you ribs and the top of you pubic bone. That gap is only about half a meter! So as you can imagine its very curled and twisted up so we can fit it all in.
The little things, the building blocks, of food that it needs t be broken down into are: sugar (which make carbohydrates like rice, pasta, bread and potatoes) this is used for energy,
Amino acids (which come from proteins like meats and nuts) these are then put back together, in a different pattern a bit like lego blocks, to make the different proteins which we need. Most of the bits of our bodies that do stuff- muscles, skin, organs, blood cells are made of protein. Fats and oils (which come from bigger fats and oils) some of which is used to make cells, some of which is a high energy source, but most of which we don't really need and only eat cos it tastes nice! If we eat more than we need as its stored around our waists and bums and makes us "fat".
Digestion starts as food as we put food in our mouth. Our teeth chomp and bite and grind the food into smaller bits.. Our spit contains enzymes. Enzymes are chemicals that break down other chemicals. You can do an experiment on this. Put some plain bread in your mouth and chew it for a long time (?minute). What started out tasting "bready" will start to taste sweet, a bit like sugar because the long carbohydrates in the bread have been broken down by your spit's enzymes into individual sugars.
Then we swallow it and our food pipe uses peristalsis to bring it down into the stomach. Our stomach is basically just a big bag of acid. Mainly hydrochloric acid with a pH of 1-3. This is so acidic that if it was anywhere else in your body it would start to dissolve it, but the stomach has evolved especially to act as a safe bag of acid). The acid has 2 effects- 1 It dissolves all the food down into smaller and smaller pieces, breaking big proteins into smaller chains of amino acids, and 2) It kills any bugs that might be in your food to stop you getting sick. If you don't have acid in your stomach either because of a disease, or medication, you are more likely to get food poisoning.
The stomach also churns your food around, a bit like a washing machine, helping to mix everything up with the acid.Want to know what the stuff in your stomach looks like? Want to know what it taste like?! Well, next time you're sick, all that puke that comes up is the contents of your stomach. When you're sick your stomach literally turns itself inside out (which is why it hurts) and everything that was being nicely digested gets shot back up the food pipe and out of your mouth (and if you're really unlucky, your nose too!)
After it's been dissolved in acid for a few hours what remains of your dinner is let out bit by bit to the next part of the GIT, your "duodenum". Next to your duodenum are two organs which most people don't know anythign about.
The pancrease- which squirts more enzymes into your GIT to further break down proteins and carbohydrates.
The gall bladder which squirts green stuff called bile into the GIT to break down fats.
It then passes into your "small intestine". This is only "small" because it's narrow. It actually make up nearly all of the 9 meters of GIT (As opposed to the "large intestine, which is only a meter or so. Weird! It is wider though).
The mixed up gunge of digesting food, saliva, acid, enzymes, water, spit etc slowly passes through the small intestine and the small bits start to get absorbed, through the wall of the GIT and into your blood. Again peristalsis keeps pushing it in one direction, down towards your bum. If it didn't when you burped you might get poo coming out! (Probably not actually true but been too long since an "Urgh" bit)
By the time it gets to your large intestine there's actually very little of your original meal left. But two important things happen here. First most of the water is reabsorbed so it changes from sludgy gunge into a solid poo. Secondarily there's squillions of "good bacteria" in your large intestine. There's about ten times more bacteria in your gut than there are human cells in your body! So really you are more bacteria than human!
These bacteria are the last bit of digestion and they are good bacteria because they help us. Thet can break down some carbohydrates that humans can't, so they digest them for us and we both get a feed off stuff that would otherwise be pooed out. The down side to this is that the processes they use to break down the carbohydrates gives off gases. Strong smelling gasses. Can you guess what they are? FARTS! So it's not really us that fart, its the bacteria. It's all their fault.
And at the end of this we're left with a nice firm brown poo, sitting in our rectums. Our rectums get stretched, which sends a message to your brain saying "time to poo". I thinks its amazing that through that entire process the only bits we have to do consciously are "swallow" and "Poo". All the rest happens without us knowing anything about it.
And as for poo, what is it? Most of it's water. Some of it is undigested food, mainly fibre, a type of carbohydrate that niether we nor the good bugs can digest. Some of it is stuff the body doesn't want or need and gets thrown into the GIT a bit like a rubbish chute. Some of it is actually cells from the lining of the GIT that have died and travel down the tube with the rest. But at least half of the solid matter is actually living or dead bacteria, from our guts! Which is why you still poo even when you haven't eaten. And why you should always wash your hands after having a poo - cos it's really just a big ball of stinky bacteria.