Large Intestine
Bacteria helping with Last Chance Digestion
Large Intestine
The chyme has been pushed by peristalsis through the 6 meters of the small intestine it has been pushed over a surface that is about the size of a badminton court.
By the time it leaves the small intestine, digestion by your digestive enzymes has finished.
The volume of chyme leaving the small intestine is far less than the volume that entered it - due to the absorption of nutrients
It pours into the large intestine for its final phase. The last chance to get nutrients out.
This last chance has a little help - 100 trillion bacteria are waiting to help.
Over this last 1.5 meters, with the help of bacterial enzymes, the last nutrients and water are absorbed.
Your Microbiome
About 0.03 % of your total body mass is bacteria!
For me, this is about 200 grams - the same as mass as 6 weetbix (a weetbix is 33g) or a whole Pie (BP pie (200g))
Almost all of this mass is in the large intestine, as over 99% of your bodies bacteria live in this warm, nutrient rich, anoxic environment. Infact
These bacteria are essential for you
They are kept where they should be by your strict Immune System and by physical barriers such as the skin and the walls of the intestines. And by mucus that moves them along and out.
These microbes are the good guys, they eat everything that is dead, thus preventing bad bacteria from having any food. The good bacteria that are yours have little genetic markers that show that they are yours. These bacteria have colonized your body completely, thus making house prices too high for any of the bad bacteria to be able to move in.
Because these bacteria are yours, they are referred to as your microbiome.
A biome is all of the live that live in a habitat. Micro meaning microorganisms.
So you are the habitat and the microorganisms that live on and in you are your microbiome
With 99% of your microbiome living in your Large Intestine, there must be a reason. So what are they doing in there?
These gut bacteria, the microbiome in your large intestine are doing something special, they are ... feeding.
As a byproduct of their feeding they create 3 extremely important things:
Short Chain Fatty Acids
Vitamin K
Vitamin B
They also make some not so useful things like the gases of farts
The bacteria do another thing that is helpful. They stop bad bacteria from surviving. Your good bacteria eat the nutrients and occupy the space that could otherwise be taken up by bad bacteria
There are thousands of different species of bacteria in your large intestine. There are two big phylum that are actively being studied.
Firmicutes
Bacteroidetes
You can see in the pie graphs that the percentage of the gut biome that each of these phylum occupy is heavily influenced by diet and disease.
What is correlation and what is causation is unknown at this stage. But it is interesting.
Bacterial Digestion
The image above is from human feces. It shows a wide range of different bacteria (artificially colored) and a very large number of them. Here they are feeding on a piece of plant material, fiber, that has managed to move all they way through the digestive system.
They feed on the fiber by doing extracellular digestion. That is that they release the enzyme Cellulase onto the fiber, it breaks down to sugar, they then absorb these nutrients.
Bacteria break down about half of the fiber you eat
Fiber is mostly cellulose. Cellulose is glucose molecules that are linked together in a way that our digestive enzymes active sites can't get to.
Amylase can only breakdown starch. You need Cellulase for Cellulose, and only bacteria make Cellulase.
Bacteria can break down all of it if given enough time, but you need a longer digestive system for them to have more time.
Cellulose
Half of the fiber you eat is turned into glucose by bacteria for bacteria
There is no oxygen in the large intestine, so the bacteria here are doing anaerobic respiration
Aerobic respiration would turn it into carbon dioxide and water. For aerobic respiration you combine 1 glucose with 6O2 to make 6CO2 and 6H2O
However, you need oxygen for this process
With anaerobic respiration their isn't enough oxygen so what do you make instead?
Anaerobic respiration, with the lack of oxygen, means that bacteria needs to break glucose into other small things. These include H2 gas, Methane CH4, Water H2O a little bit of Carbon dioxide CO2
Glucose
Water
Carbon Dioxide
Hydrogen
Methane
If we look at the composition of a fart. We can see these gases.
Most of a fart is nitrogen, this is simply air that was swallowed when you were eating, chewing and swallowing. (Indeed you also swallow some oxygen, but as the bacteria are anaerobes the oxygen will just come back out)
Next is Hydrogen gas, H2. This is Hydrogen from the glucose.
In aerobic respiration those hydrogens would be stuck onto an oxygen atom to make water. But with anaerobic respiration there are no oxygens to be stuck onto
Next is Carbon dioxide. This is the carbon and the oxygen from the glucose. However, as it takes 2 oxygens for each carbon to make CO2, we will quickly run out of oxygens
So, what is left? Well we have just Carbons and Hydrogens. (excluding any that left as Hydrogen gas). We can stick 4 hydrogens onto each Carbon. This makes CH4
If we just look at this as a nice tidy anaerobic respiration formula, we would say
Anaerobic Respiration = C6H12O6 = 3CO2 + 3CH4.
If we count our atoms, then it works
So, the end product of bacterial anaerobic respiration, is a fart.
Most of the gas of a fart is from bacterial anaerobic respiration. This includes Hydrogen gas, Carbon dioxide and Methane. A small amount of a fart is gas was air that was trapped in the food or in the bolus when swallowed. Only about 1% of the gas of the fart is the smelly sulfur compounds, but if your stuck in a small space, then that is more than enough to stink out the place. These bacteria make about a liter of gas per day.
Bacterial metabolic intermediates: The 2, 3 or 4 Carbon compounds
As the glucose turns into carbon dioxide and methane, it has to go through intermediate steps as it slowly breaks down.
These intermediaries are smaller structures.
Smaller than glucose, but bigger than carbon dioxide or methane.
These are structures that have 2, 3 or 4 Carbon atoms.
These short carbon chain compounds are called Short Chain Fatty Acids or SCFA
The production of these Short Chain Fatty acids is not exactly simple as can be indicated by the diagrams below
At the start of the first diagram we have the word Polysaccharide meaning many sugars.
Replacing that word in the second diagram is the word Carbohydrates meaning a hydrated carbon
In the diagram above the gases have been highlighted blue
You can see multiple points were H2 gas is created. remembering that Hydrogen gas is a major gas in farts.
You can also see the other major gases; CO2 and CH4
And you can see the smelly Hydrogen Sulfide H2S.
You can also see that Ethanol (alcohol) is also made. This ethanol is made all of the time in very low levels due to this anaerobic respiration. Your body quickly detoxifies this ethanol using Alcohol dehydrogenase in the liver. This is the reason that you have this alcohol dehydrogenase in your liver; to detoxify the Ethanol that gut bacteria make during anaerobic respiration. Once human civilizations discovered how to make alcoholic beverages, they could drink to get drunk. This alcohol dehydrogenase gets busy to detoxify all this alcohol as well.
Looking in the diagram above, you can see follow the arrows to the other end points: Butyrate Propionate and Acetate.
In the diagram below the Short Chain Fatty Acids have been bolded, you can also find them in the diagram above
SCFA's: Butyrate (4 Carbons), Propionate (3 Carbons), Acetate (2 Carbons)
Short Chain Fatty Acids
Bacteria break this big 6 Carbon glucose molecule into the 4 Carbon Butyrate, 3 carbon propanoate or 2 carbon Acetate.
You can see by looking at Acetate, that if we have just one spare hydrogen atom, then we can turn Acetate into Carbon dioxide and Methane
Carbon number and molecule name
6 Carbons = Glucose
4 Carbons = Butyrate
3 Carbons = Propanoate
2 Carbons = Acetate
I remember them using the acronym GBPA - 6432.
Glucose
Butyrate
C4H8O2
Propanoate
C3H6O2
side note; pyruvate is C3H3O3
Acetate
C2H3O2
side note; ethanol is C2H6O
These short chain fatty acids are not like the enormous fat molecules that you usually think of. Rather these are tiny healthy things made from the break down of cellulose
Bacteria will continue to break these short chain fatty acids down into H2, CH4 and CO2. But, some of these Short Chain Fatty Acids escape from the bacteria and leave in feces. More importantly the large intestine will absorb these SCFAs.
Along their journey to cellular respiration, these SCFA's have effects on the wall of the large intestine, on the immune system, on the heart and on the brain
The cells of the wall of the large intestine will conduct cellular respiration (burn) these Short Chain Fatty Acids.
Furthermore, some of these SCFA's will enter the blood stream where they can go to cells throughout the body where they can be used as a fuel for cellular respiration
The 2 Carbon Acetate makes up the bulk of the SCFAs at 60%
Then 20% for both the 3 Carbon Propanoate and the 4 Carbon Butyrate
Approximately 50% of the 'undigestable' fiber you is actually digested by bacteria. Of this approximately 20% of this is then absorbed by your body.
To put it another way
10% of the Carbon of 'undigestable' fiber that you eat is actually absorbed into your blood stream as very healthy SCFA's due to the digestion of this fiber by bacteria.
If your Large intestine was to be longer, then the bacteria would have more time and more would be catabolized into SCFAs and more would be absorbed
If your hunter gather tribe has lots of fruits, tubers, vegetables to eat then
Its a good idea to get as much energy from that cellulose as possible - so fiber does contribute to energy intake (but in a small and healthy way
You haven't been hunting
Inflammation is usually your immune systems quick to the trigger method of dealing with bacteria. This fiber rich diet reduces this inflammation. This could be linked back to the injuries inflicted during hunting compared with grazing.
In an agricultural society we don't hunt as much. So this quick to inflammation response of the immune system is not needed. So, this fiber rich diet of a old fashioned farmer keeps the immune system calm.
Short Chain Fatty Acids basically tell the immune system that everything is alright and it can chill out and calm down.
As you can see from the graph. The SCFA's help you to feel full (Satiety). This is in addition to how fiber makes you feel full by taking up a large amount of space in the GI track. These SCFA's help to reduce blood sugar levels, decrease fat storage in fat cells and reduce Inflammation
Evolution and Diet and the Large Intestine
The Large Intestine is responsible for the digestion of cellulose and resistive starch. But we cannot do it. No mammals can. Rather it is bacteria that do the digesting for us.
Mammals do not have the enzymes that have the active sites that can fit cellulose. But the digestive enzymes released by bacteria can fit.
Bacteria need time for this breakdown of cellulose. For us, approximately half of the cellulose eaten is turned into glucose for bacteria. During this process our large intestine absorbs roughly 10% of the atoms from the fiber as Short Chain Fatty Acids that are used as a source of energy by the cells comprising the large intestine and some makes its way to the liver and through the body to be used an energy source
Percentage of the GI tract dedicated to each part in each mammal
If you need more of your energy to come from these SCFAs and glucose from fiber, then you need more time for the bacteria to do its thing. The most effective way to give the bacteria more time is to give it more space to work in. In other words make the Large intestine longer.
The bulk of the mass of a hunter gatherer human's diet is plant based. So they need to get some SCFAs from this cellulose rich diet
The hunter gatherer will occasionally kill an animal, now there is meat
Meat has more energy (due to fat) and more protein than plants. So instead of eating all day like gorillas, the human can be more like the lion and eat meat then have a nap.
The small intestine is where protein and fat is absorbed. The villi of the small intestine move the amino acids and peptides into the capillaries. The villi of the small intestine move the fat into the lacteals.
The small intestine is also where 70% of the digestion of starch occurs for humans, so there needs to be time and space for this to happen.
Chimpanzees mainly eat plants. They will eat insects and meat if when they can.
Because plants are a bigger part of their diet than it is for us, you can see in the graph that the size of their Large intestine is bigger than ours.
Likewise on the table you can see the percentage of their GI track that is Large intestine is 52%, whereas our Large intestine is only 16% of our GI track.
For the chimpanzee, only 23% of their GI track is Small Intestine. Whereas, for us humans, it is 67%.
This is because we eat more meat than they do. We also eat more tubers and high starch plants than they do. Thus our GI track has evolved to get the most from the meat we eat
Humans are omnivores. We are an omnivore that has a starch rich diet from our tubers and grains (potato, kumara, Taro) (rice, wheat). This is supplemented with cellulose rich leaves (cabbage, lettuce) and sugar rich fruits (oranges, apples). We are also an apex predator and supplement our diet with meat.
Chimpanzees are also omnivores, but they have a cellulose (leaves) and fruit rich diet. They supplement this with insects and meat if they can, but meat is hard for them to find
Rhinos are herbivores. They eat all day long. They have a long and wide large intestine so that bacteria can break down all that cellulose so that the rhinos large intestine can absorb the glucose and SCFAs
Lions only eat meat. Meat is easily digested by the digestive enzymes that mammals have (Pepsin, Trypsin, Lipase)
Amino acids, peptides and fat are easily absorbed into the villi. Once these have been absorbed, only water minerals and vitamins are left and these occur in both the small and large intestine. As such, the lion has a very short digestive system compared to omnivores and herbivores.
If you compare the massive length of the human digestive system to the lions short digestive system, you can see that we were only able to occasionally get meat as hunter gathers. We were good hunters, but no where near as good as lions. Hunter gathers would often return to camp with no meat.
Evolution as Hunter Gatherers
As hunter gatherers we ate roots (tubers) for starch.
We ate fruits and berry's for sugar.
We ate meat if we could catch it - and this was hard work
The documentary 'Origins of us' is about human evolution. It is brilliant, but heavily blocked on YouTube. Below is a link to it on Daily motion. The Episode is called Guts. This is because it looks at the role that changes to our diet (forced onto us by climate change) had on our evolution. Watching it adds an air of clarity to this entire topic. I highly recommend it. :-)
Evolution as Farmers
As famers (agricultural civilizations) we grew crops that were starch rich. We grew fruit trees and berry shrubs for sugar. And we farmed cattle, sheep, goats, deer and pigs for the occasional 'feast' day.
Farming plants means that you have a reliable source of food - this was the original start for farming
Farming animals means that you don't have to go hunting and risk returning home with nothing
The risk of farming, when it first started, is that not everyone was farming. So you and your tribe has a farm. Then along comes a nomadic hunter gatherer tribe and they see your farm as food. This means you need to have weapons to protect your stuff. This leads to forts (and eventually castles) where harvested and dried grains (like wheat for flour and dried rice) can be stored for winter and protected from nomadic tribes.
Deciding to kill one of your beasts would usually be only for a very good reason, due to the cost of raising them.
If you could get the animals milk, then this would be a high fat high protein supplement to your otherwise plant based diet. You will get more protein and fat from milk than you will from killing it. As once it is killed you'll need to share the meat with family otherwise the meat will go off.
This feast day could be religious, superstitious, for a birthday, a birth or a wedding. We didn't eat meat everyday - only the very rich and the kings could do this. You'd sell the occasional beast to these very rich individuals. You'd only kill one of your own beasts infrequently as they are a source of income. Also, once you killed a beast, it needed to be eaten or salted as soon as possible. Even the salted cuts of meat didn't last very long. This spurred on the Salt trade. As the meat went 'off' the flavor changes, to hide the flavor of the 'off' meat you could use spices - this ability to make your meat edible for another day is valuable and drove the 'spice trade'.
Often your meat would go into a soup or a stew, that way it could stay cooking over your fire place for days, with you topping it up with starchy vegetables that would boil over they fire, breaking the starch down into sugary bits. These soups lasted days, as bacteria can't rot hot meat. Some would leave them
This is why in the very olden days the cauldron was a popular cooking pot. If you were just going to kill and barbeque, spit roast or pit cook your beast, then you'd do it on feast or celebration days because you'd have your family and friends around so it'd all get eaten whilst fresh.
The point being that we didn't eat meat very often. Just every now and then.
Evolution as Eaters of cheap processed artificial ingredient and preservative filled sugary Factory Food
The population is over 7 billion. This has placed different demands on how we get our food and has led to the rise of factory foods.
This isn't all bad, this means that lots of food can be made quickly.
However, this process is also heavily influenced by the goal of making money
All fast food beef burgers that you buy have beef patties
You can go to like Big J's and buy a steak burger, but I'm talking about the international super rich super popular chains: Wendys, BK, Carls Jr, Mcdonalds
Beef patties contain mince and flour. Mince is meet that has already been mechanically digested. This makes eating it very easy.
If we look at the other ingredients he bacon is about as thin as a piece of paper. The vegetables in it are cut very thin, again making it very easy to chew - this reduces the effort for eating and allows us to eat more. This is why you can eat more McDonalds burgers than you can your own home made 'steak' burgers (mince vs steak is important here).
The burgers are designed to be eaten easily, thus we can eat more of it, thus we buy more.
There is a guy whos eaten a Mcdonalds Big Mac burger every day since the 1970s.
He's not fat because he doesn't have the coke.
This is super important - we don't naturally drink sugar!
So, even though sugar is a food, when we drink it, our body doesn't register it as being eaten - because we are not chewing!
So all that sugar, is then absorbed in the villi, but your still hungry because you haven't eaten anything. You can drink an entire bottle of coke - more energy than is in your dinner, instead of dinner, and you will still be hungry when you go to bed.
Furthermore, you can eat more if what you are eating is sugary. This is because when a hunter gatherer finds a sweet fruit in the wild they need to eat as much as possible because the fruit wont stay fresh for long.
You eat more when you eat something sugary than something starchy or fatty.
In fact, if you eat more solid food when you have a coke with your meal than when you have water. This applies to both sugar coke and diet coke (your brain thinks its sugar).
Because sugar is high in energy, animals are rewarded when they eat some.
If you are in the wild and you find some blue berries, they are tiny and don't look very fulling. So why would a big animal like a human spend time trying to pick all these tiny berries?
Despite their tiny size, you get a lot of energy out of them due to their high sugar content.
Thus, to ensure we make the effort to pick and eat all those tiny blue berries. Or go to the effort of raiding the bee hive and being stung for honey. Animals have evolved a 'feel good' response to sugar. Our brain releases 'dopamine' when we eat sugar.
Because of this, you will climb the tree, risk falling, get stung by bees, all for a little honey... all for a tiny bit of sugar.
You don't even need to digest the sugar, simply having it in your mouth, that sweet taste, is enough. This is the same with artificially sweetened foods. You still get the good feelings.
So then big companies are all about making money. If they add a little bit of this sugar then you will feel happy when you eat their food or drink their drink. But then their competitor does the same. So then they add more. Think about how you feel emotionally when you drink coke or eat chocolate - does it make you happy?
These tasty happy foods influence the way we shop at the supermarket
Look at the change in spending over the 30 years from 1982 to 2012. What do you notice?
Food companies have to make money, so how do they do this:
Make a product you want to buy
Make the product as cheaply as possible
Make a product you want to buy
Almost all factory made food will contain sugar. This is because your brain likes sugar.
If you take a pet mouse or rat and put it in a cage with two drinking bottles, one with water and one with sugar water, they will choose the sugar water.
In fact they will choose it till become extremely obese
This is the same with the increase in fat cats and fat dogs - they are feed sweet human food as snacks and being dogs they spend most of the day sleeping - so they have high energy food and low energy lifestyle
They also target children with their advertising. Because what you did as a child, you'll do as an adult, and then your children will do. Think about the brand choices your parents make - chances are you are eating the same brands that they ate as children
They will also make it easier to chew, easier to digest so that you get the sugar hit faster. To do this, they reduce or completely remove Fiber.
The loss of fiber has massive implications
Its loss means that we need to eat more to feel full
Its loss means that we consume more calories
Its loss means that Short Chain Fatty Acids are not being made
Its loss means that the gut microbiome is less healthy
Its loss means the large intestines are less healthy
Its loss means that you are less healthy
Make a product cheaply
Replace plant based flavors and colors with artificial flavors
Replace expensive ingredients with cheap ingredients e.g. replace milk with palm oil
Add preservatives, this slows down bacterial digestion of the food, so it lasts longer. These preservatives make the food toxic to microbes so that it will extend its shelf life, thus giving the company more time to sell its product
This western diet has negative impact on gut health, on the health of the large intestine.
It has a negative impact on the microbiome and the production of those health SCFAs.
This decrease in SCFA is one of the many ways in which a 'Western diet' has a negative impact on overall health.
These companies know that the sugar makes you happy.
In fact that spend billions of dollars on advertising telling you it makes you happy
Think of the ads for Coke of for Fanta.
Look at the correlation between money spent on advertising and obesity
This is the Western Diet is one that is created in the marketing offices and board room of Western Food Companies initially. However, this 'Western diet' is becoming global with food companies in all countries, including the East, taking the same approach:
Make it easy to eat - reduce fibre
Make it something you want to buy more than the competitors product - increase sugar
Make it cheaply - artificial ingredients, palm oil, preservatives
The unspoken goal of the Western Diet is to make the people selling the food the most money possible by making the food as pleasurable as possible. Moving eating away from being something done to keep us alive. To something we do to make us happier.
If it makes a person happy, if it makes you happy, then how do you 'cut back'?
Sugar is key to making food addictive
We always get a feel good reward when we eat anything. However the more sugar that thing contains the greater the reward. So if you have to choose between a bowl of bran or a bowl of ice-cream, you already know which one will make you feel happier
But this high sugar diet has health consequences
The addition of sugar to food is everywhere.
The food companies resist any government changes and use their money to prevent changes.
We get a high reward, feeling good, for something that is only found in fruits and berries so something that is only ready at certain times of the year. Now we can have this several times a day and in far higher quantities than is found in the fruits and berries.
When we drink it our bodies don't register it as food. So we keep drinking it. Could you finish a whole bottle of coke?
This has health implications in 2 major domains:
Obesity
fat cells around your body, and the liver can turn sugar into fat
Diabetes
the cells in the pancreas get 'burnt out' producing higher levels of insulin than they were designed to produce
As companies realized how addictive sugar is, and as consumers wanted to eat more and more of it, the amount of simple sugar in our diets have skyrocketed. We eat 12 times more sugar now than we did 200 years ago. So, the sugar that an early European NZ settler would eat in a day, we eat every 2 hours!
The biggest player is sugary drinks
This graph to the side shows increases in obesity along with the increase in Sugar Sweetened Beverages (SSB) and Artificially Sweetened Beverages (ASB)
The graph below shows the consumption of soft drinks and the percentage of obese each decade
If we look at diabetes we can see that it was once a relatively rare disease. The percentage of people diagnosed with diabetes has increased 7 times since the 1950s
The main place people are getting their sugar from is sugary drinks.
Where ever sugary drinks have gone, obesity and diabetes has followed
The cheapness of soft drinks and the higher cost of fresh fruit and vegetables means that those people with lower incomes and more life challenges are effected more by the obesity and diabetes epidemic.
The Obesity and Diabetes epidemic is effecting poor people and poor countries are affected by this obesity
Once the Coca-cola company got into Mexico the levels of obesity started to increase.
Once the Coca-cola company was selling its soft drinks for less than the cost of clean water, Mexico became the fattest country in the world.
Mexico
drinks the most coke in the world
is the fattest country in the world
Once Mexico announced that it was going to introduce a sugar tax, Coca-cola spent millions on stopping the sugar tax. Bribing and doing 'favors' for officials, big businesses, people of influence, and politicians.
Food is very important for health
What you eat can help you or hurt you
Looking over the graph you can see just how important what you eat is
Be careful about what you choose to eat
A diet full of artificial, processed, factory made food full of sugar is un-naturally evolutionary
You are what you eat
This high sugar is un-natural when you look at the evolution of our diets. It is also un-natural when we realize that most of our sugar comes from sugar water (coke) and thus you realize that we are drinking a food.
It is also un-natural when you look at how fast we have increased the quantity of sugar and sugary drinks in our diets
And of course, companies do this because we like it.
This links into food addiction. Addiction to sweet foods is important for the transition from someone being Overweight, to being Obese
We might think that this 'addiction' doesn't matter healthy wise compared to other addictions.
However for some people who have developed food addiction. They will drink bottles of coke every day
This high sugar diet has an impact on all organs.
It damages the pancreas, causing Type 2 Diabetes
It contributes to Atherosclerosis and Heart Disease
It contributes to the development of Fatty liver disease
Basically, to reduce the negative effects of sugar all you have to do is eat less of it
Replace it with stuff that is high in fiber!
The fiber increases feelings of fillness
The fiber increases levels of healthy SCFA's
It's easy to eat more fiber because it is in fruits and in vegetables
Eat well for less :-)
Interesting...
Harvard School of Public Health
David L. Katz, who reviewed the most prevalent popular diets in 2014, noted:
The weight of evidence strongly supports a theme of healthful eating while allowing for variations on that theme. A diet of minimally processed foods close to nature, predominantly plants, is decisively associated with health promotion and disease prevention and is consistent with the salient components of seemingly distinct dietary approaches. Efforts to improve public health through diet are forestalled not for want of knowledge about the optimal feeding of Homo sapiens but for distractions associated with exaggerated claims, and our failure to convert what we reliably know into what we routinely do. Knowledge in this case is not, as of yet, power; would that it were so.
Marion Nestle expresses the mainstream view among scientists who study nutrition:
The basic principles of good diets are so simple that I can summarize them in just ten words: eat less, move more, eat lots of fruits and vegetables. For additional clarification, a five-word modifier helps: go easy on junk foods. Follow these precepts and you will go a long way toward preventing the major diseases of our overfed society.
The Sci Show Sugar Compilation is one of the best clips I've seen looking at Sugar and Artificial sweeteners
Absorption in the Large intestine
The chyme in the small intestine has travelled 2.5 meters. It has had most of its nutrients and water absorbed
It has shrunken in volume as these nutrients and water has been absorbed.
But the body is not done with it yet. There are still nutrients, minerals, vitamins and water to be absorbed.
50% of fiber is digested by your gut bacteria. Of this, 10% of these atoms are absorbed by the gut as Short Chain Fatty Acids (very healthy very good things). So the Large intestine provides space for this to happen. If the Large intestine is longer, than more of your fiber can be digested by the bacteria and you will get more SCFAs as a source of energy. This happens for chimpanzees and gorillas as they have longer large intestines because they have a far more plant based diet.
Peristalsis moves the chyme along in the large intestine, as it changes from liquid chyme, to semisolid feces. Peristalsis here is slow, at about 10cm per hour. This gives plenty of time for bacteria to work on the cellulose and release short chain fatty acids.
Every now and then a 'mass movement' wave of contraction will move along the large intestine, forcing everything to move forward. These big contractions ensure that everything keeps moving forward.
The Gut Bacteria also make Vitamin K. Vitamin K is required for your blood to clot after a cut. Think Vitamin K = Klot, Kut.
The Gut Bacteria also make some of the B Vitamins. These are used in the body to assist in metabolism and in the making of neurotransmitters
Vitamin K and B are made here, so they are also absorbed here.
All of the protein and fat from your diet was absorbed in the Small intestine. Under 5% of both make it into the large intestine, however bacteria will then used these as nutrients. So you miss out here. No protein is absorbed in the large intestine. This also helps to explain why if you have protein powder, you have those super smelly protein fats. The bacteria are eating any protein and releasing gas. Also, some of the amino acids in the protein powder contain sulfur atoms, and these are super smelly.
Most of your other Vitamins have already been absorbed back in the small intestine.
Almost all of the bile salts that were pumped into the small intestine by the gall bladder, are absorbed in the small intestine
Likewise most of your minerals have already been absorbed. Iron and Calcium are absorbed in the Small Intestine
Like most of the minerals, Sodium and Chloride have already been absorbed during that 2.5 meter journey that is the small intestine, however a small but important amount remain, This relates back to water.
The large intestine is the last chance to absorb the last of the water. Over the course of the day this is almost half a liter. If this water was not reabsorbed, then you would have the runs or diarrhea.
Unlike what most people think, most of the water you drink is absorbed in the small intestine. I did an experiment once where we had to drink 2 liters of water, and then pee into a cup and different moments over an hour to check out kidney function. The demand on the toilets was high as the 50 odd students all needed to use them at the same time. The point here is that the 2 liters of water was absorbed in my small intestine. It moved into the blood and then to my kidneys for removal as it was diluting my bloodstream too much and needed to be removed.
Water needs to travel with the chyme into the large intestine, or else the SCFAs being made by bacteria wouldn't be able to diffuse to the wall of the large intestine. But this water has to be absorbed, or it will run down your leg and increase the risk of you becoming dehydrated.
To absorb water, the large intestine invests energy into absorbing salt, Sodium and Chloride. Where ever salt goes, water will follow. So by actively transporting salt into the blood stream, water will follow. This is how feces goes from being watery to dry-ish.
In countries where poverty, food poisoning, water borne illness and diarrhea are a problem. One of the treatments is to use glucose-salt tablets. Both the small and large intestine absorb salt. When a kid takes the glucose-salt tablet dissolved in a glass of water, their intestines absorb the salt and because water follows salt, water is also absorbed. This doesn't fix the cause of the diarrhea, but it stops them from becoming dehydrated.
The large intestine absorbs:
Short Chain Fatty Acids made by bactiera
B Vitamins made by bacteria
Vitamin K for blood clotting, made by bacteria
Salt is actively absorbed
As a consequence of Salt absorption, Water is absorbed
The Wall of the Large Intestine
The large intestine contains NO villi
Rather the wall of the large intestine is rather smooth
It has 'crypts' that release mucus, this mucus keeps the feces moving
The tissue of the wall of the large intestine contains capillaries and lymphatic
Large Intestine Blood Supply
Blood vessels wrap around the Large Intestine and take all of the nutrients absorbed to the Liver.
The blood vessels come from the aorta, and branch of as mesenteric arteries. They branch into small arterioles and then into capillaries in the wall of the Large Intestine.
The blood vessels, these capillaries, then get loaded up with the Short Chain Fatty Acids, Vitamins K and B, Sodium Chloride and Water.
These capillaries then join up as veins, called the mesenteric veins. These then combine in to the Hepatic Portal Vein, this goes into the liver when it is redistributed into Capillaries again for filtering.
Large Intestine Parts
The image to the side has the cecum made transparent
Looking closely you can see a little blue highlight
This is where the Small Intestine enters into the Large Intestine
The sphincter that allows this one directional movement is called the ileocecal valve. Ileo= Ileum. Cecal = Cecum.
The environment is very different in the Large Intestine.
The small intestine was full of billions of tiny Villi. The large intestine has... none!
There is NO villi in the large intestine
In the small intestine almost everything was absorbed. The stuff passing into the Large Intestine is mostly Fiber (cellulose).
Any thing that has not been absorbed will now become food for bacteria. Your own gut bacteria. Your biome.
These bacteria have the enzyme cellulase, so they can digest this cellulose. In this process they release small things with just 3, 4 or 5 Carbons. These are Short Chain Fatty Acids and they are absorbed by the large intestine
These bacteria also make Vitamin K - which is absorbed in the large intestine, and is vital for blood clotting.
These guys also make some of the B vitamins that are used in regulating cellular respiration and some are used in the creation of neurotransmitters.
In the Small Intestine there are no useful bacteria. There are still some there, but very few (like people living in Fiordland)
The Large Intestine is all about these bacteria. They are there in there billions.
The first introduction of the chyme to them is in the Cecum.
The Cecum
The Cecum is about 6cm in diameter, and is a round pouch. This is were the bacteria first meet the chyme and get to work breaking down some of the cellulose
A large amount of bacterial digestion of the cellulose happens in the cecum. Again, for species that eat more plants than we do, have a larger Cecum.
The Appendix
The appendix is a long thin structure about 8 centimeters long dangling off the end of the cecum.
The roll of the appendix is two fold.
It houses bacteria so that should a diarrhea event wash out most of your good gut bacteria, they can repopulate from their 'house' before bad ones take over. In fact, people that have their appendix removed have higher occurances of this 'bad' bacteria colonizing the colon and requiring medical interventions.
The immune system is in communication with the bacteria that live there - this is so that your immune system can respond quickly if any bacteria manage to leave the large intestine (perhaps through a small cut due to a stone in your food)
The bacteria it stores feed off the chyme. They feed, grow, then reproduce. As they reproduce they leave and enter the cecum. However if it gets blocked these anaerobic bacteria will continue to divide, causing the appendix to swell and causing pain. If it ruptures the bacteria will pour into the abdominal cavity and into the blood stream
Because of the roll of the appendix as a store or a home for cellulose digesting bacteria. Some species that eat more plants than we do have a larger appendix.
The Descending Colon
After this, peristalsis pushes the chyme /feces into the Descending colon.
During this journey
Bacteria keep doing there thing
SCFA's and Vitamins K and B keep getting made by the bacteria
SCFA's and Vitamins K and B keep getting absorbed by the transverse colon
NaCl is absorbed by the Transverse colon
Water follows salt, so water is absorbed by the transverse colon.
At this stage the solid masses are large as there is less water left and so the small solid masses are joining together
The Sigmoid Colon
After this, peristalsis pushes the chyme /feces into the Sigmoid colon.
During this journey
The last of the water is being absorbed.
This area contains mushy feces as the last of the water is being absorbed
The clip above shows the Cecum and the ileocecal valve. It then goes into the ileum. The still in the image is from the ileum. You can tell that it is a still from the small intestine because of the 'carpet' of villi. Watch it and you can see the difference between the two surfaces in the small and large intestines
After siting in the Cecum, the chyme is pushed by either 'mass movement' waves or normal peristalsis into the ascending colon
The Ascending Colon
The Ascending colon (or Ascending Large Intestine if you want to use 3 words) goes upwards.
During this journey:
Bacteria use cellulase to break down fiber
SCFA's are absorbed
Vitamin K and Bs are absorbed
NaCl is absorbed
Water follows salt, so water is absorbed.
The Ascending colon absorbs the most water of the Large Intestine parts. Although this is still way less than the absorption done by the small intestine
The Transverse Colon
After this, peristalsis pushes the chyme into the Transverse colon.
If you feel the very bottom of your ribcage, this pipe runs from just under there, across the front of your puku to the other bottom of your rib cage
During this journey
Bacteria keep doing there thing
SCFA's and Vitamins K and B keep getting made by the bacteria
SCFA's and Vitamins K and B keep getting absorbed by the transverse colon
NaCl is absorbed by the Transverse colon
Water follows salt, so water is absorbed by the transverse colon
Small solid masses form as the water continues to be absorbed. These small solid masses are feces
The Rectum
Peristalsis and Mass Movements push feces into the Rectum.
Here it is compacted and stored till an opportune moment for its release. The release of feces through defecation is also known as "Egestion"
This process is done so that the animal can defecate in a single event while watching for predators.
Having a single defecation event also reduces the chance of there being material on the rectum that insects can feed on. Especially flies, as if they lay their eggs on 'daggs' then the maggots can eat their way into the rectal cavity
A singular event also helps predators with hunting. They don't have to poop and hunt at the same time.
Once the rectum is full, then a spinal reflex will cause the muscles to contract and push the feces out through the anus
Due to mass movements, once full the rectum will have to be emptied because the next mass movement will push more feces into the rectum and thus it will force feces out of the anus. A person can usually resist a maximum of 2 mass movements before they have an accident
If the feces is hard, then the person pushing will close their epiglottis. They then exhale hard, however, because their epiglottis is closed, the air does not leave the lungs, rather it stays there so the lungs are not compressed as much during this diaphragm contraction. So, the diaphragm also pushes on the abdomen. This increases pressure throughout the torso. Pressure increases in the neck and chest, as evidenced by the veins in the neck. Pressure also increases in the abdomen. This pressure helps to push the feces out.
This process of pushing one out is called the Valsalva maneuver .
If you push too hard, then you run the risk of increasing the pressure in the veins of the lower rectum and anus, thus causing hemorrhoids
Pies and Poos
If we add up all the food that a person eats in a day (excluding water), it has the same mass as 10 pies.
If we add up the mass of all the feces a person makes, its less than 2 pies.
Some of that 'less than' is due to bacteria converting some of the pies atoms into gas through anaerobic respiration (farts)
So 10 pies go in and 2 pies come out
Where is the other 8 pies?
The Intestines have absorbed 8 pies worth of atoms!
These atoms then get distributed throughout the body by the cardiovascular system
These atoms are then assimilated into cells
These atoms are then combined with oxygen and turned into carbon dioxide
These atoms are then exhaled
You breath out most of the atoms that you eat!
