The digestive system (and more)
Introduction
The digestive system is necessary to break down nutrients, so they can be absorbed into the bloodstream for energy, growth and cell and tissue repair. The digestive system you may often see on posters inside doctor's offices is known as the alimentary canal/gastrointestinal tract, which is the hollow tube that runs from the mouth to the anus. The first part of the alimentary canal is the mouth, which is obviously where food first enters the body (if it isn’t you’ve been doing it wrong), the second part is the pharynx (aka the throat), the third part is the oesophagus, the fourth part is the stomach, the fifth part is the small intestine, the sixth part is the large intestine, the seventh part is the rectum and finally the eighth part is the anus. There are other components of the digestive system known as accessory digestive components, this includes the tongue, teeth, salivary glands, the liver, the pancreas, and the gall bladder. In this article, I take you through the journey of our food through our digestive system.
The mouth, Pharynx and oesophagus
Our food first begins to undergo mechanical digestion in the mouth, when we chew on our food and the tongue moves our food around the mouth, breaking macronutrients down into smaller pieces, chemical digestion (digestion which uses water and digestive enzymes to break down complex molecules) also begins here as the parasympathetic and sympathetic nervous system stimulates the salivary glands to produce saliva as you chew, see or smell food. Saliva contains the enzyme amylase which works to break down carbohydrates to glucose so it can be absorbed into the bloodstream, it also contains small amounts of the enzyme lipase which works to break down fat into glycerol and fatty acids so it can be absorbed into the bloodstream. Once our food is well and truly “mushed up” the tongue pushes the food into the pharynx, and then the epiglottis (which is a small flexible flap of tissue) folds over the oesophagus to stop food from entering the airway, the oesophagus is a muscular tube that is about 8 inches long and it contracts with peristaltic action (this is when the muscles behind the food contract whilst the muscles in front of it relax) to move the food forward, the oesophagal sphincter at the end of the oesophagus relaxes allowing food to enter the stomach.
The stomach
The food can enter the stomach once the muscles of the upper stomach relax. The wall of the stomach is filled with gastric pits/glands. The top part of the stomach (the fundus) contains gastric pits/glands that produce mostly mucus. In the middle part of the stomach (which is the majority of the stomach, aka the body of the stomach) the gastric pits/glands contain multiple cell types, at the bottom of these gastric pits/glands there are chief cells, which produce pepsinogen, which is activated by hydrochloric acid by breaking it apart and turning it into pepsin (a type of protease) which breaks up the protein in our food into amino acids so it can be absorbed into the bloodstream, the same as all other proteases. Chief cells also produce small amounts of lipase. In the middle part of the gastric pits/ glands, there are parietal cells, which produce hydrogen and chloride ions, forming hydrochloric acid, which denatures proteins, making folded proteins change into a long line, this is so pepsin can break them apart. Parietal cells also produce intrinsic factor, which is important to let the body absorb vitamin B12 in the small intestine. Vitamin B12 is important for producing red blood cells. In the upper part of the gastric pits/glands, there are mucous neck cells, which produce mucous, which contains many different enzymes. The mucus lasts about one week before it is replaced. At the very top/most superficial portion of the gastric pits/glands, there are surface mucus cells, which also produce mucus, but a different kind to that of the mucus neck cell, they also produce bicarbonate which combines with hydrogen in hydrochloric acid to form a molecule with a neutral charge, so the stomach isn’t damaged by the acid. Now to quickly go back down to the bottom of the gastric pits/glands, as they also contain neuroendocrine cells, one of which is G cells which produce gastrin, which stimulates parietal cells to produce hydrogen and chloride ions, forming hydrochloric acid. Gastrin also stimulates other neuroendocrine cells known as ECL cells to produce histamine, which also stimulates parietal cells to produce hydrogen and chloride ions. Gastrin also stimulates other neuroendocrine cells known as the P/D 1-type endocrine cell to release ghrelin, when the stomach is not being stretched, when blood sugar is low or when body weight is low. Ghrelin goes to the hypothalamus and gives the sensation of hunger, increases gastric motility (by increasing how fast food moves through the gastrointestinal tract, via increasing peristalsis and the mixing action of food) and also goes to the parietal cells to stimulate the production of hydrogen and chloride ions. If you are wondering if you can be “hangry” well you can because ghrelin stimulates dopamine release in parts of the brain that causes aggressive behaviour. Ghrelin will drop once the stomach is stretched by a large meal. Gastrin also stimulates other neuroendocrine cells, known as D cells, to produce somatostatin which inhibits histamine from stimulating the parietal cells to produce hydrogen and chloride ions, this is important in the lower portion of the stomach (the pylorus) where the gastric pits/glands contain more D cells to stop excessive acid from being produced if they have a helicobacter pylori infection. One other neuroendocrine cell is the enterochromaffin cell which releases serotonin in response to a force, to help nutrients be absorbed into the bloodstream.
Mechanical digestion also occurs in the stomach when the stomachs muscles contract squeezing the food and mixing it with digestive juices (this is known as churning) and it makes a creamy paste called chyme which is about 3 millimetres wide, this travels into the small intestine when the pyloric sphincter at the bottom of the stomach opens once the duodenum (the first part of the small intestine) is empty, as pressure on the pyloric sphincter goes away, allowing it to open again.
The stomach has rugae, which are wrinkles on the stomach wall, this allows the stomach to stretch and grip and move food to help digestion. The lower portion of the stomach contracts to mix up the food like a blender, to further help digestion.
Stomach acid (which is usually at a 1-3 pH) helps you absorb minerals and kill microbes. If the stomach isn’t acidic enough it will inhibit the function of the gallbladder and pancreas. The pH of the stomach also controls the oesophagal sphincter, which is a valve at the top of the stomach, so if the stomach is too alkali you can get GERD or acid reflux, people often take anti-acids to help this, but it only makes it worse. If the pancreas function is inhibited, your digestion will suffer, as the pancreas is necessary to release various digestive enzymes (as you’ll find in the next section).
The small intestine, liver, gall bladder and pancreas
The pancreas will release protease and amylase into the intestine to try and finish the job by breaking down proteins and carbohydrates, but fats will still need to be broken down by lipase, with help from bile. Bile is formed when cholesterol in the liver (from the foods which we eat) is altered by digestive enzymes forming cholic acid and chenodeoxycholic acid (which are bile acids), these move to the gall bladder through ducts to be stored if they're not needed immediately, along with many other components needed to make bile (these include water, phospholipids, cholesterol, ions, bile salts and proteins). The components needed to make bile are much more concentrated in the gall bladder than in the liver. The gall bladder squeezes its contents out and into the small intestine, when the hormone cholecystokinin, is secreted when a fatty meal (containing triglycerides, monoglycerides, phospholipids and cholesterol) passes through the duodenum and stimulates cells to release bile. The bile acids that don’t get stored in the gall bladder (which are about half), move straight down to the ileum (in the small intestine), where bacteria slightly metabolises the bile acids from primary bile acids to secondary bile acids, this forms deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid. A very small amount of secondary bile acids move all the way through into our faecal material, but most of it gets reabsorbed into a part of the bloodstream, known as the portal system, where it goes back to the liver and combines with amino acids, to form bile salts. These are salts because the amino acid portion is hydrophilic, meaning it wants to combine with water, whilst the rest is hydrophobic, meaning it repels water, these factors make an amphipathic molecule. Some of the bile salts then move to the gall bladder to be stored and some moves down to the small intestine, where the bacteria there remove the amino acids, and it is then reabsorbed into the portal system, so the cycle can continue, 95% of these bile acids and bile salts go through this pathway (known as the terra hepatic circulation) and the rest are produced by the liver, a very minor amount are excreted through faecal matter. The bile salts mix with the fat in the small intestine, the hydrophobic part connects with the fat, whilst the hydrophilic part faces outwards, this begins to break the fat apart, as parts of the fat are becoming surrounded, with the hydrophilic ends facing outwards. The fat can then be broken down by lipase released from the pancreas, this makes fats possible to be absorbed into the bloodstream as well as fat-soluble vitamins (vitamins which are absorbed along with fats in the diet and are stored in the body's fatty tissue and the liver) such as vitamins A, D, E and K. Bile also helps to detoxify certain chemicals in the liver and helps to neutralise the strong acid coming in from the stomach as it is alkaline, otherwise, you’ll get an ulcer in your small intestine. The pancreas releases an alkaline fluid called bicarbonate which also helps to neutralise the acid coming in from the stomach.
The wall of the small intestine has mucosal folds, known as plicae circulares, which helps food move through the small intestine, this is where almost all nutrients are absorbed (nutrients include protein, fat, carbohydrates, vitamins, minerals and water). pliace circulares are more present in the young, and they decrease with age.
The large intestine, rectum and anus
After our food has been pushed through the small intestine it then enters the large intestine as mostly liquid. The large intestine is mainly responsible for the reabsorption of water and minerals. As food moves through the large intestine, it eventually passes through the colon, where water is absorbed, turning the liquid into stool. There are the most microbes in the large intestine, without them, you’ll get diarrhoea. Bacteria in the colon break down the remaining material in our food. After the colon, the remaining material is pushed into the rectum, where the material can exit the body through the anus as stool. The stool is made up of dead bacteria, indigestible materials (such as cellulose), cholesterol and other fats, inorganic substances (calcium phosphate and iron phosphate) and a very small amount of protein and dead cells lining the gastrointestinal tract.
The lower part of the rectum has 2 sphincters. The internal anal sphincter is a thin muscle, controlled by the autonomic nervous system (meaning it is involuntary) and it keeps the rectum closed when you are not ready to defecate, then relaxes when pressure is applied to it by a build-up of faecal matter. The external anal sphincter is a thick, voluntary muscle, meaning it can be consciously clenched or unclenched when it is appropriate for you to go to the toilet.
More on the intestines
90% of nutrition absorption occurs in the small intestine. The microbes in the intestines make a lot of B vitamins. So if you are experiencing a vitamin B deficiency it may be due to a lack of microbes not making it, or your intestines won’t absorb the vitamin, as the intestines have become damaged. The microbes in the small intestine consume fibre (preferably vegetable fibre, which is from cellulose and can be found in foods such as bananas), which they turn into butyrate, which is an acid which feeds the colon cells. There are millions of villi in the small intestine protruding outwards, which absorb nutrients, there are also large amounts of microbes in there which also help you absorb nutrients into the bloodstream, but fat enters the lymphatic system, which is located 80% around the intestines and this is why your immune system is so heavily connected to your digestive system. After the lymphatic system fat can eventually enter the bloodstream. The lymphatic system has a barrier which can decide whether to let certain microbes enter the lymphatic system or not. Without villi, the surface area of the intestines will significantly drop and certain microbes can enter the lymphatic system and create problems for the immune system, by causing stuff like autoimmune diseases (when the immune system begins attacking its own tissue) and causing inflammation. You can prevent the villi from breaking down by building up the flora in the intestines (the bacteria and other organisms living in the intestine) by consuming effective microbes, but only taking small amounts at first. Glutamine is another good supplement to build up the intestinal wall.
How long does digestion take?
It takes roughly 2-3 seconds for food to travel through the oesophagus, 2-5 hours for food to pass the stomach, 2-6 hours for food to pass the small intestine and then 10-59 hours for food to pass the large intestine. Water is easy to digest and it can enter the bloodstream 5-20 minutes after consuming it.
High-fibre foods can move through the body in less than a day, and they can help digestion become more efficient, as fibre increases the weight and size of your stool and softens it. A larger stool is easier to pass, therefore decreasing your chance of constipation. Fibre also helps to solidify watery stool because it absorbs water and adds bulk to stool. High fibre foods include beans, popcorn and avocado. One of the quickest things the body digests is sugary junk foods, as your body makes light work of simple sugars, leaving you feeling hungry quickly.
The nervous system
The intrinsic nervous system is essential to control your intestines and peristalsis (the wave of muscle contractions which push food through the intestines). This nervous system can actually work on its own, without the brain, but a lot of the information on what is going on at the intestines is transmitted to the brain, and it can cause tension in the neck, headache, depression and anxiety. The parasympathetic nervous system activity increases gastrointestinal secretion and decreases the intensity of peristalsis helping you absorb the nutrients in food, whilst sympathetic activity does the opposite, this, therefore, promotes relaxation after a big meal.
The enteric nervous system (ENS), is located in the walls of the GI tract. When food stretches the walls of the GI tract, nerves of the ENS release many different substances that speed up or delay the movement of food and the production of digestive juices. The nerves of the ENS work to control the actions of your gut by making muscles in the GI tract contract and relax to create peristalsis, to move food along the intestines. Peristalsis is what causes the “growl” of the stomach when hungry due to ghrelin, increasing peristalsis.
What causes flatulence and burping?
In case you are wondering what causes flatulence, there are many causes such as; inhaled oxygen and nitrogen that has entered and is now exiting the alimentary canal (this increases when you are nervous); when carbon dioxide leaves the body, which is formed when stomach acid is neutralized by bicarbonate; bacteria breaking down food forming gas, which is leaving the body; fibre can’t be digested and it makes the digestive enzymes work harder forming more gas as a by-product, this is why you should ease into high fibre diets; lactose intolerance not allowing the body to absorb particular sugars found in cows milk, so it must be broken down with bacteria, therefore creating gas, it is believed that up to 70% of people are affected by lactose intolerance worldwide, some people struggle to digest other carbohydrates, such as fructose. The causes of flatulence are also common causes of stomach ache.
Burping often occurs when you take in too much air, from drinking or eating too fast, or drinking carbonated beverages, and it is simply air leaving the oesophagus and some from the stomach.
Leptin
Leptin (aka the satiety hormone) is not part of the digestive system, but it is undoubtedly largely responsible for the foods which enter the digestive system. Leptin is produced by white fat and stimulated by insulin, due to high blood glucose, when white fat grows or due to emotional stress, leptin then travels to the hypothalamus and gives the sensation of fullness, leptin is inhibited by short-term fasting. Overeating, high stress, lack of sleep and too many unhealthy foods often cause a insensitivity to leptin, meaning people eat more often.
How do we taste food?
The fact that food and drink keep us alive is amazing, but the taste is just as amazing. Fascinatingly most of the taste doesn’t occur in our mouth, in fact, it begins with what we see if it is round or square, red or black, long or short, as we assume the taste before we even try it.
75-95% of what we taste is from what we smell, when we chew our food and air travels up the nasal passage and is detected by the olfactory epithelium. Both taste and smell are linked to the involuntary nervous system, this is why the smell of nice food increases the production of saliva and gastric juices, whilst an unpleasant smell can make you throw up or fill sick and why food can taste bland when our ability to smell is inhibited by a blocked up nose.
We can still sense taste in our mouth though, as certain food molecules dissolve in the saliva in our mouth, and send nerve signals to the brain giving us a perception of taste. Taste receptor cells are in groups of about 50, forming taste buds, which are located mainly on the tongue, on bumps called papillae and also on other parts of the mouth. There are multiple kinds of taste buds but all of them are similar. At the top of the taste buds, there are microvilli protruding outwards into a pit, this is known as the taste pore, they contain taste receptors, this is what chemicals bind to, depolarizing the taste receptor cell, which they’re attached to. This sends a neurotransmitter from the taste receptor cell to the sensory nerve fibres, which then send a signal to the brain. There are 5 main types of taste receptors connected to there taste receptor cell (not to say these are the only types of taste receptors/taste receptor cells), which are activated by different chemicals forming different flavours, such as salt, from sodium, sweet, from sugars, sour, from acids, savoury, from amino acids and bitter from spoiled foods, toxins and natural substances, such as caffeine. Each one of these primary taste receptors and taste receptor cells is located in each taste bud on the tongue, but some taste buds in certain parts of the tongue have more taste receptors and taste receptor cells for a particular flavour. Some people have more taste buds on their tongue making them more susceptible to certain flavours. There are different thresholds that need to be reached before different tastes are sensed, the salty and sweet thresholds are high, meaning a lot of salty and sweet compounds must be detected to get a sense of them. Sour, savoury and bitter senses have a low threshold, meaning that only a few of these compounds have to enter the mouth before you get their flavour.
As we get older our tastes can change due to changes in our taste buds and our ability to smell decreases with age.
What causes vomiting?
The emetic centre (in the brainstem) can be activated by two different neural pathways, known as the central pathway and the peripheral pathway. The central pathway comes directly from parts of the brain, whilst the peripheral pathway comes from log nerves, the splanchnic nerves and the vagus nerve which has receptors in the gastrointestinal tract. The peripheral pathway signals can go straight to the emetic centre, or they can first go to the chemoreceptor trigger zone (also in the brainstem) and then to the emetic centre. The chemoreceptor trigger zone is located outside of the blood-brain barrier meaning it is exposed to circulating toxins in the bloodstream, and if there is a lot of them it triggers signals to the emetic centre and causes vomiting and nausea. Vomiting occurs when the contents of the small intestine back up into the stomach due to reverse peristalsis and the relaxation of the pyloric sphincter. The next step is known as retching, which is when the muscles of the chest wall, the abdomen and the diaphragm undergo an intense contraction, this makes the pressure on the stomach rise, whilst the pressure inside the chest is lowered, this strongly squeezes the stomach and pushes its contents into the oesophagus and the epiglottis folds over the oesophagus, due to rapid breathing, preventing vomit from entering the airway. You produce extra saliva before your sick, protecting your teeth from the acidic vomit. After vomiting, the airways stay protected as the larynx rises and the oesophagal sphincter closes. In the process of vomiting heart rate and blood pressure rises. This process can repeat multiple times. After throwing up you feel better because muscles can now relax, heart rate returns to normal, blood pressure drops, oxygen in the blood increases and endogenous euphoriants are released as well as endorphins, similar to what happens after strenuous exercise. The clearing of the small intestine helps remove the sensation of nausea. Exercising often causes nausea or vomiting due to a lack of blood flow to the digestive system after eating, as blood flow to the muscles, brain and lungs increases. Another cause of vomiting from exercise is a lot of nerve signals being sent to the brain, to breathe, control heart rate, dilate blood vessels and a lot more, the chemoreceptor trigger zone, may get caught up in these nerve signals leading to overstimulation of this region of the brain, which may accidentally lead to vomiting. On top of this, a lack of electrolytes and dehydration may cause nausea. The good news is that the emetic centre can get more use to these signals and prevent vomiting from occurring so often in your workout. People often feel a sense of weakness after vomiting which may inhibit the quality of the rest of their exercise, so don’t aim to push yourself to this point during your workout.
Disclaimer: use the information provided in this article at your own risk, as I will not be liable for any harm that may be caused by it.
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