The process of digestion can be broken down into three phases, The Cephalic (oral) phase when you put food in your mouth that travels down your esophagus, The Gastric Phase which is when your stomach breaks down food and kills any contaminants and The Intestinal Phase which is when nutrients & water are absorbed and any waste is excreted.
Mechanical digestion is the physical action of mashing up food and breaking it into smaller parts, so there is more surface area for enzymes to attach themselves to. It includes any movement that involves muscles breaking the food into smaller parts. Mechanical digestion includes mastication which is a fancy word for chewing and peristalsis (series of wave-like motions that push the food forward through contractions).
Chemical digestion is the chemical transformation of food into smaller molecules by digestive enzymes. There are eight digestive enzymes but we are only going to cover the three main ones.
It wasn't until the 1800's that we learned whether digestion was mechanical or chemical. The main contributor to what we know was from an experiment by William Beaumont and his study using Alex St. Martin, who was a man whose gunshot wound created a window into his stomach which enabled scientists to understand digestion. If interested you can read more of the history by clicking here.
Digestion is both mechanical and chemical. In Phase I our teeth masticate and grind the food by clamping down. The tongue pushes it against the hard palette and rolls it around breaking it down further. This process is the start of mechanical digestion.
As mentioned, the goal is to increase the surface area for enzymes to attach and breakdown the food further. The process of chemical digestion begins with the salivary glands. The three pair of salivary glands include:
Sublingual glands which are under the tongue
Parotid glands which are by the cheek
Submandibular glands which are under the jaw
These glands produce two types of saliva serous and mucous. The serous cells in the mouth secrete a watery fluid and the mucous cells just like they sound secrete mucous. Our salivary glands really kick into gear when we are chomping away triggering the release of more mucous, water and enzymes to break down starches and moisten our food for its journey down the hollow tube that leads to the next phase of digestion.
The salivary glands also release our first digestive enzyme Amylase. Amylase is the digestive enzyme that breaks down starches into sugars. Once the mashed up food is heading down the esophagus, we have the next instance of mechanical digestion, peristalsis.
So to recap, for Phase I, the Oral Phase, both mechanical and chemical digestion occur. In Phase II, the Gastric Phase, there are also both mechanical and chemical digestion taking place. On its way to the stomach your food must pass through the sphincter (a ring of muscle at the opening of the tube) which relaxes to let food through and on a good day tightens to keep acid out.
Every organ in the digestive tract has a sphincter on both sides that operate like keys opening and relaxing for food to enter and leave. The sphincter between the esophagus and the stomach is called the lower esophageal sphincter. When food or bolus enters the stomach the stomach muscles act like a meat grinder pushing it around, breaking it up even smaller. As the stomach expands it sends a signal to the brain that it's time for hydrochloric acid to be released. Hydrochloric acid is released by parietal cells in the glands of the stomach and lowers the pH to kill any unwanted bacteria or potential pathogens. Remember we are taking in all sorts of things from the outside world, so our digestive process is designed to protect us. This acid helps to unwind the proteins and begins their digestion. Parietal cells also secrete intrinsic factor which is essential for the absorption of B12 which occurs later in the process.
As pH levels go down, cells in the stomach called chief cells trigger the release of a combo of enzymes that produce pepsin, a digestive enzyme that breaks down protein, that attaches to the unfolded proteins.
Proteins are made up of hundreds of linked amino acids and when broken down we can use these to build muscle and tissue. The stomach also produces an alkaline mucous that is filled with sodium bicarbonate to buffer the low acid in the stomach and prevent the stomach from digesting its own muscle lining. So why doesn't the stomach digest itself when it produces such a strong acid? As it turns out, 30-36% of our protein does come from digesting our own dead cells and mucous.
As we move into the next phase and the stomach slows down, the pH rises back up and Pepsin becomes an effective. Every enzyme requires a different pH to function and Pepsin loves a low pH of an acid stomach.
How long does this process take? Food or bolus stays in the stomach for around 2 hours if it's a carb and up to 4 hours if it's a fat. Protein falls somewhere in between.
Entering the next phase, the food now mostly liquid is released by more peristalsis into the first chamber of the small intestine, the Duodenum through the pyloric sphincter at the other end of the stomach. Here in the Duodenum we begin the 3rd Phase of the digestive process, The Intestinal Phase.
The job of the Duodenum is first to neutralize the acid with more bio carbonate from the pancreas. When the pH is higher and the small intestine expands with food, the pancreas, liver and gallbladder know they can start to deliver their own digestive enzymes. The enzymes like a higher pH and continue the digestion of fats, carbs and proteins.
An enzyme is a substance produced by the body that acts as a catalyst for sparking a specific biochemical reaction. There are three main digestive enzymes:
Amylase - breaks down carbs into sugars
Protease - breaks down proteins into amino acids
Lipase - breaks down fat into fatty acids and glycerol
These enzymes are mostly released by the pancreas, but some come from the walls of the small intestine. There are also enzymes that attach to the wall of the small intestine and break food down further as it passes by subsequently helping to activate additional enzymes. Picture it like a car wash except these enzymes called brush border enzymes stay in one place.
The pancreas has the big job of releasing the major enzymes, but what do the other helper organs do, the liver and the gallbladder? The liver produces bile (a green colored digestive enzyme that helps break down fats). Remember our end goal is to create water soluble nutrients. Fats are not soluble and they tend to clump up and cause problems. Bile acts like an emulsifier surrounding fat molecules to prevent them from clumping. Emulsifiers help keep things from binding together and in the case of bile, help prevent fat from becoming an impenetrable ball.
The bile produced by the liver is stored in the gallbladder until it gets word that there is fat to be digested at which point it is released into the Duodenum. There in the small intestine its mixed with Lipase which is produced by the pancreas and the small intestine in assisting in breaking down fat.
The primary job of the rest of the small intestine is to finish the chemical job of the digestion and absorb the resulting nutrients and minerals. Peristalsis, mechanical action, also happens here. Peristalsis moves the chyme forward, turns it over and over like a washing machine so that its evenly covered in enzymes. Contracting muscles move the chyme along until it reaches the large intestine. Intestinal goblet cells release more mucous for protection and lubrication along the way. They are called goblets because they are actually goblet shaped.
The majority of absorption and digestion happens in the small intestine, the large intestine, has the less glamorous roll of taking out the trash. To do this job, the large intestine houses a whole community of bacteria. You can think of them as a garbage compactor, chomping on our indigestible fibers, creating another chemical reaction. These thrifty bacteria transform our leftovers into vitamins, minerals and fatty acids so we don't miss a thing.
The muscles of the large intestine move the debris along with the contractions that occur 3-4 times a day, until it reaches the rectum and triggers the anal sphincter but the final move to relax and release must be a conscious choice when the time is right. The only thing left is for waste to leave the body and then do it all over again after our next meal.
To recap, the process of digestion breaks down food into fuel or tiny particles that are water soluble, that can be absorbed mostly in the small intestine. We want to break down fats, proteins and carbs into sugars, amino acids and fatty acids. The body does this through two main processes mechanical and chemical digestion. Mechanical digestion is the physical act of breaking down food into smaller pieces and chemical digestion uses enzymes to give the food a chemical bath and break it down further. These enzymes are produced in the mouth, the stomach, the pancreas and the liver. At the end of this process we are left with nutrients that our cells can absorb and use to fulfill our highest potential.
Next Monday we will move on to: Digestion and Health - Absorption of Nutrients