How Digestion Works - A Science Lesson

​Digestion is an incredible process! 

You've heard the saying, "You are what you eat".  But the TRUTH is:  "You are what you ABSORB from what you eat."  Try to make it a point to eat a nutrient rich diet every day.  If you don't, you'll end up feeling like the crap you'd be eating. (Or as they say in business... "garbage in, garbage out!) 

If you stop to think about it, it's amazing how your body knows what to do with your food to break it down and make your body work.  Every bite you take either leads you toward health OR toward disease.  It's up to you!  Learn how & why in my article: Guidelines For Good Digestion and the Relationship to Your Health.  

Enjoy this little science lesson about the digestive process that starts in your brain and ends up in the sewer system.  :-)

How digestion works

The digestive system begins in the brain.  The para-sympathetic nervous system must be engaged in order for digestion to function properly.  Food enters the mouth and is chewed and mixed with saliva.  The saliva contains a digestive enzyme called amylase, which starts to break down starches and salts in the mouth.  Saliva also contains lipase, the fat-digesting enzyme, but it does not become activated until reaching the stomach.  The saliva is made from three pairs of glands: parotid (by the ear), submandibular (inside the lower jawbone), and sublingual (under the tongue).  When food is small enough to swallow, the tongue pushes it backward into the throat (pharynx).  This is a voluntary action.  The epiglottis closes so the food, now called a bolus, does not enter the wind pipe (trachea).   The soft palate closes also, so the bolus does not enter the nasal cavity.  The bolus enters the food pipe (esophagus), which is a muscular tube that leads to the stomach.  There is a protective layer of mucus in the esophagus and entire digestive tract.

After swallowing, the digestive process is involuntary.  The food is now transported through the remainder of digestive system via peristalsis, the wavelike movement of muscles, both longitudinal and circular, which propels and churns the food along.  The bolus now gets to the bottom of the 8-10 inch long esophagus and enters the stomach via the one-way esophageal sphincter.  The stomach is a pouch and the widest part of the digestive tube.  The stomach is also a holding tank for food (now called chyme); it has folds called rugae, which expand to hold its contents.  Its job is to churn the chyme with gastric juices (hydrochloric acid (HCl), digestive enzymes, hormones, and intrinsic factor), sterilize it, and prepare it for nutrient absorption in the small intestine.  The gastric juices are stimulated by the hormone gastrin.  The upper part of the stomach is called the fundus, which is alkaline, like the mouth, and where carbohydrate digestion continues.  The lower part of the stomach is acidic and contains the HCl.  This is the start of the digestion of protein.  Protein is broken down by the pepsin enzyme, when activated by the stomach’s HCl.  The protein splits into amino-acid chains called peptides.  The presence of fat will slow down the digestive process.  The fats also begin to breakdown from the action of the fat-digesting enzyme, gastric lipase. The small and large intestines also contain billions of beneficial bacteria which aid in the digestive process. ​

​Little by little, the pyloric sphincter at the base of the stomach, allows the now microscopic food-particles into the first part of the small intestine, the duodenum.  The hormone enterogastrone regulates the speed of the peristalsis here.  It is in the duodenum where much absorption of nutrients into the bloodstream and lymphatic fluid occurs.  The gall bladder releases bile into the duodenum via the cystic and common bile ducts.  Bile contains salts that emulsify fats into tinier droplets, which allow a greater surface area for the action of enzymes.  The liver produces bile and stores it in the gall bladder.  The duodenum also receives secretions from the pancreas, which include alkalis (to neutralize HCl) and about 15 enzymes to further break down the fats, proteins, and carbohydrates in the chyme. The small intestine and beyond is again alkaline in nature.  In order to cross the intestine’s cell membrane and enter the bloodstream, fats are broken down into fatty acids and monoglycerides, carbohydrates are broken into simple sugar molecules like glucose, and proteins are broken into amino acids.  Fiber is broken down, but not absorbed.  Water is absorbed by the stomach, small, and large intestine, and the vitamins and minerals are absorbed in the small and large intestines.  

The entire small intestine is 15-20 feet long.  The surface area is the about the area of a tennis court.  The three sections are the duodenum, the jejunum, and ileum.  The lumen is the fluid-filled space inside the small intestine.  The epithelial walls are made up of fingerlike projections called villi, which are covered with micro villi.  They are one cell thick and where the nutrients are absorbed into the bloodstream via capillaries.  Nutrient-rich blood is then transported to the liver via the hepatic portal system.  The liver filters the blood in its lobules.  The liver also stores and releases blood glucose for energy, sorts and processes vitamins and minerals, breaks down toxins and eliminates them, synthesizes blood-clotting proteins and proteins for blood plasma, and recycles old blood cells.  In short, the liver allows the body to use the food ingested for energy production and storage, plus cell growth, renewal, and repair.

Next, when the chemical breakdown of foods is nearly complete, the waste chyme from this process enters the large intestine (colon) through the ileocecal valve, via more peristaltic movement.  The large intestine’s main function is to convert watery chyme into semi-solid fecal matter to be eliminated.  Fecal matter passes slowly through the colon.  The cecum is the pouch-like entrance to the large intestine and part of the ascending colon on the right side of the abdomen.  The transverse colon passes across the upper abdomen above the stomach, and the descending colon is on the left side of the abdomen.  The sigmoid colon is s-shaped and bends to meet the rectum, where elimination occurs.  Voluntary movements are used to allow waste to exit via the external anal sphincter.  

The entire process (transit time) normally takes 12-24 hours.  The process is happening continuously as food is ingested, digested, absorbed, and eliminated.

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