Digestion: Part 3 (of 4)

Hello and Happy Monday!  I want to start with some Monday vibes here…a little thoughtfulness for your day.


So we’ve covered the roles of the brain and the mouth in Part 1, and then the stomach in Part 2.  The next part of the digestive pathway is when the food moves into the small intestine.

Up until this point, the food has been chemically and mechanically broken down, whether it be by salivary enzymes, chewing, gastric juices, or peristalsis of the stomach.  We stated back in Part 1 that the goal of digestion is to break down food into small molecules so small that the body could use and absorb the nutrients.  As the food (by this point, called a chyme) enters the small intestine, this is where that absorption happens.

We left off in Part 2 with the chyme leaving the stomach through the pyloric sphincter and entering the small intestine.

When everything is working properly….

Now the chyme is this ball of acidic paste as it was mixed with the gastic juices from the stomach and mashed up as much as it can be.  It goes through the pyloric sphincter, which is the little gate that lets food pass from the stomach to the duodenum, the first section of the small intestine.

small intestine (source of photo)

At this point, the chyme is still very acidic, but the small intestine is not equipped to deal with such high acidity as the stomach is.  So as soon as the chyme enters the duodenum, mucous is secreted to start protecting the walls of the duodenum from the acid.  At the same time, the intestinal walls are also secreting two important hormones into the bloodstream: Secretin and Cholecystokinin (CCK).

This is where we are introduced to the accessory organs of the digestive system: the pancreas and gallbladder.

GB and Pancreas (source of photo)

The secretin stimulates the pancreas to release bicarbonate (think baking soda) in order to neutralize that acidic state of the chyme so it doesn’t damage any part of the small intestine as it travels through.  Once it’s neutralized (pH reaches 7.0), the pancreatic enzymes are released to come in and continue the breakdown of fats, proteins, and carbohydrates.  (There are specific enzymes to break down each macronutrient.)  While this is happening, the CCK stimulates the gallbladder to release bile, which is necessary to emulsify and absorb fats.  Then the small intestine has it’s own fashion of wave-like muscle contractions, peristalsis, that move the chyme along.

The small intestine is made up of two more parts, the jejunum and ileum.  Along this whole pathway, there are millions of villi and macrovilli, little hair-like substances, that absorb the nutrient molecules into the bloodstream and are then carried throughout the body.  At the end of the ileum is the ilocecal valve, which is the gateway into the large intestine, and also our stopping point for now.


The pH is not correct. When the chyme enters the duodenum, it MUST be a certain acidic pH in order to stimulate the mucous that contains those two important hormones we mentioned earlier.  So if this doesn’t happen, then two things occur.  1) Secretin doesn’t stimulate the release of pancreatic juice/enzymes to continue that breakdown.  2) Bicarbonate isn’t released to neutralize the chyme which can burn the lining, leading to ulcers.  (Remember, the chyme may not be acidic ENOUGH, but it’s still much more acidic than the small intestine can handle.)

Continuing with the problem of the pH, CCK won’t be released from that mucous.  If this isn’t released, then bile won’t be released either to emulsify the fats.  If the fats aren’t emulsified, they can continue on and rancidify in the colon.  This is kind of a double problem.  Aside from the rancidification in the colon happening, the fats won’t get absorbed and used which are needed to produce good healthy bile.  (The production of bile is a roundabout kind of cycle.  You need good fats to produce good quality bile, but you need good quality bile to digest the fats.)  So bile won’t be released if there’s too little fat in the diet as well.  If bile isn’t released, then it becomes this old thick (viscous) fluid.  It’s not easily released, so when the gallbladder contracts in an attempt to release it, there could very well be pain if this is a chronic condition.

Proteins don’t get digested.  Not just proteins, but all undigested foods, can affect the villi and macrovilli.  This irritates the lining, and eventually becomes damaged.  This is also known as leaky gut syndrome, where the lining loses it’s ability to select what can pass through the lining or not so proteins and fats of inappropriate sizes can pass through the intestinal lining and really overwhelm the immune system.  When this happens, the immune system comes in contact with something, and can’t decipher between “Are you an amino acid or a virus?”  So at this point, the body’s innate intelligence flips to survival mode and attacks it as if it’s a virus (or other pathogen).  This leads to all sorts of problems pertaining to “leaky gut syndrome”.

So if we back up a bit to the previous posts, we can see how this is really all coming together.  If the pH of the chyme is incorrect, it’s because the stomach acid wasn’t sufficient in doing it’s job.  If the stomach acid wasn’t sufficient, it’s because something blocked it’s production/release such as stress or acid-blockers.  If the stress blocked the HCl production, it goes all the way back to the brain.

If the food isn’t getting digested, that could go all the way back to chewing, or even further to the brain with stimulating proper saliva secretion.  Pretty interesting stuff, huh?

So we will leave here at the ileocecal valve and pick up at the large intestine for the final section of digestion.

In good health,


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