Editor's Note: This article is actually Part 2 of the sequence on Anatomy & Physiology, and the one from last week is Part 3. Please excuse the error!
The next doorway leads from the stomach into the duodenum. This is called the pyloric sphincter. Again, it is a round muscle, like the pupil of your eye, that is closed when it is relaxed. It contracts every 6 to 8 seconds and releases a small amount of the content from the stomach into the upper small intestine.
The first 5 cm of the small intestine, called the duodenum, is where most of the magic happens. It is in the duodenum that the stomach empties its contents, the pancreas adds digestive enzymes and begins the control of sugar balancing, and the liver releases bile to digest fat or the bile that has been stored in the gallbladder for larger amounts of fat ingestion.
Let's look at the pancreas first. The pancreas is an amazing organ! It is not only an endocrine gland which secretes and balances sugar and insulin in your body, but it also is what's called an exocrine gland. Exocrine glands are specialized glands that secrete substances outside your body. Since what's in your digestive tract is technically not inside of you until what you have eaten becomes absorbed into your bloodstream, what the pancreas secretes into the digestive tract, (digestive enzymes, for example) are considered an exocrine function.
Digestive enzymes come from cells called the Islets of Langerhans. They are specifically designed to break down the three types of foods we eat: proteins, fats, and carbohydrates. You can always tell when a substance is an enzyme by its name: they all end in the letters ASE. ASE at the end of a word simply means it breaks down what is in the front part of the word. So, a protease breaks down proteins, a lipase breaks down lipids, and an amylase breaks down amyloids, and so on. The pancreas is in constant communication with the brain, the stomach, the liver, and your bloodstream. It is its job to know before the food gets to the duodenum exactly what you just ate-how much protein, how much fat, how much carbohydrate. In this way, it can be prepared with the proper enzymes in the right amounts, neither over- producing or under-producing them. It's an extremely efficient gland!
As the protease breaks down proteins it turns them from long chain amino acids into smaller chain amino acids. In the same way, it takes carbohydrate molecules and breaks them into smaller bits. The hydrochloric acid in the stomach starts the breakdown process, and the enzymes cleverly cleave the molecules by hydrolyzing them (meaning they break molecules apart and add water), which makes them more water soluble and easier to pass across the digestive cell mucosa. Fats are also broken into smaller particles by lipase and by being surrounded by bile.
The liver is constantly dripping small amounts of bile into the duodenum. Bile is basically soap, and in fact, this was the source of soap for our early ancestors. When we would kill an animal, we would take the gallbladder and make soap out of the bile stored there. (Some of you may remember doing this in high school biology.) Bile bubbles up like soap, and its job is to surround the oil or fat molecule, thereby making it absorbable by the very fatty cell membranes of the digestive tract. If bile does not surround a fat molecule it cannot be absorbed by the fat-soluble cell membranes of the digestive tract, and will then become an irritant. If you have eaten a small amount of fat in your meal, just some vegetables, let's say, then the amount of bile that your liver drips into the duodenum is going to be sufficient. If you have had a larger fatty meal, some meat or cheese perhaps, then the gallbladder will be called up on to squeeze out a larger amount of stored bile. This is the gallbladder's only job; it's a small sack about the size of the tip of your thumb that sits up under the liver and stores extra bile in case you need it for a fatty meal.
Now, we have proteins broken down into smaller amino acid chains, fats surrounded by bile in little soap bubbles, and carbohydrates hydrolyzed into smaller molecules that can easily be absorbed all leaving the duodenum into the next section of small intestine called the jejunum. The jejunum and the ileum, the next sections of small intestine, are basically a long tube made up of finger-like rugae projections. If you were to stretch this out to one cell-layer thick, it would be the size of a tennis court! Under this one cell layer-that's the only thing that separates you from the food you just ate-is a layer of smooth muscle that constantly jigs and jags the food, which is now called chime, back and forth across the cell membranes to be absorbed, molecule by molecule, into your bloodstream. Proteins, now broken down into small amino acid chains, require a carrier molecule, kind of like a revolving door, to get them into the GI mucosa cells. Fats need to be surrounded by bile and carbohydrates are readily available to be absorbed directly through the gut mucosa lining.
For the next 10 to 12 hours, the food you just ate will be worked back and forth in a slightly peristaltic-like action that moves it towards the large intestine. Within the environment of the small intestine live very specific pre- and probiotic microorganisms, the highly beneficial structure of which we are just beginning to understand. We do know that if this microflora is thrown out of balance, it can not only disrupt digestion, but can also contribute to obesity, depression, anxiety and a slew of other metabolic issues.
By the time it makes its way to the final doorway of the small intestine, the ileocecal valve, all of the absorbable food molecules have been taken into the bloodstream; the only things left are the indigestible items. The ileocecal or IC valve is a one-way door leading from the small intestine into the large intestine or colon. The other sphincter valves (cardiac and pyloric, which are also one way), can be in reversed positions, but the IC valve should never, never, never, be reversed. What is in the colon must be downstream to the small intestine, otherwise pathological events take place.
The chime at this point is extremely watery, and contains undigested food and insoluble fiber. As it makes it through the one-way IC valve, there is a 6 inch, blind-ended pouch called the cecum. It is very small in humans because we don't have the ability to digest cellulose (compared with the cecum of a bunny rabbit, which is 6 feet long.) At the end of the cecum is a tiny little finger-like projection called the appendix. It is here that massive amounts of immune cells called Peyer's patches line the walls, and are the last immune defense before most of the fluid from our stool is absorbed into the bloodstream.
The colon is basically a gigantic muscular tube. The muscles are so strong and large that it is capable of pushing out a 10-pound baby. It is also capable of holding enough fecal material to collect over an entire 30 days! The colon consists of four sections: the ascending colon, which reaches from the cecum and ascends up the right side of the abdominal cavity to attach to the perineum behind the liver at the hepatic flexure. Then the transverse colon drapes across to the left, attaching at the perineum behind the spleen at the splenic flexure. It then drops down the left side of the abdominal cavity just behind the left hip, then transverses into an S shape along the sigmoid colon and finally ends at the rectum. By the time the chime becomes stool, most of the water has been absorbed, leaving a firm bowel movement.
