The Stomach

The stomach is a storage container for our ingested foods. However, in addition to the storage and the production of gastric secretions, this guy, our stomach, gets its groove on so that the formation of chyme can take place.

Not quite sure what chyme is? Let's start from the beginning.

The stomach has 3 layers of muscle and of course they have names. They are the longitudinal, the circular, and the diagonal, and to say it plainly, it is the forceful contractions of these muscles that literally beats up our ingested foods. Well, after all that chewing that took place within the mouth, of course.  

To move forward, it is the third muscle that really gets those contractions a-moving. Because, unlike the gastrointestinal tract, the stomach has the diagonal muscle that really gets the churning going, kinda like the kneading of bread, and with the help of the hormone gastrin, which I have mentioned in previous articles.

You remember don't you? The gastric pits that are lined with the endocrine cells that release gastrin into the blood, but also the exocrine cells that secrete the gastric juice into the ducts that empty into the stomach's hollow cavity...

So, back to what I was saying: Under the influence of Gastrin, the contractions of the stomach pick up their pass as well as their force and begin the mixing process, and for about the next 3-5 hours after ingestion, depending on what you ate of course, these foods become partially digested as well as mixed with gastric juice and turned into a soupy paste called chyme.

Then, the chyme is pushed towards the pyloric sphincter and is slowly released into the small intestine, and with every peristaltic wave (the contraction of the muscles that are moving the materials in the gastrointestinal tract within a forward direction), a few millimeters of chyme squeeze out of the stomach through the pyloric sphincter into the first portion of the small intestine, and chemical digestion takes place. The small intestine has the major role of chemical digestion and is the primary site for this process along with absorption.

So, we have arrived at the small intestine. This 20-foot long narrow tube has a diameter of only about 1 inch; however, the small intestine is divided into 3 regions with lovely names, and they are the duodenum, the jejunum, and the ileum.

The duodenum is the receiving end of the small intestine. The chyme, which is your chewed/ingested foods, is now mixed with gastric secretions, or we can call it vomit.

I know, not a nice image that I am throwing at you, but, descriptive? Yes it is!

The duodenum not only receives the chyme, but also the secretions from the gallbladder by way of the common bile duct, as well as the pancreatic juice by the way of the pancreatic duct.

When the gallbladder contracts, as it is a sac of a muscle that stores bile (fluid that is produced within the liver that consists of bile salts, cholesterol, and bile pigments; this fluid is necessary for the digestion of fats), it squeezes down onto the stored bile, forcing it into the cystic duct, which will ultimately join up with the common bile duct and empty into the duodenum.

Pancreatic juice is obviously secreted from the pancreas. This fluid is an alkaline solution containing sodium bicarbonate, which quickly neutralizes the acidity of the chyme as it enters into the duodenum because it is coming from the stomach. This pancreatic juice also contains the digestive enzymes needed for the process of chemical digestion. The pancreatic juice also empties into the duodenum by way of a duct, but this one is the pancreatic duct.

The small intestine has a huge surface area which is perfect for the process of digestion and nutrient absorption.

It is arranged within what is called plica, which are large circular folds that face toward the inside of the lumen (the hollow cavity). They are lined with finger-like-projections called villi, plural for villus.

Each villus consists of hundreds of absorptive cells called enterocytes. Their surface faces the inside of the lumen and are covered with very tiny microvilli.

Microvilli are the absorptive surface of the small intestine. We can also call it the brush border, as it has been described as a velvety surface because of the villi\microvilli.

The brush border of the small intestine is where the final stages of digestion and nutrient absorption take place. Each villus contains a network of blood capillaries and the lymphatic vessel contains the lymph, called a lacteal, both of which circulate nutrients away from the small intestine.