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Bit by bit, the pyloric sphincter that separates the stomach from the small intestine allows the chyme to drop into the small intestine. Here is where most of the magic occurs. Though the stomach often gets the credit, the small intestine is the primary site for digestion, and its remarkable and expansive absorptive area allow us to harness the nutrients in our foods.
Because chyme was mixed with HCl in the stomach, it has a very low pH. The mucosa of the small intestine does not have as much protective mucus as the stomach, but it does have something else to shield it from the acid - pancreatic juices. The presence of chyme in the upper portion of the small intestine triggers the pancreas to secrete bicarbonate to neutralize the acid, lipase to digest fats, amylase to digest starches, and proteases to digest proteins. The cells of the small intestine secrete additional enzymes to complete digestion.
The gallbladder gets into the act here as well. When fat is present in the chyme (as it is from the cheese and mayonnaise in our sandwich), the gallbladder contracts and secretes bile into the small intestine. Bile acts like a detergent and emulsifies the fat, breaking it into small globules, aiding fat absorption.
You need a very large surface area to absorb all of the nutrients your body requires. If the lining of your small intestine were smooth like the inside of a rubber tube, you would not be able to absorb adequate nutrition. Instead the walls of the intestinal lumen are wrinkled and folded, which increase its surface area threefold. But this is still much too little. Therefore, the folds are covered with tiny fingerlike projections called villi. which, in turn, are covered, by microscopic projections called microvilli. This combination of folds and projections increases the absorptive area of the small intestine 600-fold to the size of a tennis court! And that allows for the absorption of far more nourishment than most of us would want to eat.
Once starches and sugars have been digested into monosaccharides - glucose, galactose or fructose - they are ready for absorption. They pass through the lining of the small intestine, into the bloodstream via the portal vein and to the liver. Proteins are broken down primarily to single amino acids. They follow a similar path to the liver, as do the monosaccharides. Because of their lack of water-solubility, the majority of the fats take a much different path. Most of the dietary fat comes in the form of triglycerides and is digested into two free fatty acids and monoglycerides. While within the intestinal cells, however, these components are resynthesized into triglycerides. They do not go through the portal vein. Instead, they are packaged with cholesterol and coated with proteins to form particles called chylomicrons. The lymphatic vessels transport the chylomicrons to the junction of the lymphatic and circulatory systems where they enter the bloodstream. Only a small portion of fats is absorbed into the portal vein. These are the smaller fatty acids that are more water-soluble than the long-chain fatty acids.
Vitamins, minerals, water and many drugs are also absorbed through the intestinal mucosa.
Peristalsis pushes food waste, fiber (from your bread and vegetables) and any foreign materials through the small and large intestines. More water and salts are absorbed from the large intestine. As the contents move downward, the feces - consisting of water, fiber, intestinal secretions and dead bacteria - are formed and eventually pushed through the anus. Though many people worry they are constipated if they do not have daily bowel movements, the National Institute of Diabetes and Digestive and Kidney Diseases has the following definition for constipation: having a bowel movement fewer than three times per week. Don’t worry; it’s also normal to have two or three bowel movements each day. 
The proper functioning of the GI tract is even more intricate than described here with multiple mechanisms for communication and control. Both the endocrine and nervous systems are critical players. In fact, there are more than 80 hormones involved in regulating the GI tract. Additionally, the enteric, peripheral and central nervous systems largely regulate the contractions and secretions of the gastrointestinal tract.
You can see just how complex the GI tract is. Any problems with anatomy, inflammation, nerve disorders and more can hinder the breakdown and absorption of nutrients. Getting proper nutrition to the cells of your body is more than merely making wise food choices.