When the chyme is sufficiently liquefied, muscle/peristaltic contractions gradually push it into the upper part of the small intestine, the duodenum. The stomach empties in a slow and controlled way so as not to overwhelm all the mechanisms of digestion in the small intestine. Everything is released according to particle size.
As the small intestine fills with chyme, it signals the stomach to decrease its activity and slow down the emptying process. This is one reason a large or fatty meal stays with you. It lingers in the stomach until the small intestine is ready to process it.
The arrival of chyme in the small intestine triggers the release of a cascade of secretions. The small intestine, pancreas, liver, and gallbladder all deliver digestive enzymes and fluids that break down the food into components small enough to be absorbed. Alkaline mucus with a high concentration of bicarbonate is secreted to neutralize the gastric acid in the chyme.
All of these actions are regulated by both the nervous system and gastrointestinal hormones and called into action only when needed by the digestive system.
Most of these activities occur without any conscious awareness or control. In this sense, it is truly a “second brain” in your gut.
The Pancreas
A carrot-shaped gland that has a dual function in digestion and metabolism, the pancreas is both an endocrine gland—producing insulin, which enables the digestion and absorption of carbohydrates—and an exocrine gland—producing enzymes such as trypsin, which breaks down proteins; amylase, which breaks down starches; and lipase, which breaks down fats. When the pancreas becomes inflamed or diseased (e.g., pancreatitis), these enzymes are not secreted, and as a result carbohydrate, protein, and fat digestion is impaired.
The Liver
The liver is the energy-processing center of the body. It is the first stop for nutrients absorbed from the intestine. Its many functions include metabolizing, storing, and transporting nutrients to the body, producing chemicals necessary for digestion, and breaking down drugs and alcohol.
The liver stores glucose, iron, and vitamins A, B12, and D, sending out the nutrients and substances digested from the food to the cells of the body as they are needed. It also secretes bile, a fluid that increases the solubility of fats, enabling them to pass through the intestinal wall into the bloodstream. Bile is made in the liver and stored in the gallbladder until needed, and delivered to the small intestine when fatty foods arrive and stimulate its release through contraction of the gallbladder.
The Small Intestine
The small intestine, which is actually the longest part of the GI tract, is designed to complete digestion and much of the absorption of nutrients. It is approximately 22 feet long in adults and consists of three parts:
the duodenum (the first segment)
jejunum (the second segment)
ileum (the third segment, or distal small intestine)
All three segments have similar anatomy, but each has a specialized job, digesting and absorbing specific nutrients.
Slow waves of peristalsis push chyme from the stomach through the duodenum toward the jejunum. It actually takes several hours for an entire meal to travel the entire length of the small intestine to enable absorption to occur through the lining or mucosal wall of the intestine.
This lining has a unique structure that possesses a much larger surface area than the midsection of the body in which it is contained. (See Diagram 2.) This lining, the mucosa, consists of folds that increase its surface area. The folds are in turn covered with tiny fingerlike projections, or villi, that contain the cells that absorb nutrients and again expand the surface area. (See Diagram 3.)
The surface of each villus has a “brush” border consisting of microvilli or tiny hairs that increase the absorptive surface of the small intestine yet again. The brush border also secretes enzymes that are necessary for the digestion of specific food components.
If you were to flatten out the intestinal mucosa—all the villi, microvilli, and crypts that lie between the villi—the “small” intestine actually has a surface area about the size of a football field that is totally dedicated to absorbing food! This enormous capacity ensures that the intestine can sustain a fair amount of assault and/or damage and still feed the body.
Diagram 2
A CROSS SECTION OF THE INTESTINAL WALL
Diagram 3
Inflammatory cells normally inhabit the mucosa to protect the small intestine against toxins and bacteria. Since the food supply entering the GI tract is not sterile and may contain toxic substances, these white blood cells are another line of defense. This results in a state of constant, controlled inflammation in the mucosa.
The Villi
The villi are the workhorses of the intestine. They are the final intestinal link between your dinner plate and your bloodstream. This is where celiac disease does its primary damage and where other gluten and food-related disorders affect the body’s ability to properly absorb nutrients.
The villi play a crucial role by:
dramatically increasing the surface area of the small intestine to allow the absorption of food
releasing enzymes that continue and complete the breakdown/digestion of food
absorbing the products of digestion and transporting them into the bloodstream for distribution throughout the body
acting as a barrier that blocks bacteria, parasites, and toxins from entering the body
Each villus is an independent but intimately related part of the assembly line. It is important to understand that the final stages of digestion, absorption, and transport of nutrients occurs through—not between—these tiny, fingerlike projections. When there is inflammation, and a breakdown of the lining of the intestine, the bowel may become permeable, often termed “leaky.”
There are millions of microscopic villi in each section of the small intestine. Because of its enormous capacity to absorb, parts of it can be damaged with no obvious manifestations or symptoms. But when large sections of the lining are inflamed or destroyed, absorption, enzyme release, transport of nutrients to the body, and the defensive ability of the small intestine is compromised.
Absorption
Once the food components are sufficiently digested (broken down), they are absorbed by different parts of the small intestine.
This is why disease of or infection in one section of the small intestine is often revealed by the malabsorption of specific nutrients. Iron deficiency and metabolic bone disease (e.g., osteoporosis or osteopenia) occur when disease involves the proximal intestine, the first two segments of the small intestine. Fat and sugars are absorbed throughout the intestine. Therefore, when one part is diseased, another can