Digestion is a process whereby the body prepares food for absorption into the blood stream. The digestive system is a series of organs joined together that begin at the mouth and end with the anus. They include the mouth, esophagus, stomach, small intestine, colon, rectum and anus. Other organs such as the salivary glands, pancreas, liver and gallbladder are involved in the digestive process but are not, anatomically, considered to be part of the digestive system.
The Flow of Food within the Digestive Tract
The Flow of Food within the Digestive Tract
Mastication (chewing of food) is the first stage of digestion. Mastication initiates digestion by breaking down large aggregates of food and stimulating the release of digestive enzymes contained within the saliva of the mouth. Tiny glands along the upper digestive tract are embedded in the mucosa lining. These glands secrete digestive enzymes that catalyze the breakdown of starches and fats.
Before the food enters the stomach, it first passes through a gullet that connects the mouth to the stomach, called the esophagus. The esophagus is important, because it serves as an air lock to prevent back flow of partially digested foods and stomach acids back into the throat and mouth. GERD (Gastroesophageal reflux disease) is a condition where the lower esophageal sphincter (LES – a valve between the esophagus and the stomach) is not functioning effectively. Individuals with GERD experience a spontaneous “rise up” of stomach acid and food contents into the esophagus. This leads to heartburn or acid indigestion.
The esophagus opens into the stomach, where proteins are broken down into small peptides. The high acid content of the stomach is crucial for digestion and protecting the body from bacteria or toxins present in food, by acting as a decontamination chamber. Ingested food is stored in both the fundus (top of the stomach) and the body of the stomach. Once food enters into the stomach, hydrochloric acid (HCl) is secreted from the gastric fundal mucosa (lining of the stomach). HCl denatures proteins and activates the enzyme pepsinogen, which becomes pepsin. Pepsin then hydrolyzes the denatured proteins, resulting in several small peptides. Fats are also hydrolyzed in the stomach, but at this point they are partially broken down from their exposure to lipase in the saliva.
The “grinding” of food occurs in the antrum (lower part of the stomach). The antrum secretes gastrin (enzyme that regulates acid production) and controls the amount of food released into the intestine by way of the pyloric spinchter. Once the food-acid-enzyme mixture (chyme) enters the intestine; secretin and cholecystokinin are released, which in turn stimulates the pancreas to secrete pancreatic enzymes into the lumen (the lining) of the duodenum (the first segment of the small intestine).
The small intestine allows the body to maximize digestion and absorption because it has an expanded surface area with inner folds called plicae, villi and mircovilli. This high degree of folding enhances its ability to absorb nutrients. The duodenum is the neutralization chamber where the chyme is mixed with the pancreatic juices. The pancreatic juices include: bicarbonate along with enzymes that digest proteins (trypsin, chymotryosin, carboxypeptidase, and elastase), enzymes that digest fats (lipase and phospholipase), and amylase (enzyme that digest carbohydrates).
The jejunum (middle section of the small intestine) is where most nutrients are absorbed. Amino acids, as well as most vitamins and minerals, are absorbed in the jejunum. A few nutrients, like iron and calcium, are absorbed in the duodenum. Active fat absorption occurs in the duodenum and the jejunum. Before fat can be transported into the body and absorbed, it must first be solubilized and aggregated into small fat droplets, called miscelles. Bile, produced by the liver (and stored in the gall bladder), acts to solubilize fat. This process is particularly important for the absorption of fat-soluble vitamins (vitamins A, D, E, and K), and for cholesterol absorption. Most starch is digested in the duodenum and jejunum, the first and second segments of the small intestine.
The ileum is the end of the small intestine. It is where the digestion of nutrients is completed and bile salts are reabsorbed. Roughly 90 percent of the nutrients have been removed from most foods at this point.
The Large Intestine
The large intestine does not play a significant role in the digestive process of most foods, but it is important in the conservation of sodium and water. However, the food that does make it into the large intestine is primarily fiber. The large intestine is home to an ecosystem of bacteria called probiotics (pro-life) that can ferment much of this fiber and produce many nutrients necessary for a healthy colon. Colonic fermentation produces a series of short-chain fatty acids that are important in removing free radicals and promoting proper function.
Probiotics include the bifidobacteria and lactobaccillus genuses. Probiotic bacteria keeps pathogenic (disease-promoting bacteria) from colonizing within the colon. Certain fibers in food, called prebiotics, specifically support probiotic bacteria. Prebiotics include such molecules as inulin and fructooligosaccharides, (which are found in chicory and Jerusalem artichoke), and may include some other carbohydrates such as galactooligosaccharides, arabinogalactans, and arabinoxylans, which are found in soy, rice fibers, and in larch tree extracts.
Rectum and the Anus
The rectum and the anus allow for controlled elimination of stool.