The gastrointestinal (GI) tract is a flexible, muscular tube extending from the mouth to the anus. In a sense, the human body surrounds the GI tract. Only when a nutrient or other substance passes through the cells of the digestive tract wall does it actually enter the body.
The Digestive Organs
The process of digestion begins in the mouth. As you chew, your teeth crush and soften the food, while saliva mixes with the food mass and moistens it for comfortable swallowing. Saliva also helps dissolve the food so that you can taste it; only particles in the solution can react with taste buds. The tongue allows you not only to taste food but also to move food around the mouth, facilitating chewing and swallowing. When you swallow a mouthful of food, it passes through the pharynx, a short tube that is shared by both the digestive system and the respiratory system.
Mouth to the Esophagus
Once a mouthful of food has been chewed and swallowed, it is called a bolus. Each bolus first slides across your epiglottis, bypassing the entrance to your lungs. During each swallow, the epiglottis closes off your trachea, the air passageway to the lungs, so that you do not choke.
Esophagus to the Stomach
The esophagus has a sphincter muscle at each end. During a swallow, the upper esophageal sphincter opens. The bolus then slides down the esophagus, which conducts it through the diaphragm to the stomach. The lower esophageal sphincter closes behind the bolus so that it cannot slip back. The stomach retains the bolus for a while, adds juices to it, and transforms it into a semiliquid mass called chyme. Then, bit by bit, the stomach releases the chyme through another sphincter, the pyloric sphincter, which opens into the small intestine and then closes after the chyme passes through.
The Small Intestine
At the beginning of the small intestine, the chyme passes by an opening from the common bile duct, which secretes digestive fluids into the small intestine from two organs outside the GI tract—the gallbladder and the pancreas. The chyme travels on down the small intestine through its three segments—the duodenum, the jejunum, and the ileum. Together, the segments amount to a total of about 10 feet of tubing coiled within the abdomen. Digestion is completed within the small intestine.
The Large Intestine (Colon)
Having traveled the length of the small intestine, what remains of the intestinal contents passes through another sphincter, the ileocecal valve, into the beginning of the large intestine (colon) on the lower right-hand side of the abdomen. Upon entering the colon, the contents pass another opening.
Should any intestinal contents slip into this opening, they would end up in the appendix, a blind sac about the size of your little finger. Normally, the contents bypass this opening, however, and travel up the right-hand side of the abdomen, across the front to the left-hand side, down to the lower left-hand side, and finally below the other folds of the intestines to the back side of the body above the rectum.
As the intestinal contents pass to the rectum, the colon withdraws water, leaving semisolid waste. The strong muscles of the rectum hold back this waste until it is time to defecate. Then the rectal muscles relax, and the last sphincter in the system, the anus, opens to allow the wastes to pass. Thus, food travels through the digestive tract in this order: mouth, esophagus, lower esophageal sphincter (or cardiac sphincter), stomach, pyloric sphincter, duodenum (common bile duct enters here), jejunum, ileum, ileocecal valve, large intestine (colon), rectum, and anus.
The Involuntary Muscles and the Glands
You are usually unaware of all the activity that goes on between the time you swallow and the time you defecate. As is the case with so much else that happens in the body, the muscles and glands of the digestive tract meet internal needs without your having to exert any conscious effort to get the work done. People consciously chew and swallow, but even in the mouth, there are some processes over which you have no control. The salivary glands secrete just enough saliva to moisten each mouthful of food so that it can pass easily down your esophagus.
Once you have swallowed, materials are moved through the rest of the GI tract by involuntary muscular contractions. This motion, known as gastrointestinal motility, consists of two types of movement, peristalsis, and segmentation. Peristalsis propels, or pushes; segmentation mixes, with more gradual pushing.
Peristalsis begins when the bolus enters the esophagus. The entire GI tract is ringed with circular muscles, which are surrounded by longitudinal muscles. When the rings tighten and the long muscles relax, the tube is constricted. When the rings relax and the long muscles tighten the tube bulges. These actions alternate continually and push the intestinal contents along. If you have ever watched a bolus of food pass along the body of a snake, you have a good picture of how these muscles work.
The waves of contraction ripple through the GI tract at varying rates and intensities depending on the part of the GI tract and on whether food is present. Peristalticsis, aided by the sphincter muscles located at key places, keeps things moving along. However, factors such as stress, medicines, and medical conditions may interfere with normal GI tract contractions.
The intestines not only push but also periodically squeeze their contents. This motion, called segmentation, forces the contents back a few inches, mixing them and promoting close contact with the digestive juices and the absorbing cells of the intestinal walls before letting the contents slowly move along again.
Besides forcing the intestinal contents along, the muscles of the GI tract help to liquefy them to chyme so that the digestive juices will have access to all their nutrients. The mouth initiates this liquefying process by chewing, adding saliva, and stirring with the tongue to reduce the food to a coarse mash suitable for swallowing. The stomach then further mixes and kneads the food.
The stomach has the thickest walls and strongest muscles of all the GI tract organs. In addition to circular and longitudinal muscles, the stomach has a third layer of diagonal muscles that also alternately contract and relax. These three sets of muscles work to force the chyme downward, but the pyloric sphincter usually remains tightly closed, so that the stomach’s contents are thoroughly mixed and squeezed before being released.
Meanwhile, the gastric glands are adding juices. When the chyme is thoroughly liquefied, the pyloric sphincter opens briefly, about three times a minute, to allow small portions through. At this point, the intestinal contents no longer resemble food in the least.
Food enters the mouth and travels down the esophagus and through the lower esophageal sphincter to the stomach, then through the pyloric sphincter to the small intestine, on through the ileocecal valve to the large intestine, past the appendix to the rectum, exiting through the anus.
The wavelike contractions of peristalsis and the periodic squeezing of segmentation keep things moving at a reasonable pace.