The digestive tract, beginning at the mouth and ending at the anus, is a long tract of hollow, membrane-lined organs that are responsible for physically and chemically breaking up food into molecules that can be absorbed into the bloodstream and used to fuel cellular processes. Various accessory organs, such as the pancreas, secrete chemicals that aid digestion into the tract, but digestion is not purely a chemical process. Rather, the muscular system is an important and integral part of digestion.
Early muscular involvement
Some of the most obvious of the muscles involved in digestion are those used quite early in the process. The muscles of chewing, called the masseter muscles, are large and powerful. They run from the zygomatic arch, sometimes called the cheekbone, to the lower jaw, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." The tongue, too, is a muscle, and is important to digestion, because it helps move food around in the mouth to ensure that the teeth can break it up into small particles. Finally, the muscles of the throat push food down through the oesophagus into the stomach via the act of swallowing. All these early muscles of digestion are under voluntary control.
Muscles that churn food
The term "stomach muscles" is often taken to mean the abdominals, or muscles that form a "six pack" in a fit individual. In truth, though, the true stomach muscles are within the walls of the stomach organ itself. Unlike the abdominals and other skeletal muscles, stomach muscles are under involuntary control. Dr. Thibodeau notes that the primary role of these muscles is to mix, break up, and churn food in the stomach through waves of disorderly motion called peristalsis. The result of this churning is a soupy mixture called chyme, that then passes into the small intestine.
Muscles that limit movement
Many of the muscles of digestion are meant to move food through the digestive tract, but several special muscles, called sphincters, help keep food from progressing until the time is right. Sphincters are circular muscles that act a little like valves, explains Dr. Lauralee Sherwood in "Human Physiology." The first is the cardiac sphincter, which allows food to pass into the stomach, but prevents it--and stomach acid--from leaking back into the oesophagus. The pyloric sphincter, located at the bottom of the stomach, ensures that food doesn't leave the stomach until it has been dissolved into chyme.
Once food has passed into the small intestine, it moves through the tract and into the large intestine. Waves of peristalsis, this time of an orderly sort, accomplish this. Smooth muscles line the intestines and help to push food through in a manner analogous to that of a person pushing a tennis ball through a tube sock, hand over hand. The peristaltic waves of the intestine, like those of the stomach, are not under conscious control.
Typically, the entire process of digestion from one end of the body to the other takes eight hours to a day. The type of food consumed, quantity of food, amount of fibre consumed, and water consumption all influence the rate of digestion. Once digested food, now called faecal matter, reaches the end of the large intestine, the next step is elimination. One final sphincter, called the anal sphincter, lies between the last portion of the large intestine and the outside world. Most adult humans are able to control their anal sphincters to a point, though emotional stress and extreme need to defecate can overwhelm conscious control, notes Dr. Sherwood.