The human digestive system is the organ system responsible for extracting nutrition from consumed food items and absorbing the nutritional molecules into the bloodstream. Doing this requires breaking food into smaller pieces mechanically and chemically, so that nutritional molecules are isolated and can be absorbed. This process involves the help of the muscular system.
The purpose of the digestive system, explains Dr. Lauralee Sherwood in her book "Human Physiology," is to take in food and extract from it the macro- and micronutrients that body cells require to meet their nutritional needs. The system consists of the gastrointestinal tract, which begins at the mouth and ends at the anus, as well as a number of accessory organs and glands. Food enters the tract at the mouth, nutrients are absorbed at certain points along its length and waste exits at the anus.
Several features of the digestive tract help to demonstrate the role of the muscular system in working with the digestive system. At the mouth, the muscles of the jaws and tongue break food into pieces. Muscles of the throat move food down the esophagus. In the stomach, churning motions produced by muscles break food into smaller bits and mix it with stomach acid. Finally, muscular contractions of the intestine move food through the remainder of the digestive tract, explains Dr. Sherwood.
There are three types of contractions through which muscles aid the digestive tract in its function, explains Dr. Gary Thibodeau in his book "Anatomy and Physiology." Muscles of the mouth and throat are called skeletal muscles, and are under voluntary control. Muscles in the stomach belong to a category called smooth muscles, and are under involuntary control. The smooth muscles of the stomach contract irregularly, churning food through a grinding motion. Regular, rhythmic contractions of the small intestine--also involuntary and under unconscious control, as the muscles of the intestine are also smooth muscle--move food through the digestive tract toward the exit.
It typically takes food anywhere from several hours to a day or more to progress through the length of the digestive tract. Certain factors increase or decrease the amount of time food spends in the system, including digestibility of food, hydration level and individual differences between people. Because the digestive system has only a short period of time during which to extract nutrition from food, note Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry," the digestive system relies on enzymes to help speed digestion.
The muscular system works best in conjunction with the digestive system when food passing through has a relatively large bulk to it, explains Dr. Sherwood. As such, appropriate hydration, which helps to keep the interior of the intestine moist, and large quantities of dietary fiber, which is indigestible and adds bulk to food, improve functioning of the digestive tract. Individuals who drink plenty of water and eat a high-fiber diet typically experience less constipation than those who don't.