Digestion is the process of turning large pieces of food into its component chemicals. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines. Bile produced by the liver is also used to mechanically break fats into smaller globules. While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb. Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine thanks to the action of the pancreas. The pancreas secretes an incredibly strong digestive cocktail known as pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum , it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides, and nucleotides.
Once food has been reduced to its building blocks, it is ready for the body to absorb. Absorption begins in the stomach with simple molecules like water and alcohol being absorbed directly into the bloodstream. Most absorption takes place in the walls of the small intestine, which are densely folded to maximize the surface area in contact with digested food. Small blood and lymphatic vessels in the intestinal wall pick up the molecules and carry them to the rest of the body. The large intestine is also involved in the absorption of water and vitamins B and K before feces leave the body.
The final function of the digestive system is the excretion of waste in a process known as defecation. Defecation removes indigestible substances from the body so that they do not accumulate inside the gut. The timing of defecation is controlled voluntarily by the conscious part of the brain, but must be accomplished on a regular basis to prevent a backup of indigestible materials.
Prepared by Tim Taylor, Anatomy and Physiology Instructor
The extent of a tooth decay varies widely among meth users. A 2000 report in the Journal of Periodontology found that users who snorted the drug had significantly worse tooth decay than users who smoked or injected it, although all types of users suffered from dental problems. Anecdotal evidence also suggests that the degree of tooth decay is not necessarily dependent on the length of drug use. "[O]ne gentleman I saw said he used it for four months and there was nothing except for root tips left in his mouth," said Dr. Athena Bettger, a dentist who practices two days a week at the Multnomah County Jail in Portland, Ore. "Whereas another gentleman I saw said he was using it for four years, and … I think three teeth needed to come out and he needed a couple of fillings because of the cavities."