Digestion
Digestion is the physiological process by which food is broken down, mechanically and chemically, into particles small enough to pass through the walls of the intestinal tract and into the blood. Once in the bloodstream, these tiny particles can then be distributed throughout the body and used for nourishment. The breaking-down process takes place in almost all parts of the digestive tract (also called the alimentary canal), beginning at the mouth and ending, some fifteen feet later, in the anus.
Specifically, the digestive process starts as soon as food, taken into the mouth, begins to be chewed into smaller pieces. While being chewed, the food is mixed with saliva that contains the enzyme ptyalin, the first of many enzymes that will help convert complex and indigestible food molecules into smaller and easier-to-absorb ones. While food is in the mouth, the activity of ptyalin is already at work, converting some of the complex starches into simple sugars.
After chewing, the food is swallowed, and it passes through the esophagus and into the stomach, where the breaking-down process goes into high gear. A strong churning motion causes the food to be thoroughly mixed by the stomach's potent digestive juices, which contain both hydrochloric acid and the enzyme pepsin. The foods are all dissolved into a thick liquid called chyme but, while the protein foods are partly digested, the other nutrients are basically unchanged. Then, in the small intestine, the digestive process is completed by a combination of pancreatic juices (containing the enzymes trypsin, amylase and lipase), intestinal juices, and bile. (The bile, stored in the gall bladder, works primarily on the digestion and absorption of fat.) Thoroughly digested, the nutrient molecules can now by absorbed by blood and lymph vessels in the small intestine's walls and carried into the circulation. Indigestible food particles pass into the large intestine, where some water and minerals are absorbed and bacterial action turns the rest into feces, which are eventually eliminated as waste products.
The digestive process is clearly a highly complicated one and, until fairly recently, only vaguely understood. In ancient times, for instance, the early philosophers could see that various foods entered the body and, while some of the food remained in the system presumably to provide nourishment, the rest emerged later in a completely changed form. How did they believe all this was accomplished? The Hippocratic philosophers, in the fourth and fifth centuries b.c., concluded that food was converted through body heat first into liquid form, then into what they called the "four humors"--blood, phlegm, yellow bile and black bile--which could be absorbed into the body. Eventually, however, the theory changed.
A few centuries later, around 280 b.c., the philosopher Erasistratus pointed out that, after food was eaten, a great deal of activity soon took place in the stomach and intestines. To him, therefore, digestion was almost certainly a mechanical process. Somehow food was ground down in the stomach to liquid form, then carried to the liver where it was transformed into blood. The famous second century physician Galen, although he agreed that the liver was the major organ of digestion, decided that the Hippocratics were right: animal heat must be the guiding force behind digestion. Galen's views prevailed for hundreds of years.
In the seventeenth century, however, scientists began taking another look at the human body. A number of them, such as Franciscus Sylvius, a Dutch physician, believed that most bodily processes could be explained in purely chemical terms. Sylvius studied digestive juices, including saliva, and correctly concluded that digestion was a form of fermentation.
Other scientists of the time tended to see the human body as a kind of machine. To Giovanni Borelli, an Italian mathematician and a contemporary of Sylvius, the stomach was simply a binding device that could mechanically break food down into tiny particles. Erasistratus had been right all along, he concluded.
During the eighteenth century, several scientists made tremendous progress in settling the debate over whether digestion was a mechanical or chemical process. Through his experiments on animal tissues, the Swiss physiologist Albrecht von Heller (1708-1777) discovered that bile was the key element in the body's digestion of fats. The debate was essentially settled by the French physiologist Rene de Reaumur (1683-1757) in 1752. De Reaumur fed a hawk two small metal cylinders filled with meat and covered at their ends by gauze. Unable to digest the cylinders, the hawk eventually regurgitated them, and de Reaumur deduced that the meat could only have been dissolved by chemicals present in internal fluids.
Before long, a number of eighteenth-century scientists were able to study the role of digestive juices. The Italian physiologist Lazzaro Spallanzani (1729-1799) performed experiments similar to de Reaumur's and coined the term "gastric juices." Spallanzani discovered that these juices helped inhibit and prevent the putrefaction of food within the body. He also helped to further define the important role that saliva plays in the digestive process. Additionally, in 1822, the German chemist Leopold Gmelin (1788-1853) continued to study the chemical processes involved in food digestion by investigating stomach and pancreatic juices. And in 1842, William Prout proved that the most potent acid in gastric juices was actually hydrochloric acid. The discoveries made by these scientists validated the proposition that chemical processes governed the body's digestive system.
A particularly notable contribution to the understanding of digestion was made by the American army surgeon William Beaumont. Beaumont, who joined the army in 1812, was sent a few years later to a frontier post in Michigan. While there, he treated a young French-Canadian, Alexis St. Martin, who had been accidentally shot in the side. Although St. Martin recovered, his bullet wound never fully closed. He retained an inch-wide opening (or fistula) in his side that led to his stomach. Through this opening, Beaumont could not only observe changes in the stomach under varying conditions, but could remove samples of gastric juices. Beginning in 1825, then, the army surgeon conducted over two hundred experiments and, by so doing, provided the medical world with a great deal of previously unknown information about gastric physiology and the digestive process in a living human being.
Inspired by Beaumont's work, the French physiologist Claude Bernard began, in the mid-1800s, to create artificial fistulas in laboratory animals. Through these openings, Bernard made a number of important discoveries, among them that the small intestine, rather than the stomach, was the major site of digestion and that pancreatic juices were important digestive agents, particularly where fat molecules were concerned.
A few years later, in the 1870s, Willy Kuhne's research clarified the role of the intestines in the absorption of digested foods. And, toward the end of the nineteenth century, the Russian physiologist Ivan Petrovich Pavlov, by experimenting on living dogs, worked out the nervous mechanism that controls the secretion of gastric juices by the digestive glands.
Because digestion is such a complex process, there are many things that can go wrong. Today 60 to 70 million people a year in the United States are affected by digestive diseases, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Among the most prevalent digestive diseases are abdominal wall hernias, constipation, gallstones, indigestion, hemorrhoids, diarrhea, irritable bowel syndrome, lactose intolerance, and peptic ulcers.
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