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. The digestive system of humans consists of a one-way digestive tract with several specialized chambers along the way--mouth, stomach, small intestine, and large intestine. Each chamber has a specific function. In the human digestive system, digestion starts in the mouth. There food is physically digested by the action of jaws and teeth, which breaks it down into smaller pieces, increasing the surface area for the digestive enzymes to work on. While being chewed, the food is mixed with saliva. Saliva makes the food slippery for swallowing, and also contains the enzyme amylase, a carbohydrase that breaks down starch into smaller polysaccharides. Once in the bloodstream, these tiny particles can be distributed throughout the body and used for nourishment. The process of disassembling food into useable molecules takes place in almost all parts of the digestive tract, which begins at the mouth and ends at the anus.
The digestive process starts as soon as food is taken into the mouth and chewed into smaller pieces. This mechanical chopping is the first step. Ingestion occurs in the mouth where food is chewed and mixed with saliva. In fact, "The teeth have different shapes to perform different tasks; the incisors are used to cut into food, the canines are used to tear or pierce food, and the premolars and molars are used for crushing and grinding food" (Biology, 2009). Chewing of food is accompanied by mixing the food with saliva. The mouth is normally kept moist by the production of small quantities of saliva by numerous intrinsic salivary glands located in the inner lining of the mouth. During chewing, much greater quantities of saliva are secreted by three pairs of extrinsic salivary glands, namely the parotid glands , the submandibular glands, and the sublingual glands. Saliva is a watery fluid containing several components including lysozyme, an enzyme that kills bacteria, and salivary amylase, an enzyme that begins the digestion of starch. The food mixes with saliva that contains the enzyme ptyalin, the first of many enzymes that help convert complex and indigestible food substances into smaller, simpler, and easier-to-absorb molecules. While food is in the mouth, ptyalin is already at work, converting some complex starches into simple sugars. Once the food has been chewed into a soft, flexible mass called a bolus, it is swallowed for delivery to the stomach (2009). On its journey, the bolus passes through the pharynx and then through the esophagus, a straight muscular tube that descends through the thoracic cavity, anterior to the spine. Each bolus of food is propelled through the esophagus by gravity, and by the process of peristalsis, a wave of muscular contraction that pushes the bolus downward. The lower end of the esophagus, which passes through a hole in the diaphragm to meet the stomach within the abdominal cavity, has a lower esophageal sphincter which briefly relaxes to allow the bolus of food to enter the stomach.
The stomach is a muscular sac that is located in the upper left portion of the abdominal cavity. The inner lining of the stomach wall contains millions of tiny gastric glands that secrete gastric juice, which dissolves the food to form a thick liquid called chyme. Gastric juice contains several substances including hydrochloric acid and other enzymes (2007). The hydrochloric acid has several functions including destroying ingested bacteria, and converting pepsinogen, a type of enzyme. At the lower end of the stomach is the pyloric sphincter, a valve through which chyme must flow to enter the small intestine. In fact, "Most meals are gradually emptied into the small intestine after two to six hours due to peristaltic contractions that travel toward the lower end of the stomach" (World of Anatomy and Physiology, 2007). Most digestion and absorption occur within the small intestine. Once digestion has been completed, the digested nutrients are absorbed into blood vessels and lymphatic vessels within the wall of the small intestine. Peristaltic contractions move materials through the small intestine and into the large intestine. The large intestine consists of three major segments, the cecum, the colon, and the rectum. As peristalsis moves material through the colon, water is absorbed to gradually convert the "chyme" into material called feces. The feces contain indigestible food molecules and intestinal bacteria that live in the colon. Peristalsis delivers the feces into the rectum where they are stored until they are expelled through the anus by the process of defecation.
Research on the human digestive system has been prevalent amongst researchers throughout history. In the seventeenth century, h scientists began taking another look at the human body. A number of them, including Dutch physician Franciscus Sylvius, believed that most bodily processes could be explained in purely chemical terms. Sylvius studied digestive secretions and correctly concluded that digestion was a form of fermentation. During the eighteenth century, several scientists made tremendous progress in settling the debate over whether digestion was a mechanical process or a chemical process. In fact, "Swiss physiologist Albrecht von Heller performed experiments on animal tissues which showed that bile was the key element in the body's digestion of fats" (World of Anatomy and Physiology, 2007). A number of eighteenth-century scientists provided further insights into the role of digestive secretions. The Italian physiologist Lazzaro Spallanzani performed experiments and coined the term "gastric juices." Spallanzani discovered that these juices help prevent food from decaying in the body. He also helped to further define saliva's important role in the digestive process. German chemist Leopold Gmelin continued to study the chemical processes involved in food digestion by investigating stomach and pancreatic secretions. In 1842, English physician and chemist William Prout proved that the most potent substance in gastric juices was actually hydrochloric acid. These scientists' discoveries validated the proposition that chemical processes governed the body's digestive system. In 1825, American army surgeon William Beaumont began a series of experiments that would make a particularly notable contribution to the scientific understanding of digestion. In 1822, he treated a young man, who had been accidentally shot in the side. The man recovered, but his bullet wound never fully closed. There remained an inch-wide opening in his side that led to his stomach (2007). Through this opening, Beaumont could observe how the stomach changed under varying conditions, and he could remove samples of gastric secretions. Beginning in 1825, Beaumont conducted over 200 experiments, thereby providing the medical world with a great deal of new information about gastric physiology and the digestive process in living human beings.
The digestive process starts as soon as food is taken into the mouth and chewed into smaller pieces. This mechanical chopping is the first step. Ingestion occurs in the mouth where food is chewed and mixed with saliva. In fact, "The teeth have different shapes to perform different tasks; the incisors are used to cut into food, the canines are used to tear or pierce food, and the premolars and molars are used for crushing and grinding food" (Biology, 2009). Chewing of food is accompanied by mixing the food with saliva. The mouth is normally kept moist by the production of small quantities of saliva by numerous intrinsic salivary glands located in the inner lining of the mouth. During chewing, much greater quantities of saliva are secreted by three pairs of extrinsic salivary glands, namely the parotid glands , the submandibular glands, and the sublingual glands. Saliva is a watery fluid containing several components including lysozyme, an enzyme that kills bacteria, and salivary amylase, an enzyme that begins the digestion of starch. The food mixes with saliva that contains the enzyme ptyalin, the first of many enzymes that help convert complex and indigestible food substances into smaller, simpler, and easier-to-absorb molecules. While food is in the mouth, ptyalin is already at work, converting some complex starches into simple sugars. Once the food has been chewed into a soft, flexible mass called a bolus, it is swallowed for delivery to the stomach (2009). On its journey, the bolus passes through the pharynx and then through the esophagus, a straight muscular tube that descends through the thoracic cavity, anterior to the spine. Each bolus of food is propelled through the esophagus by gravity, and by the process of peristalsis, a wave of muscular contraction that pushes the bolus downward. The lower end of the esophagus, which passes through a hole in the diaphragm to meet the stomach within the abdominal cavity, has a lower esophageal sphincter which briefly relaxes to allow the bolus of food to enter the stomach.
The stomach is a muscular sac that is located in the upper left portion of the abdominal cavity. The inner lining of the stomach wall contains millions of tiny gastric glands that secrete gastric juice, which dissolves the food to form a thick liquid called chyme. Gastric juice contains several substances including hydrochloric acid and other enzymes (2007). The hydrochloric acid has several functions including destroying ingested bacteria, and converting pepsinogen, a type of enzyme. At the lower end of the stomach is the pyloric sphincter, a valve through which chyme must flow to enter the small intestine. In fact, "Most meals are gradually emptied into the small intestine after two to six hours due to peristaltic contractions that travel toward the lower end of the stomach" (World of Anatomy and Physiology, 2007). Most digestion and absorption occur within the small intestine. Once digestion has been completed, the digested nutrients are absorbed into blood vessels and lymphatic vessels within the wall of the small intestine. Peristaltic contractions move materials through the small intestine and into the large intestine. The large intestine consists of three major segments, the cecum, the colon, and the rectum. As peristalsis moves material through the colon, water is absorbed to gradually convert the "chyme" into material called feces. The feces contain indigestible food molecules and intestinal bacteria that live in the colon. Peristalsis delivers the feces into the rectum where they are stored until they are expelled through the anus by the process of defecation.
Research on the human digestive system has been prevalent amongst researchers throughout history. In the seventeenth century, h scientists began taking another look at the human body. A number of them, including Dutch physician Franciscus Sylvius, believed that most bodily processes could be explained in purely chemical terms. Sylvius studied digestive secretions and correctly concluded that digestion was a form of fermentation. During the eighteenth century, several scientists made tremendous progress in settling the debate over whether digestion was a mechanical process or a chemical process. In fact, "Swiss physiologist Albrecht von Heller performed experiments on animal tissues which showed that bile was the key element in the body's digestion of fats" (World of Anatomy and Physiology, 2007). A number of eighteenth-century scientists provided further insights into the role of digestive secretions. The Italian physiologist Lazzaro Spallanzani performed experiments and coined the term "gastric juices." Spallanzani discovered that these juices help prevent food from decaying in the body. He also helped to further define saliva's important role in the digestive process. German chemist Leopold Gmelin continued to study the chemical processes involved in food digestion by investigating stomach and pancreatic secretions. In 1842, English physician and chemist William Prout proved that the most potent substance in gastric juices was actually hydrochloric acid. These scientists' discoveries validated the proposition that chemical processes governed the body's digestive system. In 1825, American army surgeon William Beaumont began a series of experiments that would make a particularly notable contribution to the scientific understanding of digestion. In 1822, he treated a young man, who had been accidentally shot in the side. The man recovered, but his bullet wound never fully closed. There remained an inch-wide opening in his side that led to his stomach (2007). Through this opening, Beaumont could observe how the stomach changed under varying conditions, and he could remove samples of gastric secretions. Beginning in 1825, Beaumont conducted over 200 experiments, thereby providing the medical world with a great deal of new information about gastric physiology and the digestive process in living human beings.