Digestive System
The functions of the digestive system are to:
- Ingest the food
- Break it down into small molecules that can cross plasma membranes
- Absorb these nutrient molecules
- Eliminate nondigestible wastes
Mouth
The mouth receives the food in its oral cavity. The taste of food results from the stimulation of chemoreceptors in taste buds on the tongue and in the nasal cavities. Therefore, smell is a very important component of taste perception.
The roof of the mouth has two parts: An anterior hard palate separates the oral cavity from the nasal passages, and a posterior soft palate separates the oral cavity from the nasopharynx. The hard palate contains several skull bones-namely, portions of the maxillae and palatine bones. The soft palate is a muscular structure and ends in the uvula, a cone-shaped process.
The tongue is made up of skeletal muscle covered by a mucous membrane. Intrinsic muscles that have their origin and insertion in the tongue itself change the shape of the tongue. Extrinsic muscles insert into the tongue but have their origin outside the tongue, such as on a skull bone. The extrinsic muscles move the tongue about and account for its maneuverability. A fold of mucous membrane (lingual frenulum) on the underside of the tongue attaches the tongue to the floor of the mouth. If the frenulum is too short, the individual cannot speak clearly and is said to be tongue-tied. The floor of the oral cavity and underside of the tongue are richly supplied with blood vessels, and soluble medications will enter the circulation directly if placed beneath the tongue.
There are three pairs of salivary glands called the parotid, sublingual, and submandibular glands. The
parotid glands are located in front of and below the ears. Each parotid gland has a duct that opens on the inner surface of the cheek, just at the location of the second upper molar. Mumps, an acute viral infection of the parotid saIivary glands, causes them to swell, making it difficult to open the mouth. The sublingual glands lie beneath the tongue proper, and their ducts open into the floor of the oral cavity. The submandibular glands lie in the posterior floor of the oral cavity beneath the base of the tongue. The ducts from the submandibular glands open on either side of the lingual frenulum.
The salivary glands produce about 1 liter of saliva a day, which enters the mouth by way of their ducts. Saliva contains mucus and a digestive enzyme called salivary amylase. Mucus binds and lubricates the food so it is easier to swallow, and salivary amylase begins the process of digesting the food by breaking down starch. Saliva also contains chemicals that protect against bacterial infection.
The teeth carry out mastication, or chewing offood. The tongue assists in mastication by moving the food between the teeth. Mastication breaks down the food into small portions, making the work of the digestive enzymes more efficient. The tongue forms the chewed food into a small mass called a bolus in preparation for swallowing.
The salivary glands produce saliva, vvhich enters the mouth. vvhere teeth masticate the food and the tongue forms a bolus prior to svvallovving.
Pharynx
Swallowing, a reflex action that moves food into the esophagus, occurs in the pharynx (far'inks), a region that opens into the nose, mouth, and larynx. During swallowing, food normally enters only the esophagus, a long, muscular tube that extends to the stomach, because the nasal and laryngeal passages are blocked. The nasopharyngeal openings are covered when the soft palate moves back. The opening to the larynx at the top of the trachea, called the glottis, is covered when the trachea moves up under a flap of tissue, called the epiglottis. Therefore, breathing does not occur when swallowing. This process is easy to observe in the up-and-down movement of the Adam's apple, the ventral cartilage of the larynx, when a person eats.
The air and food passages merge in the pharynx. When a person svvallovvs. the air passage is usually blocked off. and food must enter the esophagus.
Esophagus
The esophagus is a tube with a pharyngoesophageal sphincter near the pharynx and a gastroesophageal sphincter near the stomach. Sphincters are muscles that encircle tubes and act as valves; tubes close when sphincters contract, and they open when sphincters relax.
Swallowing pushes the bolus past the pharyngoesophageal sphincter into the esophagus, which conducts it through the thoracic cavity to the stomach. Rhythmical contraction of the digestive tract walL called peristalsis, begins in the esophagus and pushes food along the entire tract. Occasionally, peristalsis begins even though there is no food in the esophagus. This produces the sensation of a lump in the throat.
The esophagus stretches in three smooth curves from the back of the pharynx to just below the diaphragm, where it meets the stomach at an angle. When food reaches the stomach, the gastroesophageal sphincter relaxes, allowing the bolus to pass through. Normally, this sphincter prevents the acidic contents of the stomach from entering the esophagus. However, heartburn, which feels like a burning pain rising up into the throat, occurs when some of the contents of the stomach escape into the esophagus. When
vomIting occurs, contraction of the abdominal muscles and diaphragm propels the contents of the stomach upward through the esophagus.
The esophagus takes food to the stomach. Peristalsis. a rhythmical contraction of the digestive tract vvall. pushes food through the esophagus. as vvell as the rest of the digestive tract.
Stomach
The stomach is a thick-walled, J-shaped organ that lies in the upper left quadrant of the abdominal cavity beneath the diaphragm. It is continuous with the esophagus superiorly and the duodenum of the small intestine inferiorly. The stomach stores food and starts the digestion of protein. The wall of the stomach has three layers of muscle and contains deep folds called rugae (roo'je), which flatten out as the stomach fills. The muscular wall of the stomach churns, mixing its contents.
The mucosal lining of the stomach contains millions of microscopic digestive glands, called gastric glands (the word gastric always refers to the stomach). The gastric glands produce gastric juice, which contains a digestive enzyme, called pepsin, and hydrochloric acid (HCI). Pepsin digests protein. The acidity of the stomach is beneficial in that it kills most bacteria present in food. Although HCI does not digest food, it breaks down the connective tissue of meat and activates gastric enzymes present in gastric juice. It also stimulates the stomach to produce a hormone called gastrin. Gastrin goes into the bloodstream, and when it circulates back to the stomach, the gastric glands continue to secrete.
Normally, the wall of the stomach is protected by a thick layer of mucus, but if, by chance, HCI penetrates this mucus, an ulcer can form. An ulcer is an open sore in the stomach wall caused by the gradual disintegration of a mucous membrane. The most frequent cause of an ulcer has been shown to be an infection caused by the bacterium Helicobacter pylori.
Normally, the stomach empties in 2-6 hours. When food leaves the stomach, it is a pasty, semisolid, acidic mixture called chyme (kim). Chyme leaves the stomach and enters the small intestine by way of the pyloric sphincter. This sphincter repeatedly opens and closes, allowing chyme to enter the small intestine in small amounts only. This assures that digestion in the small intestine will proceed at a slow and thorough rate.
The stomach expands and stores food. While food is in the stomach. the stomach churns, mixing food vvith the acidic gastric juices.
Small Intestine
The small intestine is named for its small diameter (compared to that of the large intestine), but perhaps it should be called the long intestine. The small intestine averages about 3 meters (10 feet) in length, compared to the large intestine, which is about 1.5 meters (5 feet) in length. The small intestine is found in the central and lower portion of the abdominal cavity, where it is supported by a fan-shaped mesentery. It receives secretions from the liver and pancreas, chemically and mechanically breaks down chyme, absorbs nutrient molecules, and transports undigested material to the large intestine.
The small intestine has these regions:
Duodenum (dun o-de'num) The first 25 centimeters (10 inches) contain distinctive glands that secrete mucus and also receive the pancreatic secretions and the bile from the liver through a common duct. Folds and villi are more numerous at the end than at the beginning.
Jejunum (je-joo'num) The next 1 meter (3 feet) contains folds and villi, more at the beginning than at the end.
Ileum (il'e-um) The last 2 meters (6-7 feet) contain fewer folds and villi than the jejunum. The ileum wall contains reyer's patches, aggregates oflymph nodules. The small intestine has the following layers, as does
the rest of the digestive tract:
Serosa The serosa is a very thin, outermost layer of squamous epithelium supported by connective tissue. The serosa secretes a serous fluid that keeps the outer surface of the intestines moist so that the organs of the abdominopelvic cavity slide against one another.
Muscularis Two layers of smooth muscle comprise this section: The inner, circular layer of cells encircles the intestines; the outer, longitudinal layer lies in the same direction as the intestines.
Submucosa This broad band of loose connective tissue contains blood vessels and nerves.
Mucosa This layer of columnar epithelium, supported by connective tissue and smooth muscle, lines the central cavity and contains glandular epithelial cells that secrete mucus. Digestive glands, if present, are in this layer.
The wall of the small intestine has circular folds with fingerlike projections, called villi (vil'i) (sing., villus). The epithelial cells of each villus have extensions called microvilli. A large number of villi with their microvilli increase the small intestine's surface area for nutrient absorption and give the intestinal wall a soft, velvety appearance.
Each villus contains blood vessels and a small lymph vessel called a lacteal. The nutrient molecules that result from digestion cross the walls of the villi until all have been absorbed. Sugar and amino acids cross the epithelial cells to enter the blood; fats enter the lacteals.
As already mentioned, the plasma membranes of epithelial cells making up the villi contain microvilli, thousands of microscopic projections. In electron micrographs, the microvilli give the cells a fuzzy border, collectively called a brush border. The microvilli bear the intestinal digestive enzymes, which are therefore called the brush-border enzymes. These enzymes finish the digestion of chyme to small molecules that can be absorbed.
The intestinal wall also secretes the hormones secretin and cholecystokinin (CCK). These hormones travel in the bloodstream and stimulate the pancreas to secrete its juices. CCK also stimulates the gallbladder to contract and release bile.
