THE DIGESTIVE SYSTEM
The digestive system includes organs that digest and absorb food substances, and eliminate the unused residuals. The digestive system consists of the alimentary canal and several accessory organs. The accessory organs release secretions into the canal. These secretions assist in preparing food for absorption and use by body tissues. Table 6-8 illustrates principal digestive juices (secretions) produced by alimentary and accessory organs.
Digestion is both mechanical and chemical. Mechanical digestion occurs when food is chewed, swallowed, and propelled by a wavelike motion called peristalsis. When peristalsis occurs, a ring of reflex contraction appears in the walls of the alimentary canal. As the wave moves along, it pushes the canal's contents ahead of it (Fig. 6-74).
Chemical digestion consists of changing the various food substances, with the aid of digestive enzymes, into solutions and simple compounds. Complex carbohydrates (starches and sugars) change into simple sugars (glucose); fats change into fatty acids; and proteins change into amino acids. Once the food substances have been broken down into simple compounds, the cells of the body can absorb and use them.
THE ALIMENTARY CANAL
The alimentary canal (tract) is 9 meters in length, tubular, and includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
The mouth, which is the first portion of the alimentary canal, is adapted to receive food and prepare it for digestion (Fig. 6-75). The mouth mechanically reduces the size of solid particles and mixes them with saliva; this process is called mastication. Saliva, produced by the salivary gland, moistens food making it easier to chew (Fig. 6-76). Saliva also lubricates the food mass to aid swallowing. The tongue assists with both mastication and swallowing.
Image reprinted from: Thibedeau, G. A., & Patton, K. T. (2006). Anatomy & Physiology (6th ed.). St. Louis: Elsevier Health Sciences.
Mastication and Deglutition
The mastication process includes the biting and tearing of food into manageable pieces. This usually involves using the incisors and cuspid teeth. The grinding of food is usually performed by the molars and premolars. During the mastication process, food is moistened and mixed with saliva.
Deglutition is the swallowing of food and involves a complex and coordinated process. It is divided into three phases; the first phase of swallowing is voluntarily; phases two and three are involuntary. Phase One, Oral Stage: the collection and swallowing of masticated food Phase Two, Pharyngeal Stage: Passage of food through the pharynx into the beginning of the esophagus Phase Three, Esophageal Stage: The passage of food into the stomach
The process of moving food from the pharynx into the esophagus requires that three openings must be blocked: the mouth, nasopharynx, and the larynx (Fig. 6-77).
Pharynx The pharynx is the passageway between the mouth and the esophagus and is shared with the respiratory tract. The epiglottis is a cartilaginous flap that closes the opening to the larynx when food is being swallowed down the pharynx. Food is deflected away from the trachea to prevent particle aspiration (inhalation).
The esophagus is a muscular tube about 25 cm (10 inches) long and pierces the diaphragm on its way to the stomach (Fig. 6-78). It is the passageway between the pharynx and the stomach. “Each end of the esophagus is encircled by muscular sphincters that act as valves to regulate passage of material. The upper esophageal sphincter in the cervical part of the esophagus helps prevent air entering the esophagus during respiration2;” it is also the valve that is relaxed when a person belches. The lower esophageal sphincter is at the junction with the stomach which help keeps food in, when this is damaged or does not work properly a patient gets heartburn or gastroesophageal reflux disease (GERD). By means of peristalsis, food is pushed along this tube to the stomach. When peristalsis is reversed, vomiting occurs.
The stomach acts as an initial storehouse for swallowed material and helps in the chemical breakdown of food substances. The stomach is a saccular enlargement of the gastrointestinal tube and lies in the left upper quadrant of the abdomen (Fig. 6-79). It connects the lower end of the esophagus with the first portion of the small intestine (the duodenum). The stomach is divided into the cardiac, fundus, body, and pyloric regions (Fig. 6-79). At each end of the stomach, muscular rings (or sphincters) form valves to close off the stomach. The sphincters prevent the stomach's contents from escaping in either direction while food substances are being mixed by peristaltic muscular contractions of the stomach wall. The sphincter at the esophageal end is the cardiac sphincter or lower esophageal sphincter; at the duodenal end it is the pyloric sphincter.
The chemical breakdown of food in the stomach is accomplished through the production of digestive juices (enzymes) by small (gastric) glands in the wall of the stomach. The principal digestive enzyme produced by the gastric glands is pepsinogen along with a secondary enzyme, hydrochloric acid. Hydrochloric acid activates pepsin from pepsinogen, kills bacteria that enter the stomach, inhibits the digestive action of amylase, and helps regulate the opening and closing of the pyloric sphincter. Pepsin is a protein-splitting enzyme capable of beginning the digestion of nearly all types of dietary protein.
Most food absorption takes place in the small intestine. In general, food is not absorbed in the stomach. An exception is alcohol, which is absorbed directly through the stomach wall. It is for this reason that intoxication occurs quickly when alcohol is taken on an empty stomach.
The stomach and intestines are enclosed in the abdominal cavity, the space between the diaphragm and the pelvis. This cavity is lined with a serous membrane called the peritoneum. The peritoneum covers the intestines and the organs; by secreting a serous fluid, it prevents friction between adjacent organs. The mesentery (double folds of peritoneum) extends from the cavity walls to the organs of the abdominal cavity, suspending them in position and carrying blood vessels to the organs.
The small intestine is a muscular, convoluted, coiled tube, about 7 meters (23 feet) long and attached to the posterior abdominal wall by its mesentery.
The small intestine is divided into three contiguous parts: the duodenum, jejunum, and ileum. It receives digestive juices from three accessory organs of digestion: the pancreas, liver, and gallbladder.
DUODENUM.—The duodenum is approximately 25 cm (10 inches) long and forms a C-shaped curve around the head of the pancreas, posterior to the liver. It has enzymes that start the breakdown of foods and receives enzymes from the pancreas that assist in digestion.
JEJUNUM.—The jejunum is the middle part of the small intestine; it is approximately 2.5 meters (8.2 feet) long. Its enzymes continue the digestive process.
ILEUM.—The ileum is the last and longest part of the small intestine; it is approximately 3.5 meters (12 feet) long.
Most of the absorption of food occurs in the small intestines, where fingerlike projections (villi) provide a large absorption surface. After ingestion, it takes 20 minutes to 2 hours for the first portion of the food to pass through the small intestine to the beginning of the large intestine.
The large intestine is so called because it is larger in diameter than the small intestine. It is considerably shorter, being about 1.5 meters (5 feet) long. It is divided into three parts: cecum, colon, and rectum.
CECUM AND COLON.—The unabsorbed food or waste material passes through the cecum into the ascending colon, across the transverse colon, and down the descending colon through the sigmoid colon to the rectum. Twelve hours after the meal, the waste material passes slowly through the colon, building in mass and reaching the rectum 24 hours after the food is ingested.
The appendix, a long narrow tube with a blind end, is a pouch-like structure of the cecum located near the junction of the ileum and the cecum. There is no known function of this structure. The appendix can become infected, causing inflammation to develop. This inflammation of the appendix is known as appendicitis (Fig. 6-80) and requires surgery to correct.
RECTUM.—The rectum is approximately 17-20 cm (7 or 8 inches) long and follows the contour of the sacrum and coccyx until it curves back into the short 2.5 cm (inch) anal canal. The anus is the external opening at the lower end of the digestive system. Except during bowel movement (defecation), it is kept closed by two sphincters. An internal one made of smooth muscle and external one made of striated muscle (Fig. 6-81).
ACCESSORY ORGANS OF DIGESTION
The accessory organs of digestion include the salivary glands, pancreas, liver, and gallbladder. As stated earlier, during the digestive process, the accessory organs produce secretions that assist the organs of the alimentary canal.
Salivary Glands The salivary glands are located in the mouth (Fig. 6-76). Within the salivary glands are two types of secretory cells, serous cells and mucous cells. The serous cells produce a watery fluid containing a digestive juice called amylase. Amylase splits starch and glycerol into complex sugars. The mucous cells secrete thick, sticky liquid called mucus. Mucus binds food particles together and acts to lubricate during swallowing. The fluids produced by the serous and mucous cells combine to form saliva. The salivary glands produce 1.7 liters of saliva daily, greatly aiding in the digestion process. Enzymes are present in saliva; they act on food, and start the breakdown process. In dentistry, location of the saliva glands and ducts (openings) is important in keeping the mouth dry during certain dental procedures. Table 6-5 lists the three major salivary glands.
The pancreas is a large, elongated gland lying posterior to the stomach (Fig. 6-82). As discussed earlier in "The Endocrine System," the pancreas has two functions: It serves both the endocrine system and the digestive system. The digestive portion of the pancreas produces digestive juices (amylase, proteinase, and lipase) that are secreted through the pancreatic duct to the duodenum. These digestive juices break down carbohydrates (amylase), proteins (proteinase), and fats (lipase) into simpler compounds.
The liver is the largest gland in the body. It is located in the upper abdomen on the right side, just under the diaphragm and superior to the duodenum and pylorus.
Of the liver's many functions, the following are important:
The gallbladder is a pear-shaped sac, stained dark green by the bile it contains. It is located in the hollow underside of the liver (Fig. 6-83). Its duct, the cystic duct, joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum (Fig. 6-83). The gallbladder receives bile from the liver and then concentrates and stores it. It secretes bile when the small intestine is stimulated by the entrance of fats.
|David L. Heiserman, Editor||
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Revised: June 06, 2015