LEARNING OBJECTIVE: Identify the location and function of each part of the digestive system.
The digestive system includes the 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 the tissues of the body. Table 1-5 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 wave-like motion called peristalsis. When peristalsis occurs, a ring of contraction appears in the walls of the alimentary canal. At the same time, the muscular wall just ahead of the ring relaxes. This phenomenon is called receptive relaxation. As the wave moves along, it pushes the canal's contents ahead of it. Chemical digestion consists of changing the various food substances, with the aid of digestive enzymes, into solutions and simple compounds. 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 (fig. 1-53).
The mouth, which is the first portion of the alimentary canal, is adapted to receive food and prepare it for digestion (fig. 1-53). 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. Saliva also lubricates the food mass to aid swallowing. The tongue assists with both mastication and swallowing.
The pharynx (covered earlier in “The Respiratory System”) is the passageway between the mouth and the esophagus and is shared with the respiratory tract (fig. 1-53). 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).
Figure 1-53.—The digestive system.
Table 1-5.—Principal Digestive Juices
Substance Acted Upon
|Amylase||Salivary glands and pancreas||Starch||Complex sugars (maltose)|
|Hydrochloric acid||Gastric glands||Pepsinogen (Proteins)||Pepsin (Split proteins)|
|Proteinase||Pancreas||Proteins and split proteins||Peptides and polypeptides|
|Lipase||Pancreas||Fats (triglycerides)||Fatty acids and glycerol|
|Carbohydrase||Intestinal glands||Complex sugars (maltose, sucrose, and lactose)||Simple sugars (glucose, fructose, and galactose)|
|Peptidase||Intestinal glands||Peptides and polypeptides||Amino acids|
The esophagus is a muscular tube about 25 cm (10 inches) long (fig. 1-53). It is the passageway between the pharynx and the stomach. 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. 1-53). 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, fundic, body, and pyloric regions (fig. 1-54). 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; 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 enzymes produced by the gastric glands are hydrochloric acid and pepsinogen. Hydrochloric acid activates pepsin from pep sinogen, 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 ofbeginning the digestion ofnearly 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.
Figure 1-54.—Major regions of the stomach.
The stomach and intestines are enclosed in the abdominal cavity, the space between the diaphragm and the pelvis. This cavity is lined with serous membrane called the peritoneum. The peritoneum covers the intestines and the organs and, by secreting a serous fluid, 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 (fig. 1-53).
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 is lined with a mucous membrane that contains small glands. These glands secrete intestinal juices containing the enzymes carbohydrase, peptidase, and lipase.
JEJUNUM.—The jejunum is the middle part of the small intestine and 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 (11.5 feet) long. Most of the absorption of food occurs in the ileum, 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 (fig. 1-53). It is considerably shorter, however, being about 1.5 meters (5 feet) long. It is divided into three distinct parts: the 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 pouchlike structure of the cecum located near the junction of the ileum and the cecum (fig. 1-53). There is no known function of this structure. Occasionally, the appendix becomes infected, causing inflammation to develop. This inflammation of the appendix is known as appendicitis.
RECTUM.—The rectum is approximately 12.5 cm (5 inches) long and follows the contour of the sacrum and coccyx until it curves back into the short (2.5 to 4 cm) 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 a strong muscular ring, the anal sphincter.
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.
The salivary glands are located in the mouth (fig. 1-53). Within the salivary glands are two types of secretory cells, serous cells and mucous cells. Theserous cells produce a watery fluid that contains a digestive juice called amylase. Amylase splits starch and glycerol into complex sugars. The mucous cells secrete a 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. Approximately 1 liter of saliva is secreted daily.
The pancreas is a large, elongated gland lying posteriorly to the stomach (fig. 1-53). 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 (fig. 1-53).
Of the liver's many functions, the following are important to remember:
The gallbladder is a pear-shaped sac, usually stained dark green by the bile it contains. It is located in the hollow underside of the liver (fig. 1-53). Its duct, the cystic duct,joins the hepatic duct from the liver to form the common bile duct, which enters the duodenum. 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.