DIGESTIVE SYSTEM: Alimentary Canal

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DIGESTIVE SYSTEM


Alimentary Canal




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Digestive System


The digestive system consists of two parts: the accessory glands and the alimentary canal. The accessory glands, which include the salivary glands, liver, gallbladder, and pancreas, secrete digestive enzymes into the alimentary canal. The alimentary canal is a musculomembranous tube that extends from the mouth to the anus. The regions of the alimentary canal vary in diameter according to functional requirements. The greater part of the canal, which is about 29 to 30 ft (8.6 to 8.9 m) long, lies in the abdominal cavity. The component parts of the alimentary canal (Fig. 17-1) are the mouth, in which food is masticated and converted into a bolus by insalivation; the pharynx and esophagus, which are the organs of swallowing; the stomach, in which the digestive process begins; the small intestine, in which the digestive process is completed; and the large intestine, which is an organ of egestion and water absorption that terminates at the anus.




Esophagus


The esophagus is a long, muscular tube that carries food and saliva from the laryngopharynx to the stomach (see Fig. 17-1). The adult esophagus is approximately 10 inches (24 cm) long and ¾ inch (1.9 cm) in diameter. Similar to the rest of the alimentary canal, the esophagus has a wall composed of four layers. Beginning with the outermost layer and moving in, the layers are as follows:



The esophagus lies in the midsagittal plane. It originates at the level of the sixth cervical vertebra, or the upper margin of the thyroid cartilage. The esophagus enters the thorax from the superior portion of the neck. In the thorax, the esophagus passes through the mediastinum, anterior to the vertebral bodies and posterior to the trachea and heart (see Fig. 17-1, B). In the lower thorax, the esophagus passes through the diaphragm at T10. Inferior to the diaphragm, the esophagus curves sharply left, increases in diameter, and joins the stomach at the esophagogastric junction, which is at the level of the xiphoid tip (T11). The expanded portion of the terminal esophagus, which lies in the abdomen, is called the cardiac antrum.



Stomach


The stomach is the dilated, saclike portion of the digestive tract extending between the esophagus and the small intestine (Fig. 17-2). Its wall is composed of the same four layers as the esophagus.



The stomach is divided into the following four parts:



The cardia of the stomach is the section immediately surrounding the esophageal opening. The fundus is the superior portion of the stomach that expands superiorly and fills the dome of the left hemidiaphragm. When the patient is in the upright position, the fundus is usually filled with gas and in radiography is referred to as the gas bubble. Descending from the fundus and beginning at the level of the cardiac notch is the body of the stomach. The inner mucosal layer of the body of the stomach contains numerous longitudinal folds called rugae. When the stomach is full, the rugae are smooth. The body of the stomach ends at a vertical plane passing through the angular notch. Distal to this plane is the pyloric portion of the stomach, which consists of the pyloric antrum, to the immediate right of the angular notch, and the narrow pyloric canal communicating with the duodenal bulb.


The stomach has anterior and posterior surfaces. The right border of the stomach is marked by the lesser curvature. The lesser curvature begins at the esophagogastric junction, is continuous with the right border of the esophagus, and is a concave curve ending at the pylorus. The left and inferior borders of the stomach are marked by the greater curvature. The greater curvature begins at the sharp angle at the esophagogastric junction, the cardiac notch, and follows the superior curvature of the fundus and then the convex curvature of the body down to the pylorus. The greater curvature is four to five times longer than the lesser curvature.


The entrance to and the exit from the stomach are each controlled by a muscle sphincter. The esophagus joins the stomach at the esophagogastric junction through an opening termed the cardiac orifice. The muscle controlling the cardiac orifice is called the cardiac sphincter. The opening between the stomach and the small intestine is the pyloric orifice, and the muscle controlling the pyloric orifice is called the pyloric sphincter.


The size, shape, and position of the stomach depend on body habitus and vary with posture and the amount of stomach contents (Fig. 17-3). In persons with a hypersthenic habitus, the stomach is almost horizontal and is high, with its most dependent portion well above the umbilicus. In persons with an asthenic habitus, the stomach is vertical and occupies a low position, with its most dependent portion extending well below the transpyloric, or interspinous, line. Between these two extremes are the intermediate types of bodily habitus with corresponding variations in the shape and position of the stomach. The habitus of 85% of the population is either sthenic or hyposthenic. Radiographers should become familiar with the various positions of the stomach in the different types of body habitus so that accurate positioning of the stomach is ensured.



The stomach has several functions in the digestive process. The stomach serves as a storage area for food until it can be digested further. It is also where food is broken down. Acids, enzymes, and other chemicals are secreted to break food down chemically. Food is also mechanically broken down through churning and peristalsis. Food that has been mechanically and chemically altered in the stomach is transported to the duodenum as a material called chyme.



Small Intestine


The small intestine extends from the pyloric sphincter of the stomach to the ileocecal valve, where it joins the large intestine at a right angle. Digestion and absorption of food occur in this portion of the alimentary canal. The length of the adult small intestine averages about 22 ft (6.5 m), and its diameter gradually diminishes from approximately 1½ inches (3.8 cm) in the proximal part to approximately 1 inch (2.5 cm) in the distal part. The wall of the small intestine contains the same four layers as the walls of the esophagus and stomach. The mucosa of the small intestine contains a series of fingerlike projections called villi, which assist the process of digestion and absorption.


The small intestine is divided into the following three portions:



The duodenum is 8 to 10 inches (20 to 24 cm) long and is the widest portion of the small intestine (Fig. 17-4). It is retroperitoneal and relatively fixed in position. Beginning at the pylorus, the duodenum follows a C-shaped course. Its four regions are described as the first (superior), second (descending), third (horizontal or inferior), and fourth (ascending) portions. The segment of the first portion is called the duodenal bulb because of its radiographic appearance when it is filled with an opaque contrast medium. The second portion is about 3 or 4 inches (7.6 to 10 cm) long. This segment passes inferiorly along the head of the pancreas and in close relation to the undersurface of the liver. The common bile duct and the pancreatic duct usually unite to form the hepatopancreatic ampulla, which opens on the summit of the greater duodenal papilla in the duodenum. The third portion passes toward the left at a slight superior inclination for a distance of about 2½ inches (6 cm) and continues as the fourth portion on the left side of the vertebrae. This portion joins the jejunum at a sharp curve called the duodenojejunal flexure and is supported by the suspensory muscle of the duodenum (ligament of Treitz). The duodenal loop, which lies in the second portion, is the most fixed part of the small intestine and normally lies in the upper part of the umbilical region of the abdomen; however, its position varies with body habitus and with the amount of gastric and intestinal contents.



The remainder of the small intestine is arbitrarily divided into two portions, with the upper two fifths referred to as the jejunum and the lower three fifths referred to as the ileum. The jejunum and ileum are gathered into freely movable loops, or gyri, and are attached to the posterior wall of the abdomen by the mesentery. The loops lie in the central and lower part of the abdominal cavity within the arch of the large intestine.



Large Intestine


The large intestine begins in the right iliac region, where it joins the ileum of the small intestine, forms an arch surrounding the loops of the small intestine, and ends at the anus (Fig. 17-5). The large intestine has four main parts, as follows:




The large intestine is about 5 ft (1.5 m) long and is greater in diameter than the small intestine. The wall of the large intestine contains the same four layers as the walls of the esophagus, stomach, and small intestine. The muscular portion of the intestinal wall contains an external band of longitudinal muscle that forms into three thickened bands called taeniae coli. One band is positioned anteriorly, and two are positioned posteriorly. These bands create a pulling muscle tone that forms a series of pouches called the haustra. The main functions of the large intestine are reabsorption of fluids and elimination of waste products.


The cecum is the pouchlike portion of the large intestine and is below the junction of the ileum and the colon. The cecum is approximately 2½ inches (6 cm) long and 3 inches (7.6 cm) in diameter. The vermiform appendix is attached to the posteromedial side of the cecum. The appendix is a narrow, wormlike tube that is about 3 inches (7.6 cm) long. The ileocecal valve is just below the junction of the ascending colon and the cecum. The valve projects into the lumen of the cecum and guards the opening between the ileum and the cecum.


The colon is subdivided into ascending, transverse, descending, and sigmoid portions. The ascending colon passes superiorly from its junction with the cecum to the undersurface of the liver, where it joins the transverse portion at an angle called the right colic flexure (formerly hepatic flexure). The transverse colon, which is the longest and most movable part of the colon, crosses the abdomen to the undersurface of the spleen. The transverse portion makes a sharp curve, called the left colic flexure (formerly splenic flexure), and ends in the descending portion. The descending colon passes inferiorly and medially to its junction with the sigmoid portion at the superior aperture of the lesser pelvis. The sigmoid colon curves to form an S-shaped loop and ends in the rectum at the level of the third sacral segment.


The rectum extends from the sigmoid colon to the anal canal. The anal canal terminates at the anus, which is the external aperture of the large intestine (Fig. 17-6). The rectum is approximately 6 inches (15 cm) long. The distal portion, which is about 1 inch (2.5 cm) long, is constricted to form the anal canal. Just above the anal canal is a dilation called the rectal ampulla. Following the sacrococcygeal curve, the rectum passes inferiorly and posteriorly to the level of the pelvic floor and bends sharply anteriorly and inferiorly into the anal canal, which extends to the anus. The rectum and anal canal have two AP curves; this fact must be remembered when an enema tube is inserted.



The size, shape, and position of the large intestine vary greatly, depending on body habitus (see Fig. 17-3). In hypersthenic patients, the large intestine is positioned around the periphery of the abdomen and may require more radiographs to show its entire length. The large intestine of asthenic patients, which is bunched together and positioned low in the abdomen, is at the other extreme.



Liver and Biliary System


The liver, the largest gland in the body, is an irregularly wedge-shaped gland. It is situated with its base on the right and its apex directed anteriorly and to the left (Fig. 17-7). The deepest point of the liver is the inferior aspect just above the right kidney. The diaphragmatic surface of the liver is convex and conforms to the undersurface of the diaphragm. The visceral surface is concave and molded over the viscera on which it rests. Almost all of the right hypochondrium and a large part of the epigastrium are occupied by the liver. The right portion extends inferiorly into the right lateral region as far as the fourth lumbar vertebra, and the left extremity extends across the left hypochondrium.



At the falciform ligament, the liver is divided into a large right lobe and a much smaller left lobe. Two minor lobes are located on the medial side of the right lobe: the caudate lobe on the posterior surface and the quadrate lobe on the inferior surface (Fig. 17-8, A). The hilum of the liver, called the porta hepatis, is situated transversely between the two minor lobes.



The portal vein and the hepatic artery, both of which convey blood to the liver, enter the porta hepatis and branch out through the liver substance (see Fig. 17-8, C). The portal vein ends in the sinusoids, and the hepatic artery ends in capillaries that communicate with sinusoids. In addition to the usual arterial blood supply, the liver receives blood from the portal system.


The portal system, of which the portal vein is the main trunk, consists of the veins arising from the walls of the stomach, from the greater part of the intestinal tract and the gallbladder, and from the pancreas and the spleen. The blood circulating through these organs is rich in nutrients and is carried to the liver for modification before being returned to the heart. The hepatic veins convey the blood from the liver sinusoids to the inferior vena cava.


The liver has numerous physiologic functions. The primary consideration from the radiographic standpoint is the formation of bile. The gland secretes bile at the rate of 1 to 3 pints (½ to 1½ L) each day. Bile, the channel of elimination for the waste products of red blood cell destruction, is an excretion and a secretion. As a secretion, it is an important aid in the emulsification and assimilation of fats. The bile is collected from the liver cells by the ducts and either carried to the gallbladder for temporary storage or poured directly into the duodenum through the common bile duct.


The biliary, or excretory, system of the liver consists of the bile ducts and gallbladder (see Fig. 17-8). Beginning within the lobules as bile capillaries, the ducts unite to form larger and larger passages as they converge, finally forming two main ducts, one leading from each major lobe. The two main hepatic ducts emerge at the porta hepatis and join to form the common hepatic duct, which unites with the cystic duct to form the common bile duct. The hepatic and cystic ducts are each about 1½ inches (3.8 cm) in length. The common bile duct passes inferiorly for a distance of approximately 3 inches (7.6 cm). The common bile duct joins the pancreatic duct, and they enter together or side by side into an enlarged chamber known as the hepatopancreatic ampulla, or ampulla of Vater. The ampulla opens into the descending portion of the duodenum. The distal end of the common bile duct is controlled by the choledochal sphincter as it enters the duodenum. The hepatopancreatic ampulla is controlled by a circular muscle known as the sphincter of the hepatopancreatic ampulla, or sphincter of Oddi. During interdigestive periods, the sphincter remains in a contracted state, routing most of the bile into the gallbladder for concentration and temporary storage; during digestion, it relaxes to permit the bile to flow from the liver and gallbladder into the duodenum. The hepatopancreatic ampulla opens on an elevation on the duodenal mucosa known as the major duodenal papilla.


The gallbladder is a thin-walled, more or less pear-shaped, musculomembranous sac with a capacity of approximately 2 oz. The gallbladder concentrates bile by absorption of the water content; stores bile during interdigestive periods; and, by contraction of its musculature, evacuates the bile during digestion. The muscular contraction of the gallbladder is activated by a hormone called cholecystokinin. This hormone is secreted by the duodenal mucosa and released into the blood when fatty or acid chyme passes into the intestine. The gallbladder consists of a narrow neck that is continuous with the cystic duct; a body or main portion; and a fundus, which is its broad lower portion. The gallbladder is usually lodged in a fossa on the visceral (inferior) surface of the right lobe of the liver, where it lies in an oblique plane inferiorly and anteriorly. Measuring about 1 inch (2.5 cm) in width at its widest part and 3 to 4 inches (7.5 to 10 cm) long, the gallbladder extends from the lower right margin of the porta hepatis to a variable distance below the anterior border of the liver. The position of the gallbladder varies with body habitus, being high and well away from the midline in hypersthenic persons and low and near the spine in asthenic persons (Fig. 17-9). The gallbladder is sometimes embedded in the liver and frequently hangs free below the inferior margin of the liver.




Pancreas and Spleen


The pancreas is an elongated gland situated across the posterior abdominal wall. Extending from the duodenum to the spleen (Fig. 17-10; see Fig. 17-8), the pancreas is about 5½ inches (14 cm) long and consists of a head, neck, body, and tail. The head, which is the broadest portion of the organ, extends inferiorly and is enclosed within the curve of the duodenum at the level of the second or third lumbar vertebra. The body and tail of the pancreas pass transversely behind the stomach and in front of the left kidney, with the narrow tail terminating near the spleen. The pancreas cannot be seen on plain radiographic studies.



The pancreas is an exocrine and an endocrine gland. The exocrine cells of the pancreas are arranged in lobules with a highly ramified duct system. This exocrine portion of the gland produces pancreatic juice, which acts on proteins, fats, and carbohydrates. The endocrine portion of the gland consists of clusters of islet cells, or islets of Langerhans, which are randomly distributed throughout the pancreas. Each islet comprises clusters of cells surrounding small groups of capillaries. These cells produce the hormones insulin and glucagon, which are responsible for glucose metabolism. The islet cells do not communicate directly with the ducts but release their secretions directly into the blood through a rich capillary network.


The digestive juice secreted by the exocrine cells of the pancreas is conveyed into the pancreatic duct and from there into the duodenum. The pancreatic duct often unites with the common bile duct to form a single passage via the hepatopancreatic ampulla, which opens directly into the descending duodenum.


The spleen is included in this section only because of its location; it belongs to the lymphatic system. The spleen is a glandlike but ductless organ that produces lymphocytes and stores and removes dead or dying red blood cells. The spleen is more or less bean-shaped and measures about 5 inches (13 cm) long, 3 inches (7.6 cm) wide, and 1½ inches (3.8 cm) thick. Situated obliquely in the left upper quadrant, the spleen is just below the diaphragm and behind the stomach. It is in contact with the abdominal wall laterally, with the left suprarenal gland and left kidney medially, and with the left colic flexure of the colon inferiorly. The spleen is visualized with and without contrast media.






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Mar 3, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on DIGESTIVE SYSTEM: Alimentary Canal
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Condition Definition
Achalasia Failure of smooth muscle of alimentary canal to relax
Appendicitis Inflammation of the appendix
Barrett esophagus Peptic ulcer of lower esophagus, often with stricture
Bezoar Mass in the stomach formed by material that does not pass into the intestine
Biliary stenosis Narrowing of bile ducts
Carcinoma Malignant new growth composed of epithelial cells
Celiac disease or sprue Malabsorption disease caused by mucosal defect in the jejunum
Cholecystitis Acute or chronic inflammation of gallbladder
Choledocholithiasis Calculus in common bile duct
Cholelithiasis Presence of gallstones
Colitis Inflammation of the colon
Diverticulitis Inflammation of diverticula in the alimentary canal
Diverticulosis Diverticula in the colon without inflammation or symptoms
Diverticulum Pouch created by herniation of the mucous membrane through the muscular coat
Esophageal varices Enlarged tortuous veins of lower esophagus, resulting from portal hypertension
Gastritis Inflammation of lining of stomach
Gastroesophageal reflux Backward flow of stomach contents into the esophagus
Hiatal hernia Protrusion of the stomach through the esophageal hiatus of the diaphragm
Hirschsprung disease or congenital aganglionic megacolon Absence of parasympathetic ganglia, usually in the distal colon, resulting in the absence of peristalsis
Ileus Failure of bowel peristalsis
Inguinal hernia Protrusion of the bowel into the groin
Intussusception Prolapse of a portion of the bowel into the lumen of an adjacent part
Malabsorption syndrome Disorder in which subnormal absorption of dietary constituents occurs
Meckel diverticulum Diverticulum of the distal ileum, similar to the appendix
Pancreatitis Acute or chronic inflammation of the pancreas
Pancreatic pseudocyst Collection of debris, fluid, pancreatic enzymes, and blood as a complication of acute pancreatitis
Polyp Growth or mass protruding from a mucous membrane
Pyloric stenosis Narrowing of pyloric canal causing obstruction
Regional enteritis or Crohn disease Inflammatory bowel disease, most commonly involving the distal ileum
Ulcer Depressed lesion on the surface of the alimentary canal
Ulcerative colitis