SPMD 201 Digestive System
Esophogus
-The esophagus is the part of the digestive tract that extends between the pharynx and the stomach. It is about 25 cm long and lies in the mediastinum, anterior to the vertebrae and posterior to the trachea. It passes through the esophageal hiatus (opening) of the diaphragm and ends at the stomach. The esophagus transports food from the pharynx to the stomach. -The esophagus has thick walls consisting of the four tunics common to the digestive tract: mucosa, submucosa, muscularis, and adventitia. The muscular tunic has an outer longitudinal layer and an inner circular layer, as is true of most parts of the digestive tract, but it differs by having skeletal muscle in the superior part of the esophagus and smooth muscle in the inferior part
Gallbladder
-The gallbladder is a saclike structure on the inferior surface of the liver; it is about 8 cm long and 4 cm wide (see figure 24.18). Three tunics form the gallbladder wall: (1) an inner mucosa folded into rugae that allow the gallbladder to expand; (2) a muscularis, which is a layer of smooth muscle that allows the gallbladder to contract; and (3) an outer covering of serosa. The cystic duct connects the gallbladder to the common bile duct. -The liver continually secretes bile, which flows to the gallbladder, where 40-70 mL of bile can be stored. While the bile is in the gallbladder, water and electrolytes are absorbed, and bile salts and pigments become as much as 5-10 times more concentrated than they were when secreted by the liver. Shortly after a meal, the gallbladder contracts in response to stimulation by cholecystokinin and, to a lesser degree, in response to vagal stimulation, thereby dumping large amounts of concentrated bile into the small intestine
Mucosa and Submucosa
-The innermost tunic, the mucosa (mū-kō′să), or mucous membrane, consists of three layers: (1) the inner mucous epithelium, which is moist stratified squamous epithelium in the mouth, oropharynx, esophagus, and anal canal and simple columnar epithelium in the remainder of the digestive tract; (2) a loose connective tissue called the lamina propria (lam′i-nă prō′prē-ă); and (3) a thin outer layer of smooth muscle, the muscularis mucosae. -The epithelium extends deep into the lamina propria in many places to form intestinal glands and crypts. Two types of specialized cells in the mucosa are mechanoreceptors involved in peristaltic reflexes and chemoreceptors that detect the chemical composition of food. -Beneath the mucosa lies the submucosa, a thick connective tissue layer containing nerves, blood vessels, lymphatic vessels, and small glands. A network of nerve cells in the submucosa forms the submucosal plexus (plek′sŭs), or Meissner plexus, consisting of axons, many scattered neuron cell bodies, and neuroglial cells.
Muscularis & Serosa
-The next tunic is the muscularis, which consists of an inner layer of circular smooth muscle and an outer layer of longitudinal smooth muscle. Two exceptions are the upper esophagus, where the muscles are striated, and the stomach, which has three layers of smooth muscle. Another nerve plexus, the myenteric (mī-en-ter′ik) plexus, or Auerbach plexus, also consists of axons, many scattered neuron cell bodies, and neuroglial cells and is between these two muscle layers (figure 24.4). The myenteric plexus, which is much more extensive than the submucosal plexus, controls the motility of the intestinal tract -When the outer layer of the digestive tract is derived from adjacent connective tissue, the tunic is called the adventitia and consists of a connective tissue covering that blends with the surrounding connective tissue
Peritoneum part 2
Although mesentery is a general term referring to the serous membranes attached to the abdominal organs, it is also applied specifically to the mesentery associated with the small intestine, sometimes called the mesentery proper. The mesenteries of parts of the colon are the transverse mesocolon, which extends from the transverse colon to the posterior body wall, and the sigmoid mesocolon. The vermiform appendix has its own little mesentery, called the mesoappendix. -The mesentery connecting the lesser curvature of the stomach and the proximal end of the duodenum to the liver and diaphragm is called the lesser omentum (ō-men′tŭm; membrane of the bowels), and the mesentery extending as a fold from the greater curvature and then to the transverse colon is called the greater omentum.
Liver Histology
Hepatic cords radiate out from the central vein of each lobule like the spokes of a wheel. The hepatic cords are composed of hepatocytes, the functional cells of the liver. The spaces between the hepatic cords are blood channels called hepatic sinusoids. The sinusoids are lined with a very thin, irregular squamous endothelium consisting of two cell populations: (1) extremely thin, sparse endothelial cells and (2) hepatic phagocytic cells, Kupffer cells. A cleftlike lumen, the bile canaliculus (kan-ă-lik′ū-lŭs; little canal), lies between the cells within each cord
Stomach Part 2
The body narrows to form the funnel-shaped pyloric part of the stomach. The wider part of the funnel, toward the body of the stomach, is the pyloric antrum. The narrow part of the funnel is the pyloric canal. The pyloric canal opens through the pyloric orifice into the small intestine. The pyloric orifice is surrounded by the pyloric sphincter, or pylorus, a relatively thick ring of smooth muscle, which helps regulate the movement of gastric contents into the small intestine. Hypertrophic pyloric stenosis is a common defect of the stomach in infants, in which the pyloric sphincter is greatly thickened and thus interferes with normal stomach emptying. -The serosa, or visceral peritoneum, is the outermost tunic of the stomach. It consists of an outer layer of simple squamous epithelium and an inner layer of connective tissue. The muscularis of the stomach consists of three layers: an outer longitudinal layer, a middle circular layer, and an inner oblique layer (figure 24.11a). In some areas of the stomach, such as the fundus, the three layers blend with one another and cannot be separated. -Deep to the muscular layer are the submucosa and the mucosa, which are thrown into large folds called rugae (roo′gē; wrinkles) when the stomach is empty. These folds allow the mucosa and submucosa to stretch, and the folds disappear as the stomach volume increases as it is filled.
Large Intestine part 2
The colon (kō′lon), about 1.5-1.8 m long, consists of four parts: the ascending colon, transverse colon, descending colon, and sigmoid colon (figure 24.25). The ascending colon extends superiorly from the cecum and ends at the right colic flexure (hepatic flexure) near the right inferior margin of the liver. The transverse colon extends from the right colic flexure to the left colic flexure (splenic flexure), and the descending colon extends from the left colic flexure to the superior opening of the true pelvis, where it becomes the sigmoid colon. The sigmoid colon forms an S-shaped tube that extends into the pelvis and ends at the rectum.
Anatomy
The digestive system consists of the digestive tract, a tube extending from the mouth to the anus, and its associated accessory organs—primarily glands located outside the digestive tract that secrete fluids into it. The digestive tract is also called the alimentary tract, or alimentary canal. Technically, the term gastro-intestinal tract refers only to the stomach and intestines, but it is often used as a synonym for digestive tract.
Histology
The digestive tube consists of four major tunics, or layers: an internal mucosa and an external serosa, with a submucosa and a muscularis in between. These four tunics are present in all areas of the digestive tract, from the esophagus to the anus. Three major types of glands are associated with the intestinal tract: (1) unicellular mucous glands in the mucosa, (2) multicellular glands in the mucosa and submucosa, and (3) multicellular glands (accessory glands) outside the digestive tract.
Duodenum
The duodenum nearly completes a 180-degree arc as it curves within the abdominal cavity (figure 24.16a), and the head of the pancreas lies within this arc. The duodenum begins with a short, superior part, where it exits the pylorus of the stomach, and ends in a sharp bend, where it joins the jejunum. Within the duodenum, about two-thirds of the way down the descending part, are two small mounds: the major duodenal papilla and the minor duodenal papilla. Ducts from the liver and/or pancreas open at these papillae. -The surface of the duodenum has several modifications that increase its area about 600-fold to allow for more efficient digestion and absorption of food. The mucosa and submucosa form a series of folds called the circular folds, or plicae (plī′sē) circulares (figure 24.16a), which run perpendicular to the long axis of the digestive tract. Tiny, fingerlike projections of the mucosa form numerous villi (vil′ī), which are 0.5-1.5 mm in length -The mucosa of the duodenum is simple columnar epithelium with four major cell types: (1) Absorptive cells are cells with microvilli that produce digestive enzymes and absorb digested food; (2) goblet cells produce a protective mucus; (3) granular cells, or Paneth cells, may help protect the intestinal epithelium from bacteria; and (4) endocrine cells produce regulatory hormones.
Pancreas pt. 2
The exocrine part of the pancreas is a compound acinar gland (see chapter 4). The acini (as′i-nī; figure 24.22b) produce digestive enzymes. Clusters of acini form lobules that are separated by thin septa. Lobules are connected by small intercalated ducts to intra-lobular ducts, which leave the lobules to join interlobular ducts between the lobules. The interlobular ducts attach to the main pancreatic duct, which joins the common bile duct at the hepatopancreatic ampulla, or Vater's ampulla -The exocrine secretions of the pancreas, called pancreatic juice, have an aqueous component and an enzymatic component. Pancreatic juice is delivered to the small intestine through the pancreatic ducts, where it functions in digestion. The aqueous component is produced principally by columnar epithelial cells that line the smaller ducts of the pancreas. It contains Na+ and K+ in about the same concentration found in extracellular fluid. Bicarbonate ions (HCO3−) are a major part of the aqueous component, and they neutralize the acidic chyme that enters the small intestine from the stomach.
Jejenum and Ileum
The jejunum (jĕ-joo′nŭm) and ileum (il′ē-ŭm) are similar in structure to the duodenum (see figure 24.15). However, progressing from the duodenum through the ileum, there are gradual decreases in the diameter of the small intestine, the thickness of the intestinal wall, the number of circular folds, and the number of villi. The duodenum and jejunum are the major sites of nutrient absorption, although some absorption occurs in the ileum. Lymphatic nodules called Peyer patches are numerous in the mucosa and submucosa of the ileum. Peyer patches and other mucosa-associated lymphoid tissue in the digestive tract initiate immune responses against microorganisms that enter the mucosa from ingested food.
Large Intestine
The large intestine is the portion of the digestive tract extending from the ileocecal junction to the anus. It consists of the cecum, colon, rectum, and anal canal. Normally, 18-24 hours are required for material to pass through the large intestine, in contrast to the 3-5 hours required for chyme to move through the small intestine. Thus, the movements of the colon are more sluggish than those of the small intestine. While in the colon, chyme is converted to feces. The formation of feces involves the absorption of water and salts, secretion of mucus, and extensive action of microorganisms. -The cecum is the proximal end of the large intestine, where it meets the small intestine at the ileocecal junction. The cecum extends inferiorly about 6 cm past the ileocecal junction in the form of a blind sac (figure 24.25). Attached to the cecum is a smaller, blind tube about 9 cm long called the vermiform (ver′mi-fōrm; worm-shaped) appendix. The walls of the appendix contain many lymphatic nodules, which contribute to immune functions
Liver
The liver is the largest internal organ of the body, weighing about 1.36 kg (3 pounds). It is in the right-upper quadrant of the abdomen, tucked against the inferior surface of the diaphragm (figure 24.17; see figure 24.1). The liver consists of two major lobes, the right lobe and left lobe, which are separated by a connective tissue septum, the falciform ligament. Two minor lobes, the caudate lobe and the quadrate lobe, can be seen from an inferior view, along with the porta. -The porta (gate) is on the inferior surface of the liver, where the various vessels, ducts, and nerves enter and exit the liver. The hepatic (he-pat′ik) portal vein, the hepatic artery, and a small hepatic nerve plexus enter the liver through the porta (figure 24.17b). Lymphatic vessels and two hepatic ducts, one each from the right and left lobes, exit the liver at the porta. -A smooth muscle sphincter surrounds the common bile duct where it enters the hepatopancreatic ampulla. The gallbladder is a small sac on the inferior surface of the liver that stores bile. Bile can flow from the gallbladder through the cystic duct into the common bile duct, or it can flow back up the cystic duct into the gallbladder.
Pancreas
The pancreas is a complex organ composed of both endocrine and exocrine tissues that perform several functions. The pancreas consists of a head, located within the curvature of the duodenum (figure 24.22a), a body, and a tail, which extends to the spleen. -The endocrine part of the pancreas consists of pancreatic islets, or islets of Langerhans (figure 24.22b). The islet cells produce insulin and glucagon, which are very important in controlling the blood levels of nutrients, such as glucose and amino acids, and somatostatin, which regulates insulin and glucagon secretion and may inhibit growth hormone secretion
Pharynx
The pharynx, described in detail in chapter 23, consists of three parts: the nasopharynx, oropharynx, and laryngopharynx. Normally, only the oropharynx and laryngopharynx transmit food. The oropharynx communicates with the nasopharynx superiorly, with the larynx and laryngopharynx inferiorly, and with the mouth anteriorly. The laryngopharynx extends from the oropharynx to the esophagus and is posterior to the larynx. The epiglottis covers the opening of the larynx and keeps food and drink from entering the larynx. The posterior walls of the oropharynx and laryngopharynx consist of three muscles: the superior, middle, and inferior pharyngeal constrictors, which are arranged like three stacked flowerpots, one inside the other. The oropharynx and the laryngopharynx are lined with moist stratified squamous epithelium, and the nasopharynx is lined with ciliated pseudostratified columnar epithelium.
Large Intestine part 3
The rectum is a straight, muscular tube that begins at the termination of the sigmoid colon and ends at the anal canal (see figure 24.25). The mucosal lining of the rectum is simple columnar epithelium, and the muscular tunic is relatively thick, compared with the rest of the digestive tract. -The last 2-3 cm of the digestive tract is the anal canal (see figure 24.25). It begins at the inferior end of the rectum and ends at the anus (external digestive tract opening). The smooth muscle layer of the anal canal is even thicker than that of the rectum and forms the internal anal sphincter at its superior end. Skeletal muscle forms the external anal sphincter at the inferior end of the canal. The epithelium of the superior part of the anal canal is simple columnar, and that of the inferior part is stratified squamous. Rectal veins that supply the anal canal can become enlarged or inflamed, a condition known as hemorrhoids. Hemorrhoids cause pain, itching, and bleeding around the anus. They can usually be treated by changes in diet or medications.
Small Intestine
The small intestine consists of three parts: the duodenum, the jejunum, and the ileum. The entire small intestine is about 6 m long. The duodenum is about 25 cm long (duodenum means 12, suggesting that it is 12 inches long). The jejunum, constituting about two-fifths of the total length of the small intestine, is about 2.5 m long. The ileum, constituting three-fifths of the small intestine, is about 3.5 m long. Two major accessory glands, the liver and the pancreas, are associated with the duodenum
Stomach
The stomach is an enlarged segment of the digestive tract that primarily functions as a storage and mixing chamber. It is located in the left superior part of the abdomen. Its shape and size vary from person to person, even within the same individual from time to time, depending on food content and body posture. Nonetheless, several general anatomical features can be described. -The opening from the esophagus into the stomach is the gastroesophageal opening, or cardiac (located near the heart) opening, and the region of the stomach around the cardiac opening is the cardiac part -A part of the stomach to the left of the cardiac part, the fundus (fŭn′dŭs), is actually superior to the cardiac opening. The largest part of the stomach is the body, which turns to the right, creating a greater curvature and a lesser curvature.
Stomach Part 3
The stomach is lined with simple columnar epithelium. The epithelium forms numerous, tubelike gastric pits, which are the openings for the gastric glands (figure 24.11b). The epithelial cells of the stomach are of five types. The first type, surface mucous cells, are found on the surface and lining the gastric pit. -The remaining four cell types are in the gastric glands. They are mucous neck cells, which produce mucus; parietal cells, which produce hydrochloric acid and intrinsic factor; chief cells, which produce pepsinogen; and endocrine cells, which produce regulatory hormones and paracrine factors. The mucous neck cells are located near the openings of the glands, whereas the parietal, chief, and endocrine cells are interspersed in the deeper parts of the glands. There are several types of endocrine cells
Peritoneum
The walls and organs of the abdominal cavity are lined with serous membranes. These membranes are very smooth and secrete a serous fluid, which provides a lubricating film between the layers of membranes. The membranes and fluid reduce friction as organs move within the abdomen. The serous membrane that covers the organs is the visceral peritoneum (per′i-tō-nē′ūm; to stretch over), and the one that covers the interior surface of the wall of the abdominal cavity is the parietal peritoneum -Connective tissue sheets called mesenteries hold many of the organs in place within the abdominal cavity. The mesenteries consist of two layers of serous membranes with a thin layer of loose connective tissue between them. They provide a route by which vessels and nerves can pass from the abdominal wall to the organs. -Other abdominal organs lie against the abdominal wall, have no mesenteries, and are referred to as retroperitoneal (re′trō-per′i-tō-nē′ăl; behind the peritoneum; see chapter 1). The retroperitoneal organs include the duodenum, pancreas, ascending colon, descending colon, rectum, kidneys, adrenal glands, and urinary bladder.
