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McKinley/O’Loughlin Human Anatomy, 2nd Edition CHAPTER 26 Answers to “What Did You Learn?” 1. The GI tract includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine. 2. Secretion is the process of producing and releasing fluid products, such as acid, bile, digestive enzymes, and mucus, in order to facilitate chemical digestion or passage of material through the GI tract lumen. Absorption involves either the passive movement or active transport of electrolytes, products of digestion, vitamins, and water, through the GI tract epithelium and into the lymph vessels and blood vessels. 3. The components of saliva are water (moisturizes food and cleanes mouth), ions, immunoglobulin A (a class of secreted antibodies that inhibit bacterial growth), lysozyme (an antibacterial enzyme), mucin, and salivary amylase (initiates chemical digestion of carbohydrate in the mouth). 4. Permanent teeth include the incisors, canines, premolars, and molars. The incisors are designed for slicing or cutting into food. Canines have pointed tips for puncturing and tearing food. Premolars have flat crowns with cusps that are used to crush and grind ingested materials. Molars have large, broad flat crowns with distinctive cusps, and are also used for grinding and crushing. 5. Sequential contraction of the pharyngeal constrictors decreases the diameter of the pharynx beginning at its superior end and moving toward its inferior end, thus forcing swallowed material toward the esophagus. McKinley/O’Loughlin 6. Human Anatomy, 2nd Edition Intraperitoneal refers to organs that are completely surrounded by visceral peritoneum. They include the stomach, part of the duodenum of the small intestine, the jejunum and the ileum of the small intestine, the cecum, appendix and transverse and sigmoid colon of the large intestine. Retroperitoneal refers to organs that typically lie directly against the posterior abdominal wall, so only their anterolateral portions are covered with peritoneum. Retroperitoneal organs include most of the duodenum, the pancreas, and the ascending and descending colon of the large intestine. 7. The four main tunics and their default patterns are a mucosa (typically lined with simple columnar epithelium), submucosa (formed from areolar or dense irregular connective tissue), muscularis (typically formed from two layers of smooth muscle), and an adventitia or serosa. 8. The esophageal tunics differ from the default tunic pattern in that the mucosa has a stratified squamous epithelium and the muscularis has a mixture of skeletal and smooth muscle. 9. The three phases of swallowing are the voluntary phase, the pharyngeal phase, and the esophageal phase. The voluntary phase occurs after the ingestion of food. Food mixed with saliva forms a bolus that is pushed into the archway leading into the oropharynx. The appearance of the food bolus at the oropharynx initiates the pharyngeal phase. The bolus passes quickly and involuntarily through the pharynx to the esophagus. The pharyngeal phase includes the soft palate elevation to block the nasopharynx, the reception of the food bolus into the oropharynx, and the movement of the larynx toward the epiglottis. The McKinley/O’Loughlin Human Anatomy, 2nd Edition esophageal phase (involuntary stage) moves the bolus through the esophagus and into the stomach. 10. The four regions of the stomach are: the cardia, fundus, body, and pylorus. The cardia is a small, narrow, superior entryway into the stomach lumen from the esophagus. The fundus is the dome-shaped region lateral and superior to the esophageal connection with the stomach. Its superior surface contacts the diaphragm. The body is the largest region of the stomach; it is inferior to the cardiac orifice and the fundus. The pylorus is a narrow, medially directed, funnelshaped region that forms the terminal part of the stomach. 11. The five types of epithelial cells in the stomach are surface mucous cells (secrete mucin) , mucous neck cells (secrete alkaline mucin), parietal cells (secrete hydrochloric acid and intrinsic factor) , chief cells (secrete pepsinogen), and enteroendocrine cells (secrete gastrin). 12. The duodenum forms the first region of the small intestine. It is mostly retroperitoneal and is approximately 10 inches long. It is arched into a C-shape around the head of the pancreas. The duodenum begins at the pyloric sphincter and ends at the duodenojejunal flexure where it connects with the jejunum. The jejunum is the middle region of the small intestine and forms approximately 2/5 the length of the small intestine. The ileum is the last region of the small intestine and forms approximately 3/5 the length of the small intestine. It extends from its origin at the end of the jejunum and its distal end terminates at the ileocecal valve. The jejunum and ileum are intraperitoneal and suspended in the abdomen by a mesentery. Circular folds are mucosal and submucosal folds in all three regions of McKinley/O’Loughlin Human Anatomy, 2nd Edition the small intestine, but are best developed in the duodenum and jejunum. Additionally, villi are larger and more numerous in the jejunum. Peyer's patches are much more numerous in the ileum than in the jejunum. 13. The ascending colon, descending colon, rectum, and anal canal are retroperitoneal. The cecum, vermiform appendix, transverse colon, and sigmoid colon are intraperitoneal organs. 14. The movements and reflexes that propel material through the large intestine are peristaltic movements, haustral churning, and mass movement. 15. Portal triads are composed of a branch of the hepatic portal vein, hepatic artery, and the bile duct. 16. The gallbladder stores and concentrates the bile produced by the liver. The gallbladder will release this concentrated bile in response to ingesting a fatty meal. 17. Pancreatic acini are cells that secrete pancreatic juice, which is an alkaline fluid that also contains digestive enzymes. 18. A decrease in mucin secretion results in a decrease in thickness and volume of mucus that coat and protect the luminal lining of GI tract organs. Without this protection the organ lining will be subject to abrasion by a moving bolus or chyme and damage by acid or enzymes. 19. The liver parenchyma, gallbladder, pancreas and biliary apparatus develop from buds or outgrowths from the endoderm of the duodenum. Answers to “Content Review” McKinley/O’Loughlin 1. Human Anatomy, 2nd Edition The oral cavity initiates the process of both mechanical and chemical digestion. The amylase in saliva begins the mechanical digestion of carbohydrates. Saliva moistens the food, while the teeth break apart the food into smaller components. The tongue helps mix the saliva and food, and pushes the material against the palate of the mouth, where it is transformed into a bolus. A bolus is eventually swallowed. 2. The general structural plan of the GI tract from the esophagus through the large intestine is a tube composed of four concentric layers, called tunics. From innermost to outermost are the tunica mucosa, submucosa, muscularis, and serosa (or adventitia). The mucosa is composed of an epithelium (typically simple columnar), a lamina propria, and a muscularis mucosae. The submucosa is formed from areolar or dense irregular connective tissue, while the muscularis typically contains two smooth muscle layers (an inner circular and outer longitudinal). The adventitia is areolar connective tissue, while serosa also has a layer of peritoneum covering this connective tissue. The stomach’s mucosa is lined with simple columnar epithelium, but no goblet cells. The mucosa also has gastric pits which are lined by gastric glands. The muscularis layer has three layers of smooth muscle instead of the usual two layers. 3. Gastric juice and pancreatic juice both assist with the chemical digestion of food. Gastric juice is formed by the gastric glands of the stomach. It is highly acidic and contains hydrochloric acid, pepsinogen, mucin, gastrin and intrinsic factor (the latter assists in vitamin B12 absorption). Pancreatic juice, in contast, is alkaline due to the high percentage of bicarbonate. The bicarbonate acts to McKinley/O’Loughlin Human Anatomy, 2nd Edition neutralize the acidic chyme that enters the duodenum from the stomach. Other components of pancreatic juice include mucin and digestive enzymes. 4. Internally, the mucosal and submucosal tunics of the small intestine are thrown into circular folds. They help increase the surface area of the small intestine, allowing a greater opportunity for nutrients to be absorbed, and slow down movement of chyme to ensure that this material remains within the small intestine for maximal nutrient absorption. Along these circular folds are smaller fingerlike projections of mucosa only, called villi, that act to increase surface area for secretion and absorption. Microvilli are apical membrane surface folds to increase surface area at the absorptive and secretory surface of each cell. 5. The teniae coli in the wall of the large intestine helps form sacs called haustra. Movement of digested materials through these sacs is called haustral churning. It occurs after a relaxed haustrum fills with digested/fecal material to a point where its distension stimulates reflex contractions in the muscularis causing a churning and movement of the material to more distal hasutra. 6. At the periphery of hepatic lobules, there are several portal triads, composed of a least a branch of the hepatic portal vein, hepatic artery, and the bile duct. The hepatic portal vein carries blood that has already passed through the capillary beds of the GI tract, spleen, and pancreas. This blood is rich in nutrients and other absorbed substances but relatively poor in oxygen. The hepatic artery, a branch of the celiac trunk, carries well-oxygenated blood and supplies the remaining blood to the liver. Blood from branches of these two vessels mixes in passing to and through the liver lobules and drained into the central veins. Bile produced by McKinley/O’Loughlin Human Anatomy, 2nd Edition hepatocytes is secreted into bile canaliculi: it flows through these tiny channels to the portal triad where it enters the bile ducts and eventually exits the liver. Bile emulsifies fat arriving in the small intestine. Without this emulsification, the fat in our consumed food could not be chemically digested. 7. Attached to the inferior surface of the liver, a sac-like organ, called the gallbladder, stores and concentrates bile produced by the liver. Bile is a yellowgreen fluid produced by hepatocytes; its primary digestive function is to aid in the emulsification of fat. 8. Ingested material enters the GI tract via the oral cavity. The material is mechanically digested by the teeth and tongue, while chemical digestion of carbohydrates occurs from the amylase in saliva. The ingested material is now called a bolus. When the bolus is swallowed, it leaves the oral cavity and enters the oropharynx, laryngopharynx, and esophagus. The bolus is then propelled into the stomach, where both mechanical and chemical ingestion of the material occur. The paste-like material that leaves the stomach is called chyme. Chyme leaves the stomach and enters the small intestine, where mechanical and chemical digestion will be completed. The chyme first enters the duodenum, where it will mix with bile and pancreatic juices secreted via the major duodenal papilla. Then the material travels through the jejunum and ileum, and then enters the large intestine, where the material is referred to as feces, or fecal matter. Material first enters the cecum of the large intestine, and then travels into the ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal. Feces leaves the body through the anus. McKinley/O’Loughlin Human Anatomy, 2nd Edition 9. The mucin-producing cells throughout the GI tract collectively produce a covering layer of viscous mucus. This mucus lubricates the lining and prevents desiccation of the lining cells. Additionally the mucus protects the epithelial lining from abrasion, or damage as a result of exposure to acid or enzymes in the contents of the tract. 10. Most of the small intestine and the proximal part of the large intestine are formed from a section of the embryologic gut tube called the midgut. During the fifth week of development, the midgut elongates and forms a primary intestinal loop. The upper portion of the loop is called the cranial loop, while the lower part is called the caudal loop. The cranial loop will form most of the small intestine, while the caudal loop will form the proximal part of the large intestine. Beginning the sixth week, the intestinal loop grows and herniates into the umbilicus (due to space constraints in the abdominal cavity). As it herniates, it undergoes a 90 degree counterclockwise rotation, as viewed from the front of the embryo. The loop remains herniated until week 10, when the abdominal cavity has grown spacious enough to house all of the intestines. During weeks 10-11, the intestinal loop retracts into the body and in so doing, it rotates another 180 degrees counterclockwise. Now the cranial limb is on the left side of the body and the caudal limb is on the right side of the body. These limbs will attain their postnatal position and develop into most of the small and large intestine.