dynamic study module chapter 22 ( digestive) part 2
What are rugae?
Rugae are folds in the mucosa of the stomach. They increase the surface area of the stomach and allow the stomach expand considerably when it receives food and fluid without raising its internal pressure.
The acid pH of the gastric juice is caused by the secretion of hydrochloric acid from the _____.
The parietal cells secrete the hydrochloric acid (HCl) that is responsible for the acidic pH of gastric juice. Acid is an important component of gastric juice because it is required to activate a precursor enzyme called pepsinogen and also because it destroys most disease-causing organisms we ingest. In addition, parietal cells produce the chemical intrinsic factor, which is required for intestinal absorption of vitamin B12, found in various foods. The cells located near the top, or "neck," of the gland are called mucous neck cells. As their name implies, these cells secrete mucus much like the surface epithelial cells. However, these surface cells secrete alkaline mucus, whereas mucous neck cells secrete acidic mucus. This prevents their mucus from neutralizing the acid produced by other cells known as parietal cells. The chief cells secrete the inactive precursor enzyme pepsinogen. When pepsinogen encounters an acidic pH, it becomes the active enzyme pepsin, which begins protein digestion in the stomach. Enteroendocrine cells, or diffuse neuroendocrine system cells are endocrine cells found in many places through the body—hence the term diffuse—that secrete many of the same products as neurons. DNES cells are closest to the blood vessels in the underlying submucosa, which gives the hormones they produce ready access to the blood. DNES cells are found throughout the digestive system, and several different types are located within gastric glands.
The ____ are the smallest folds in the small intestine that create the appearance of a brush border.
The smallest folds, the microvilli, are found in the plasma membrane of the enterocytes. Each enterocyte has as many as 3000 microvilli, which gives the cell the appearance of a bristle brush, or brush border, on microscopic examination. Associated with the brush border are numerous digestive enzymes produced and secreted by enterocytes, such as sucrase, maltase, and lactase, which catalyze reactions that break down disaccharides, and peptidases, which catalyze reactions that break down peptides. The largest type of folds, which are visible with the naked eye as ridges in the wall, are called circular folds (or plicae circulares). Circular folds involve both the mucosa and submucosa of the small intestine. These folds not only increase surface area but also slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested, and the small intestine cells, called enterocytes (EN-tehr-oh-sytz), more time to absorb nutrients. The smaller two types of folds are not visible to the naked eye. The mucosa folds into projections called villi. Notice that each villus consists of a layer of enterocytes and occasional mucus-secreting goblet cells surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal. Between villi, the mucosa indents to form intestinal crypts, which house glands that also contain hormone-secreting DNES cells.
The main functions of the ____ are churning, peristalsis, storage, and chemical digestion with the enzyme pepsin.
The stomach performs three primary functions: secretion, propulsion, and digestion. Note that absorption of digested nutrients is not a main role of the stomach, but small amounts of chemicals such as alcohol may be absorbed here. The gastric glands of the stomach secrete many chemicals, including hormones, acid, mucus, and pepsinogen, which becomes the digestive enzyme pepsin. Pepsin catalyzes the beginning of the chemical digestion of proteins, and the stomach's churning actions begin mechanical digestion. When the stomach has completed churning, it propels the food, now chyme, through the pyloric sphincter into the duodenum. Secretion from the stomach, particularly of acid, and its motility are tightly regulated by the endocrine and nervous systems. The internal surface of the small intestine contains three progressively smaller types of folds. Most of the absorption that occurs within the alimentary canal takes place in the small intestine, and these folds increase the surface area available for absorption. The large intestine functions to absorb significant quantities of water, electrolytes, and vitamins. In addition to nutrients, the alimentary canal absorbs large quantities of water, electrolytes (such as sodium, potassium, and calcium ions), and vitamins. Much of this absorption occurs in the small intestine, but a significant quantity occurs in the large intestine as well. The liver has one of the most diverse sets of functions of any organ in the body. Like the pancreas, it releases both endocrine and exocrine secretions, and it converts harmful chemicals into nontoxic substances that the body can eliminate. The liver's main digestive function is to produce bile, a liquid that contains multiple components, including water, electrolytes, and organic compounds. Other functions of the liver include nutrient metabolism, detoxification, excretion, production of plasma proteins, and storage of nutrients.
The ____ is the terminal portion of the stomach.
The terminal portion of the stomach is the pylorus, which abuts the first portion of the small intestine, the duodenum. Like the cardia, the pylorus contains a sphincter that controls the flow of ingested food. In the pylorus, a thick ring of smooth muscle called the pyloric sphincter regulates the flow of materials between the stomach and the small intestine. The region where the esophagus empties into the stomach is the cardia. The cardia receives the bolus when the gastroesophageal sphincter relaxes. The dome-shaped top of the stomach is its fundus. The largest portion of the stomach is its body.
What can we consciously control about the defecation reflex?
We can consciously control the relaxation of the external anal sphincter during the defecation reflex. The terminal portion of the anal canal has two sphincters. The first is the involuntary internal anal sphincter, which is simply the thickened circular layer of the muscularis externa. The internal anal sphincter is supplied by parasympathetic motor neurons. The second is the voluntary external anal sphincter, which is composed of skeletal muscle. As this sphincter is voluntary, it is innervated by somatic motor neurons controlled by the cerebral cortex. Stretch receptors transmit the sensation of rectal distention to the spinal cord. The walls of the rectum house stretch receptors that are activated by the presence of fecal material stretching the wall. Sensory neurons transmit these impulses to the spinal cord. At the esophagus's inferior end is another sphincter, the gastroesophageal sphincter, also known as the lower esophageal sphincter, that regulates the passage of the bolus into the stomach. This sphincter also prevents the contents of the stomach from reentering the esophagus. When food or liquid is swallowed, the gastroesophageal sphincter and smooth muscle of the fundus and body of the stomach relax to allow the stomach to fill. This is known as receptive relaxation, and is mediated by both the medulla (as part of the swallowing reflex) and the vagus nerve.
Which of the following statements is incorrect?
Defecation involves the sympathetic nervous system is the incorrect statement. Parasympathetic neurons cause smooth muscle in the sigmoid colon and rectum to contract, and the internal anal sphincter to relax. In an infant or young child, relaxation of the internal anal sphincter results in release of fecal matter from the anal canal. In older children and adults, the defecation reflex ends here and there is conscious contraction of the external anal sphincter if it is not an appropriate time to defecate.
What region of the stomach does food first enter after its passage through the relaxed gastroesophageal sphincter?
Food first enters the cardia region of the stomach after its passage through the relaxed gastroesophageal sphincter. The esophagus pierces the diaphragm and passes through the esophageal hiatus to empty into the J-shaped organ known as the stomach. The dome-shaped top of the stomach is its fundus. The largest portion of the stomach is its body. The inferior portion of the stomach is the pyloric antrum. The terminal portion of the stomach is the pylorus, which abuts the first portion of the small intestine, the duodenum. Like the cardia, the pylorus contains a sphincter that controls the flow of ingested food. In the pylorus, a thick ring of smooth muscle called the pyloric sphincter regulates the flow of materials between the stomach and the small intestine.
Gastric pressure increases and stomach and intestinal contents are propelled in the opposite direction during ____.
Gastric pressure increases and stomach and intestinal contents are propelled in the opposite direction during vomiting. Normally, the stomach empties its contents into the duodenum. But occasionally stomach contents move backward, an unpleasant process known as vomiting, or emesis (EM-uh-sis). Vomiting involves a complex motor response mediated by several different groups of neurons in the brain. The first part of this motor response involves relaxation of the smooth muscle of the stomach and small intestine, which slows and then stops propulsion through the alimentary canal. Next, the smooth muscle begins retrograde contractions, which propel the contents of the stomach and intestine in the opposite direction. At the same time, the abdominal skeletal muscles and diaphragm muscle contract to increase intra-abdominal pressure; the upper and lower esophageal sphincters relax; the soft palate closes off the nasopharynx; and the larynx elevates so the epiglottis covers the glottis. The gastroesophageal sphincter normally remains closed except during swallowing. When this mechanism fails, acidic chyme from the stomach regurgitates into the esophagus. If this occurs on a chronic basis, it is called gastroesophageal reflux disease (GERD) and may lead to pain, difficulty swallowing, vocal cord damage, respiratory problems, and even esophageal cancer. Multiple factors contribute to the development of GERD, including increased acid secretion, obesity, caffeine consumption, a hiatal hernia (abnormal enlargement of the esophageal hiatus in the diaphragm muscle), and medications such as anti-inflammatory drugs. Conditions that decrease the amount of mucus secreted by these cells can lead to a gastric ulcer (also called a peptic ulcer), in which acid eats away at the mucosa and exposes the underlying tissues. Ulcers are often associated with the presence of certain bacteria or excessive production of acid.
What is released by the enteroendocrine cells of the gastric glands?
Gastrin is released by the enteroendocrine cells of the gastric glands. Enteroendocrine cells called G cells secrete gastrin, which stimulates acid secretions from the parietal cells. Chief cells secrete the inactive precursor enzyme pepsinogen. When pepsinogen encounters an acidic pH, it becomes the active enzyme pepsin, which begins protein digestion in the stomach. The parietal cells secrete the hydrochloric acid (HCl) that is responsible for the acidic pH of gastric juice. Acid is an important component of gastric juice because it is required to activate a precursor enzyme called pepsinogen and also because it destroys most disease-causing organisms we ingest. In addition, parietal cells produce the chemical intrinsic factor, which is required for intestinal absorption of vitamin B12, found in various foods.
Which of the following features does not increase surface area in the small intestine?
Rugae do not increase surface area in the small intestine. The interior of the stomach contains folds called rugae. Rugae allows the stomach to expand considerably. The internal surface of the small intestine contains three progressively smaller types of folds. Most of the absorption that occurs within the alimentary canal takes place in the small intestine, and these folds increase the surface area available for absorption about 400 to 600 times—another example of the Structure-Function Core Principle. The largest type of folds, which are visible with the naked eye as ridges in the wall, are called circular folds (or plicae circulares). Circular folds involve both the mucosa and submucosa of the small intestine. These folds not only increase surface area but also slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested. The smaller two types of folds are not visible to the naked eye. The mucosa folds into projections called villi. Notice that each villus consists of a layer of enterocytes and occasional mucus-secreting goblet cells surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal. The smallest folds, the microvilli, are found in the plasma membrane of the enterocytes. Each enterocyte has as many as 3000 microvilli, which gives the cell the appearance of a bristle brush, or brush border, on microscopic examination.
Which process is a squeezing motion of the circular layer of smooth muscle in the small intestine?
Segmentation is a squeezing motion of the circular layer of smooth muscle in the small intestine. Segmentation, also known as intestinal churning, involves contractions of only the circular layer of smooth muscle, which produces a squeezing motion. The primary functions of segmentation are mechanical digestion and mixing the chyme with intestinal and pancreatic enzymes as well as bile. As with other regions of the alimentary canal, peristalsis in the small intestine is accomplished by alternating contractions of the longitudinal and circular layers of smooth muscle in the muscularis externa. Its primary function is to propel chyme toward the ileum and ultimately through the ileocecal valve to the cecum. Certain ingested materials are not digestible or usable by the body. Such materials continue their transit through the alimentary canal until they exit the body as feces through defecation. Defecation also provides the body with a way to eliminate certain metabolic wastes. Note that defecation is simply a specialized form of propulsion. The process of swallowing, or deglutition, is a specialized type of propulsion that pushes a bolus of food from the oral cavity through the pharynx and esophagus to the stomach. Swallowing relies on the coordinated action of the upper alimentary canal, including the soft palate, pharynx, and esophagus. The tongue also plays a role in this process, and is the only accessory organ to directly participate in motility. Swallowing consists of three phases: voluntary, pharyngeal, and esophageal.
The _____ of the small intestine facilitate its role in digestion and absorption.
The circular folds of the small intestine facilitate its role in digestion and absorption. Most of the absorption that occurs within the alimentary canal takes place in the small intestine, and these folds increase the surface area available for absorption. The largest type of folds, which are visible with the naked eye as ridges in the wall, are called circular folds (or plicae circulares). Circular folds involve both the mucosa and submucosa of the small intestine. These folds not only increase surface area but also slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested, and the small intestine cells, called enterocytes, more time to absorb nutrients. Rugae are folds in the mucosa of the stomach. They increase the surface area of the stomach and allow the stomach expand considerably when it receives food and fluid without raising its internal pressure. The muscularis externa of the large intestine is unique in that its longitudinal layer is not continuous throughout most of its length. Instead, this layer is gathered into three bands or ribbons of muscle called taeniae coli. Their constant tension bunches the colon into pockets referred to as haustra.
Place the structures of the large intestine listed in their correct sequence, starting at the cecum and ending at the rectum. right colic flexure sigmoid colon ascending colon transverse colon left colic flexure descending colon
The correct sequence for the structures of the large intestine are: 3 (ascending colon), 1 (right colic flexure or hepatic flexure), 4 (transverse colon), 5 (left colic flexure or splenic flexure), 6 (descending colon), and 2 (sigmoid colon).
Which section of the large intestine begins at the splenic flexure?
The descending colon begins at the splenic flexure. At the spleen, the transverse colon takes a sharp turn inferiorly at a junction called the splenic flexure, also known as the left colic flexure. The splenic flexure gives rise to the retroperitoneal descending colon, which passes along the left side of the abdominal cavity. The retroperitoneal ascending colon travels superiorly along the right side of the abdomen from the right lower quadrant to the right upper quadrant. When it reaches the liver, it makes a sharp left-hand turn at a junction called the hepatic flexure, also known as the right colic flexure. At the hepatic flexure, the ascending colon becomes the intraperitoneal transverse colon, so named because it passes transversely across the superior abdominal cavity. In the left lower quadrant, the descending colon becomes the S-shaped sigmoid colon, which is intraperitoneal and passes toward the sacrum.
Which segment of the small intestine receives chyme from the stomach?
The duodenum is the segment of the small intestine that receives chyme from the stomach. The duodenum is the initial segment of the small intestine that begins at the pylorus. When the stomach has completed churning, it propels the food, now chyme, through the pyloric sphincter into the duodenum. The small intestine consists of three divisions—the duodenum, jejunum, and ileum. The jejunum is the middle portion of the small intestine, measures about 2.5 meters (7.5 feet) in length. It begins at the duodenojejunal flexure and sits within the peritoneal cavity. It is the most active site for chemical digestion and absorption. The ileum is the small intestine's final segment, it is also intraperitoneal. About 3.6 meters (10.8 feet) in length, it terminates at the portion of the large intestine called the cecum. A sphincter known as the ileocecal valve controls the movement of materials from the ileum into the cecum. This sphincter also prevents materials in the large intestine from flowing backward into the ileum. This function is key because, as we discuss shortly, the large intestine houses a great number of bacteria that could cause illness if they entered the ileum.
The final process to occur in the alimentary canal is __________.
The final process to occur in the alimentary canal is defecation. Certain ingested materials are not digestible or usable by the body. Such materials continue their transit through the alimentary canal until they exit the body as feces through defecation. Defecation also provides the body with a way to eliminate certain metabolic wastes. Note that defecation is simply a specialized form of propulsion. Once food particles are mechanically and chemically digested, nutrients move through the wall of the alimentary canal into blood or lymphatic vessels by a process called absorption. Water, electrolytes, and vitamins are also absorbed into the blood in the same manner. Digestive organs contain both endocrine and exocrine glands that secrete a variety of substances—such as mucus, enzymes, acid, and hormones—to aid other digestive processes. Segmentation, also known as intestinal churning, involves contractions of only the circular layer of smooth muscle, which produces a squeezing motion. The primary functions of segmentation are mechanical digestion and mixing the chyme with intestinal and pancreatic enzymes as well as bile.
The smell, sight, or even thought of food can activate the ____ of gastric acid secretion in the stomach.
The initial cephalic phase is mediated by the sight, smell, taste, or even thought of food. This phase, which is directed by the central nervous system (CNS), prepares the stomach to receive food by increasing the release of hydrogen ions into it. These stimuli trigger output from the vagus nerve, the main nerve of the parasympathetic nervous system, resulting in four physiological effects. The gastric phase begins when food enters the stomach and continues the stimulation provided during the cephalic phase. There are two stimuli that trigger acid secretion during the gastric phase. The first is the simple presence of food in the stomach. Distention of the stomach wall stimulates neurons of the enteric nervous system (ENS) and sensory receptors involved with vagus nerve reflexes. The second stimulus is the presence of partially digested proteins in gastric juice. These protein fragments stimulate G cells to produce and release gastrin, which in turn stimulates acid secretion. The final phase of gastric acid secretion, the intestinal phase, is responsible for the remaining approximately 10% of acid secretion, after which further acid secretion is inhibited. The intestinal phase is triggered by the presence of partially digested proteins in the fluid entering the duodenum. As in the stomach, these partially digested proteins trigger duodenal DNES cells to release intestinal gastrin. This hormone has the same effect as gastrin produced by the stomach and stimulates hydrogen ion secretion from parietal cells. The voluntary phase occurs during swallowing. During the voluntary phase, the tongue pushes the bolus superiorly against the hard palate and posteriorly toward the oropharynx. The voluntary phase is the only stage of swallowing under conscious control.
The most active site for chemical digestion and absorption is the ____.
The jejunum, the middle portion of the small intestine, measures about 2.5 meters (7.5 feet) in length. It begins at the duodenojejunal flexure and sits within the peritoneal cavity. It is the most active site for chemical digestion and absorption. The duodenum, the initial segment of the small intestine, begins at the pylorus. The shortest of the three divisions, it is only about 25 cm (10 in.) long. The duodenum arches into a "C" shape as it curves around the pancreas. Internally, the duodenum houses the major duodenal papilla, which is where secretions from the gallbladder and pancreas enter the small intestine. The duodenal submucosa contains specialized glands called duodenal (Brunner's) glands, which produce an alkaline mucus to protect the duodenum from the acidic chyme. The large intestine functions to absorb significant quantities of water, electrolytes, and vitamins. In addition to nutrients, the alimentary canal absorbs large quantities of water, electrolytes (such as sodium, potassium, and calcium ions), and vitamins. Much of this absorption occurs in the small intestine, but a significant quantity occurs in the large intestine as well. The stomach performs three primary functions: secretion, propulsion, and digestion. Note that absorption of digested nutrients is not a main role of the stomach, but small amounts of chemicals such as alcohol may be absorbed here.
The longitudinal ribbons of smooth muscle visible on the outer surfaces of the colon just beneath the serosa are the _____.
The longitudinal ribbons of smooth muscle visible on the outer surfaces of the colon just beneath the serosa are the taeniae coli. The muscularis externa of the large intestine is unique in that its longitudinal layer is not continuous throughout most of its length. Their constant tension bunches the colon into pockets referred to as haustra. The serosa, or visceral peritoneum, of the large intestine contains fat-filled pouches known as epiploic appendages (epiploic = "membrane-covered"). Their function is unknown. The cecum features a smaller blind-ended pouch extended from its posteroinferior end, called the vermiform appendix, which is generally shortened to simply appendix. The appendix was long considered a vestigial organ or one left over from evolution that served no function. However, it is now understood that it houses multiple lymphatic nodules and plays a role in the immune system.
