Chapter 28: Structure and Function of the Gastrointestinal System Porth

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Chapter 28: Structure and Function of the Gastrointestinal System Porth - Essentials of Pathophysiology

The major physiologic function of the _____ is to digest food and absorb nutrients into the bloodstream.

gastrointestinal (GI) system

The upper esophageal sphincter, the ____ sphincter, consists of a circular layer of striated muscle.

pharyngoesophageal

The lower esophageal sphincter, the ____ sphincter, lies just above the area where the esophagus joins the stomach.

gastroesophageal

The ____ lies in the left side of the abdomen and serves as a food storage reservoir during the early stages of digestion.

stomach

The small intestine, which forms the middle portion of the digestive tract, consists of three subdivisions: the _____, _____, and _____.

duodenum, jejunum, ileum

Blue and pancreatic juices enter the intestine through openings for the common bile duct and the main pancreatic duct in the _____.

jejunum

The _____ cells carry out the secretory and absorptive functions of the GI tract and they produce the ____ that lubricates and protects the inner surface of the alimentary canal.

epithelial, mucus

____ fluid forms a moist and slippery surface that prevents friction between the continuously moving abdominal structures.

Serous

The ____ contains the blood vessels, nerves, and lymphatic vessels that supply the interstitial wall.

mesentery

Like the self-excitable cardiac muscle cells in the heart, some smooth muscle cells of the GI tract function as _____ cells.

pacemaker

The ____ nervous system consists of the myenteric and submucosal plexuses in the wall of the GI tract.

enteric

____ monitor the stretch and distention of the GI tract wall, and ____ monitor the chemical composition of its contents.

Mechanoreceptors, chemoreceptors

Numerous ____ reflexes influence motility and secretions of the digestive tract.

vagovagal

Swallowing consists of three phases: an _____ phase, a _____ phase, and an _____ phase.

oral, pharyngeal, esophageal

The ____ is the major site for the digestion and absorption of food.

small intestine

____ normally is initiated by the mass movements of the large intestine.

Defecation

The GI tract produces _____ that act locally, pass into the general circulation for distribution to more distant sites, and interact with the central nervous system by way of the enteric and autonomic nervous systems.

hormones

The primary function of ____ is the stimulation of gastric acid secretion.

gastrin

____ has potent growth hormone-releasing activity and has a stimulatory effect on food intake and digestive function, while reducing energy expenditure.

Ghrelin

____ potentiates the action of secretin, increasing the pancreatic bicarbonate response to low circulating levels of secretin, stimulates biliary secretion of fluid and bicarbonate, and regulates gallbladder contraction and gastric emptying.

Cholecystokinin

The ____ cells secrete hydrochloric acid and intrinsic factor, which is necessary for the absorption of _____.

parietal, vitamin B12

The chief cells secrete _____, an enzyme that initiates proteolysis or breakdown of proteins.

pepsinogen

G cells secrete _____.

gastrin

____ secrete large amounts of alkaline mucus that protect the duodenum from the acid content in the gastric chyme and from the action of the digestive enzymes.

Brunner glands

The stomach and small intestine contain only a few species of _____, probably because of the composition of luminal contents.

bacteria

The major metabolic function of colonic microflora is the fermentation of ____ and endogenous mucus produced by the epithelial cells.

indigestible dietary residue

____ is the process of dismantling foods into their constituent parts.

Digestion

_____ is the process of moving nutrients and other materials from the external environment of the GI tract into the internal environment.

Absorption

Each villus is covered with cells called _____ that contribute to the absorptive and digestive functions of the small bowel, and goblet cells that provide mucus.

enterocytes

The enterocytes secrete ____ that adhere to the border of the villus structures.

brush border enzymes

Triglycerides are broken down by pancreatic _____.

lipase

_____ represents a loss of appetite.

Anorexia

_____ is the conscious sensation resulting from stimulation of the medullary vomiting center that often precedes or accompanies vomiting.

Nausea

____ is the sudden and forceful oral expulsion of the contents of the stomach.

Vomiting

Define amylase

Breaks down starch

Define mastication

Result of chemical breakdown of proteins in stomach

Define mesentery

Blood vessels, nerves, and lymphatic vessels that supply the intestinal wall

Define interstitial cells of Cajal

Generate slow waves of electrical activity

Define peritoneum

Chewing of food

Define submucosal plexus

Controls function of each segment of intestinal tract

Define haustrations

Segmental mixing movements of the large intestine

Define chyme

Result of chemical breakdown of proteins in stomach

Define myenteric plexus

Responsible for motility along the length of the gut

Define secretin

Inhibits gastric acid secretion

Describe the functional divisions of the GI tract.

The upper part - the mouth, esophagus, and the stomach - act as an intake source and receptacle through which food passes and in which initial digestive processes take place. The middle portion - the duodenum, jejunum, and ileum - the place where most digestive and absorptive processes occur. The lower segment - the cecum, colon, and rectum, serves as a storage channel for the efficient elimination of waste.

What factors are involved in stimulating the emptying of the stomach?

The emptying of the stomach is regulated by hormonal and neural mechanisms. The hormones cholecystokinin and glucose-dependent insulinotropic polypeptide are thought to partly control gastric emptying, which are released in response to the pH and the osmolar and fatty acid composition of the chyme. Afferent receptor fibers synapse with the neurons in the intramural plexus or trigger intrinsic reflexes by means of vagal or sympathetic pathways that participate in extrinsic reflexes.

Describe the two types of contractions seen in the small intestine.

With segmentation waves, slow contractions of the circular muscle layer occlude the lumen and drive the contents forward and backward. Most of the contraction that produce segmentation waves are local events involving only 1 to 4 cm of intestine at a time. They function mainly to mix the chyme with the digestive enzymes from the pancreas and to ensure adequate exposure of all parts of the chyme to the mucosal surface of the intestine, where absorption takes place. Peristaltic movements are rhythmic propulsive movements designed to propel the chyme along the small intestine toward the large intestine.

Describe the incretin effect.

The incretin effect is the increase in insulin release after an oral glucose load. The two hormones that account for about 90% of the incretin effect are GLP-1, which is released from L cells in the distal small bowel, and GIP, which is released by K cells in the upper gut (mainly the jejunum). Because increased levels of GLP-1 and GIP can lower blood glucose levels by augmenting insulin release in a glucose-dependent manner (i.e. at low blood glucose levels no further insulin is secreted, minimizing the risk of hypoglycemia), these hormones have been targeted as possible antidiabetic drugs. Moreover, GLP-1 can exert other metabolically beneficial effects, including suppression of glucagon release, slowing of gastric emptying, augmenting of net glucose clearance, and decreasing appetite and body weight.

What are the three functions of saliva?

The first function of saliva is protection and lubrication. Saliva is rich in mucus, which protects the oral mucosa and coats the food as it passes through the mouth, pharynx, and esophagus. The sublingual and buccal glands produce only mucus-type secretions. The second function of saliva is its protective antimicrobial action. The saliva cleans the mouth and contains the enzyme lysozyme, which has an antibacterial action. Third, saliva contains ptyalin and amylase, which initiate the digestion of dietary starches.

What is the mechanism of acid secretion by parietal cells of the stomach?

The cellular metabolism for hydrochloric acid secretion by the parietal cells in the stomach involves the hydrogen/potassium adenosine triphosphatase transporter and chloride channels located in their luminal membrane. During the process of hydrochloric acid secretion, carbon dioxide produced by aerobic metabolism combines with water, catalyzed by carbonic anhydrase, to form carbonic acid, which dissociates into hydrogen and bicarbonate. The hydrogen is secreted with chloride into the stomach and the biocarbonate moves out of the cell and into blood from the basolateral membrane. At the luminal side of the membrane, hydrogen is secreted into the stomach via the hydrogen/potassium adenosine triphosphatase transporter and chloride follows hydrogen into the stomach by diffusing through chloride channels in the luminal membrane.

How are carbohydrates broken down to absorbable units?

Digestion of starch begins in the mouth with the action of amylase. Pancreatic secretions also contain an amylase. Amylase breaks down starch into several disaccharides, including maltose, isomaltose, and alpha-dextrins. The brush border enzymes convert the disaccharides into monosaccharides that can be absorbed.

Describe protein digestion and absorption.

Protein digestion begins the stomach with the action of pepsin. Proteins are broken down further by pancreatic enzymes, such as trypsin, chymotrypsin, carboxypeptidase, and elastase. The pancreatic enzymes are secreted as precursor molecules. Trypsinogen, which lacks enzymatic activity, is activated by an enzyme located on the brush border cells of the duodenal enterocytes. Activated trypsin activates additional trypsinogen molecules and other pancreatic precursor proteolytic enzymes. The amino acids are liberated on the surface of the mucosal surface of the intestine by brush border enzymes that degrade proteins into peptides that are one, two, or three amino acids long. Similar to glucose, many amino acids are transported across the mucosal membrane in a sodium-linked process that uses ATP as an energy source. Some amino acids are absorbed by facilitated diffusion processes that do not require sodium.

The nurse is preparing an educational event for a group of children in elementary school who are studying the gastrointestinal tract. What facts should the nurse know to include for these children?

The gastrointestinal tract is the largest endocrine gland in the body. Many nerves make the GI tract work. The stomach begins digestion by kneading and churning the food we eat. Food then progresses to the small intestine, where most of the food is digested and absorbed. Our food then goes into the large intestine, where it is compacted into the feces that we expel from our bodies.

The circular layer of smooth muscle that lies between the stomach and the small intestine is called what?

Pyloric sphincter. At the end of the pyloric channel, the circular layer smooth muscle thickens to form the pyloric sphincter. This muscle serves as a valve that controls the rate of stomach emptying and prevents the regurgitation of intestinal contents back into the stomach. There is no cardiac sphincter in the GI tract. The antrum is a portion of the stomach that is the wider, upper portion of the pyloric region. The cardiac orifice is the opening between the esophagus and the stomach.

Where in the gastrointestinal tract is food digested and absorbed?

It is in the jejunum and ileum that food is digested and absorbed.

Some smooth muscle cells in the gastrointestinal tract serve as pacemakers. They display rhythmic and spontaneous oscillations in membrane potentials. What are these called?

Slow waves. No contraction can occur without an action potential and an action potential cannot occur unless the slow wave brings the membrane potential to threshold.

Defecation is controlled by both an internal and an external sphincter. What nerve controls the external sphincter?

Pudendal nerve. The external sphincter is controlled by nerve fibers in the pudendal nerve, which is part of the somatic nervous system and therefore under voluntary control.

The stomach secretes two important hormones in the gastrointestinal tract. One is gastrin. What is the second hormone secreted by the stomach?

Ghrelin. Ghrelin is a newly discovered peptide hormone produced by endocrine cells in the mucosal layer of the fundus of the stomach. It displays potent growth hormone-releasing activity and has a stimulatory effect on food intake and digestive function, while reducing energy expenditure. The isolation of this hormone has led to new insights into the gut-brain regulation of growth hormone secretion and energy balance.

Saliva has more than one function. What are the functions of saliva?

Protection, lubrication, antibacterial, and initiate digestion of starches. Saliva has three functions. The first is protection and lubrication. Saliva is rich in mucus, which protects the oral mucosa and coats the food as it passes through the mouth, pharynx, and esophagus. The second function of saliva is its protective antimicrobial action. Third, saliva contains ptyalin and amylase, which initiate the digestion of dietary starches.

The colon is home to between 300 and 500 different species of bacteria. What is their main metabolic function?

Fermentation of undigestible dietary residue. The major metabolic function of colonic microflora is the fermentation of undigestible dietary residue and endogenous mucus produced by the epithelial cells.

Absorption is a major function of the gastrointestinal tract. How is absorption accomplished in the gastrointestinal tract?

Absorption is accomplished by active transport and diffusion.

Nausea and vomiting can be side effects of many drugs as well as physiologic disturbances within the body. What is the common cause of nausea?

A common cause of nausea is distention of the duodenum or upper small intestinal tract.

Several neurotransmitters have been identified with nausea and vomiting. In this capacity they act as neuromediators. What neuromediator is thought to be involved in the nausea and vomiting that accompanies chemotherapy?

Serotonin. Serotonin is believed to be involved in the nausea and emesis associated with cancer chemotherapy and radiation therapy. Serotonin antagonists (e.g. graniestron and ondansetron) are effective in treating the nausea and vomiting associated with these stimuli.

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