Digestive System-Chapter 26

Question 1

Describe the major functions of the digestive tract

Answer 1

The major functions are ingestion, motility, secretion, digestion (mechanical digestion and chemical digestion), absorption and elimination.

Question 2

Define mechanical digestion

Answer 2

It occurs when ingested material is physically broken down into smaller units by chewing and mixing without changing their chemical structure.

Question 3

Define chemical digestion

Answer 3

It involves specific enzymes to break chemical bonds to change larger complex molecules into smaller molecules that can then be absorbed.

Question 4

Describe the organization of the digestive system.

Answer 4

It has two separate categories of organs: the gastrointestinal (GI) tract (also called the digestive tract, or alimentary canal) form a continuous tube that includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine and ends at the anus. The accessory digestive organs include the salivary glands, liver, and pancreas. Also, the teeth, tongue and gallbladder.

Question 5

Identify the layers of the gastrointestinal tract wall

Answer 5

They are the mucosa (inner lining), the submucosa, the muscularis, and the outermost tunic may be either the adventitia or a serosa.

Question 6

Describe the structure and function of the mucosa

Answer 6

The mucosa is the inner-lining mucous memebrane. It consists of an epithelium, an underlying lamina propria, and a thin layer of muscularis mucosae. The epithelium is in contact with the contents of the lumen and it allows for secretion and absorption.

Question 7

Describe the structure and function of the submucosa

Answer 7

The submucosa is composed of areolar and dense irregular connective tissue. Many large blood vessels, lymph vessels, and nerves are withing the submucosa. In the last region of the small intestine, MALT is present.

Question 8

Describe the structure and function of the muscularis

Answer 8

The muscularis contains two layers of smooth muscle. The cells of the inner layer are oriented circumferentially around the GI tract and are called the inner circular layer. The cells of the outer layer are oriented lengthwise along the GI tract and are called the outer longitudinal layer. The enteric nervous system are located between the two layers and detect both changes in the GI tract wall and the chemical makup of the contents of the lumen. The function of the muscularis is to mix and propel the contents within the GI tract.

Question 9

Describe the two types of movement that occur in the digestive system.

Answer 9

1) Peristalsis-the alternating contraction sequence of both the inner circular and outer longitudinal muscle layers for the purpose of propelling ingested materials through the GI tract. 2) Mixing-the “back-and-forth,” or kneading motion that occurs at any point in time within different regions but lacks directional movement. Mixing is for the purpose of blending ingested materials with the secretions within the GI tract.

Question 10

Explain how the enteric nervous system is involved in regulating digestion.

Answer 10

Both the submucosal nerve plexsus and the myenteric nerve plexus together compse the enteric nervous system. Sensory neurons within these plexuses (housed within the GI tract wall) detect both changes in the GI tract wall (e.g., stretch), and chemical makeup of the contents of the lumen. THus, this system is composed of both sensory and motor neurons of the autonomic nervous system.

Question 11

Explain how the central nervous system is involved in regulating digestion

Answer 11

Sensory input from both mechanoreceptors and chemoreceptors is relayed to the CNS in response to stimulation. Autonomic motor output is then relayed through three cranial, and vagus nerves, to different digestive system effectors, such as salivary glands, the pancreas, and the muscularis layers in the GI tract wall. The result is coordinated secretory and smooth muscle contractions involved in digestive responses.

Question 12

Explain how hormones are involved in regulating digestion.

Answer 12

Gastrin in the stomach stimulates the contractile activity of muscle in the gastric wall, increases release of HCl from parietal cells, and also stimulates contraction of the pyloric sphincter to slow stomach emptying. Secretin is released from the small intestine in response to an increase in chyme activity. It causes the release of an alkaline solution from both the liver and ducts of the pandreas that help neutralize the acidic chyme. Chymottrypsinis activated by trypsin (from chymotrypsinogen) and it breaks the bonds between specific amino acids within the protein to produce smaller strands of amino acides called peptides

Question 13

Describe the structure and function of mesenteries.

Answer 13

The structure of mesenteries refers to the double-layer of peritoneum (connective tissue) that supports, suspends and stabilizes the intraperitoneal GI tract organs. Blood vessels, lymph vessels, and nerves are sandwiched between the two folds and supply the digestive organs.

Question 14

Identify the four major mesentaries.

Answer 14

The greater omentum (which covers most of the abdominal organs); the lesser omentum (connects the superomedial surface of the stomach and the proximal end of the duodenum to the liver); the falciform ligament (a peritoneal fold that attaches the liver to the internal surface of the anterior abdominal wall); and the mesentery proper (a fan-shaped fold of peritoneum that suspends most of the small intestine from the internal surface of the posterior abdominal wall).

Question 15

Describe the structure of the oral cavity and its accessory structures.

Answer 15

The oral cavity is the mouth and is the entrance to the GI tract. There are two distinct spatial regions: the vestibule (or buccal cavity) is the space between the gums, lips, and cheeks, and the oral cavity proper lies central to the teeth.The oral cavity is bound laterally by the cheeks, anteriorly by the teeth and lips, and it leads posteriorly into the oropharynx. The cheeks contain the buccinator muscles. The internal surfaces of both the superior and inferior lips are attached to the gingivae (gums) by a thin mucosa fold in the midline, called the labial frenulum. The palate forms the superior boundar of the oral cavity and acts as a barrier to separate it from the nasal cavity. The anterior 2/3 is hard and bony (the hard palate), and the posterior 1/3 is soft and muscular (soft palate). The hard palate exhibits transverse palatine folds (or friction ridges); the uvula is the posterior part of the soft palate and is a conical median projection; the fauces represent the opening between the oral cavity and the oropharynx; the tongue is formed primarily from skeletal muscle. Both extrinsic and intrinsic muscles move the tongue; papillae are small projections that cover the superior surface of the tongue; The lingual frenulum is a thin vertical mucous membrane that attaches the tongue to the floor of the oral cavity; collectively the intrinsic salivary glands are the buccal glands of the cheeks, lingual glands of the tongue, and labial glands of the lips.

Question 16

Describe the function of the oral cavity and its accessory structures.

Answer 16

The main function of the oral cavity is to ingest food, and here it undergoes the initial processes of mechanical and chemical digestion. The buccinator muscles of the cheeks compress the cheeks against the teeth to hold solid materials in place during mastication; the palatine folds assist the tongue in manipulating ingested materials prior to swallowing; when swallowing the soft palate and the uvula elevate to close off the posterior entrance into the nasopharynx and prevent ingested materials from entering the nasal region; the tongue manipulates and mixes ingested materials during chewing and helps compress the partially digested materials against the palate to assist in mechanical digestion. The tongue also performs important function in both swallowing and in speech production. The teeth are responsible for mastication. The intrinsic salivary glands contribute to the production of saliva.

Question 17

Explain what mastication is.

Answer 17

Mastication is mechanical digestion in the oral cavity, or chewing. It requires the coordinated activities of teeth, skeletal muscles in lips, tongue, cheeks, and jaws that are controlled by nuclei within the medulla oblongata and the pons, collectively called the mastication center.

Question 18

Describe the components of saliva.

Answer 18

Saliva is composed of 99.5% water and a mixture of solutes. Saliva is formed as water and electrolytes are filtered from capillaries through cells (acini) of a salivary gland. Other components are added by acinar cells, including salivary amylase, mucin, and lysozyme.

Question 19

Explain how salivation is activated.

Answer 19

The salivary nuclei within the brainstem regulate salivation. Input to the salivary nuclei is received from chemoreceptors or mechanoreceptors in the upper GI tract. THese receptors detect various types of stimuli, including the introduction of sbstances into the oral cavity, especially those that are acidic; and arrival of foods into the stomach lumen, especially foods that are spicy or acidic. Input is also received by the salivary nuclei from the higher brain center in response to the thought, smell, or sight of food.

Question 20

Describe the structure and function of the pharynx.

Answer 20

The pharynx is a funnel-shaped, muscular passageway with distensible lateral walls, Three skeletal muscle pairs called the superior, middle, and inferior pharyngeal constrictors form the wall of the pharynx. The oropharynx and laryngopharynx are lined with nonkeratinized stratified squamous epithelium that provides protection against the abrasiive activities associated with swallowing ingested materials. The function of the pharynx is to serve as the passageway for both air and food.

Question 21

Describe the structure and function of the espophagus.

Answer 21

The esophagus is a normally collapsed, tubular passageway. It is about 25 cm long in an adult. The superior esophageal sphincter is a contracted ring of circular skeletal muscle at the superior end of the esophagus. It is the area where the esophagus and the pharynx meet. It is closed during inhalation of air, so air does not enter the esophagus and instead enters the larynx and trachea. The inferior esophageal sphincter is at the inferior end of the stomach. It isn’t strong enough alone to prevent materials from refluxing back into the esophagus; instead, the muscle of the diaphragm at the esophageal opening contract to help prevent materials from regurgitating from the stomach. The mucosa of the esophagus is lined with a nonkeratinized stratified squamous epithelium. There is a submucosa and muscularis. THe muscularis is unique in that it contains a blend of both skeletal and smooth muscle. The function of the esophagus is as a passageway for food into the stomach.

Question 22

List the phases of deglutition (swallowing).(swallowing).

Answer 22

The voluntary phase, the pharyngeal phase, and the esophageal phase.

Question 23

Describe the voluntary phase of deglutition.

Answer 23

The voluntary phase occurs after ingestion. Ingested materials and saliva mix in the oral cavity. Chewing forms a bolus. Transverse palatine folds help direct the bolus posteriorly toward the oropharynx.

Question 24

Describe the pharyngeal phase of deglutition.

Answer 24

The arrival of the bolus at the entryway to the oropharynx initiates the pharyngeal phase. It is involuntary. Tactile sensory receptors around the fauces are stimulated by the bolus and initiate sensory input ot the swallowing center in the medulla. Newve signals are then relayed to effectors to cause the following responses: 1) entry of bolus into the oropharynx; 2) elevation of the soft palate and uvula to block the passageway between the oropharynx and nasopharynx; 3) elevation of the larynx by the hyoid muscles in the neck to move the epiglottis to cover the laryngeal opeining (this prevents ingestede material from entering the larynx and trachea); 4) inhibition of the respiratory center in the medulla to assure that a breath is not taken during swallowing. The bolus passes quickly and involuntarily through the pharynx to the esophagus-about 1 second elapses in this phase.Sequential contraction of the pharyngeal constrictors decreases the diameter of the pharynx, beginning at its superior end and moving toward its inferior end. This creates a pressure difference, forcing swallowed material from the pharynx into the esophagus.

Question 25

Describe the esophageal phase of deglutition.

Answer 25

It is also involuntary. It is the time during which the bolus passes through the esophagus and into the stomach-about 5-8 seconds. The presence of the bolus within the lumen of the esophagus stimulates sequential peristaltic waves of muscular contraction that assist in propelling the bolus toward the stomach. The superior and inferior esophageal sphincters are usually closed at rest. . When the bolus is swallowed, these sphincters relax to allow it to pass through the esophagus. Nerve signals to the inferior esophageal sphincter after passage of the bolus cause it to contract, preventing reflux of materials and fluids from the stomach into the esophagus.

Question 26

Describe the anatomy of the stomach.

Answer 26

The stomach is a muscular J-shaped organ. It is composed of 4 regions: 1) cardia-a small narrow superior entryway into the stomach lumen from the esophagus; 2) fundus-the dome-shaped region lateral and superior to the esophageal connection with the stomach; 3) body-the largest region of the stomach; 4) pylorus-a narrow, medially directed, funnel-shaped pouch that forms the terminal region of the stomach. It’s opening into the duodenum of the small intestine is called the pyloric orifice. Surrounding this is a thick ring of circular smooth muscle called the pyloric sphincter. THe internal stomach lining is composed of numerous gastric folds. THe greater omentum and the lesser omentum are the two serous membranes associated with the stomach.

Question 27

List the cells found in the gastric epithelium and gastric glands.

Answer 27

1) surface mucous cells; 2) mucous neck cells; 3) parietal cells; 4) chief cells; and 5) G-cells.

Question 28

Describe the secretions of surface cells.

Answer 28

They continuously secrete an alkaline product containing mucin. Mucin becomes hydrated, producing a 1 to 3 mm mucus layer. This layer helps to prevent ulceration of the stomach lining upon exposure to both the high acidity of the gastric fluid and gastric enzymes.

Question 29

Describe the secretions of mucous neck cells.

Answer 29

They produce an acidic mucin that helps maintain the acidic conditions resulting from the secretion of HCl by parietal cells.

Question 30

Describe the secretions of parietal cells (also called oxyntic cells).

Answer 30

They secrete two substances into the lumen of the stomach. 1) intrinsic factor-it is required for absorption of vitamin B12 in the ileum. 2) HCl is not formed within the parietal cell (it would destroy the cell). Instead, it forms from H and Cl secreted across the parietal cells’ surface. HCl converts pepsinogen into active pepsin; kills most microorganisms that enter the stomach; contributes to the breakdown of plant cell walls and animal connective tissue; and denatures proteins.

Question 31

Describe the secretions of chief cells (also called zymogenic cells, or peptic cells.

Answer 31

They produce and secretepackets of zymogen granules primarily containing pepsinogen. Pepsinogen is the inactive precursor of the proteolytic enzyme pepsin. Pepsin must be produced in this inactive from to prevent the destruction of chief cell proteins. Pepsin chemically digests denatured proteins into smaller peptide fragments (oligopeptides). Chief cells also produce gastric lipase, an enzyme that has a limited role in fat digestion.

Question 32

Describe the secretions of G-cells (enteroendocrine cells).

Answer 32

THey secrete gastrin hormone into the bloodstream. Gastrin stimulates stomach secretions and motility. Other enteroendocrine cells produce other hormones, such as somatostatin, a peptide hormone that modulates the function of nearby enteroendocrine and exocrine cells.

Question 33

Distinguish between gastric mixing and gastric emptying.

Answer 33

1) Gastric mixing-a form of mechanical digestion that changes the semidigested bolus into chyme. Contractions of the stomach’s thick muscularis layer churn and mix the bolus with the gastric secretions, leading to a reduction in the size of swallowed particles. 2) gastric emptying-the movement of acidic chyme from the stomach through the pyloric sphincter into the duodenum, which is facilitated by the progressive thickening of the muscularis layer in the pylorus region. As a wave of peristaltic muscular contraction moves through the pylorus toward the pyloric sphincter, a pressure gradient is established that drives the stomach contents toward the small intestine.

Question 34

List the three phases involved in the regulation of gastric activity.

Answer 34

1) Cephalic phase; 2) gastric phase; and 3) intestinal phase.

Question 35

Describe the cephalic phase.

Answer 35

It primarily involves the cephalic reflex, which is a nervous system reflex initiated by the thought, smell, sight, or taste of food. Nerve signals from the higher region of the brain are sent to the hypothalamus, which then relays nerve signals to the medulla, which increases vagal stimulation of the stomach, causing both an increase in contractile force in the gastric wall and secretory activity of the gastric glands. The stomach ‘growling’ is this phase.

Question 36

Describe the gastric phase.

Answer 36

It involves the processes following the bolus reaching the stomach. It is regulated by both the nervous system via the gastric reflex and the endocrine system through the release of gastrin hormone. Baroreceptors in the wall of the stomach detect increased distension in the wall, and chemoreceptors detect both protein and an increase in pH of gastric contents. Nerve signals are relayed directly to the medulla, resulting in an increase in both stomach motility and secretory activity of gastric cells. The presence of food in the stomach also causes release of gastrin. Gastrin further stimulates the contractile activity of muscle in the gastric wall and increases the release of HCl from parietal cells. Gastrin also stimulates contraction of the pyloric sphincter to slow stomach emptying.

Question 37

Describe the intestinal phase.

Answer 37

It involves the processes following the chyme reaching the small intestine, a phase regulated by both the nervous system and the endocrine system. It involves the intestinal reflex and the release of cholecystokinin (CCK) and secretin. The intestinal reflex opposes the gastric and cephalic reflexes. It protects the small intestine from being overloaded with chyme. CCK and secretin are hormones that decrease both stomach motility and secretory activity and inhibit the release of gastrin. This slows down the emptying of the stomach.

Question 38

Gastrin-describe the stimulus for secretion, the secreting organ/cell type, and the function.

Answer 38

Gastrin are secreted by G-cells. The presence of food (especially protein) causes the release of gastrin. Gastrin enters the bloodstream and circulates back to the stomach to further stimulate the contractile activity of muscle in teh gastric wall and it increases the release of HCl from parietal cells. It also stimulates contraction of the pyloric sphincter to slow stomach emptying.

Question 39

Cholecystokinin (CCK)-describe the stimulus for secretion, the secreting organ/cell type, and the function.

Answer 39

It is released by the small intestine. It is sttimulated by fatty chyme. It stimulates the smooth muscle in the gallbladder wall to strongly contract, causing the release of concentrated bile. It also stimulates the pancreas to release pancreatic juice, and stimulates to relaxation of the smooth muscle within the hepatopancreatic ampulla, allowing entry of the bile and pancreatic juice into the small intestine. Finally, it also inhibits both stomach motility and release of gastric secretions.

Question 40

Secretin-describe the stimulus for secretion, the secreting organ/cell type, and the function.

Answer 40

It is released from the small intestine primarily in response to an increase in chyme acidity. It causes the release of an alkaline solution that contains HCO3- from both the liver and ducts of the pnacreas. This alkaoline fluid helps neutralize acidic chyme. Secretin also hinhibits gastric secretions and motility.

Question 41

Describe the organization of hepatocytes in the liver.

Answer 41

The liver is partitioned into thousands of small, polyhedral hepatic lobules, with are the structural and functional units of the liver. WIthin the hepatic lobules are the liver cells, the hepatocytes.In cross section, a hepatic lobule loods like a side view of a bicycle wheel. The central vein is like the hub of the wheel. At the circumference of the wheel (where the tire would be) are the portal triads. Cords of hepatocytes make up the numerous spokes of the wheel, and they are bordered by hepatic sinusoids.

Question 42

Describe the flow of blood through the liver.

Answer 42

The cells of the liver are served by a dual blood supply; one is oxygenated and the other is deoxygenated. The hepatic artery is a branch of the celiac trunk that carries oxygenated blood to the liver. The hepatic portal vein, as part of teh hepatic portal system, carries blood from the capillary beds of the GI tract, spleen and pncreas. It brings approx. 75% of the blood volume to the liver. Blood from the branches of these two vessels mixes in the hepatic sinusoids as it passes into and through the hepatic lobules The blood then flows slowly toward the central vein, that drains the blood flow from the lobule. Central veins collect the blood and merge throughout the liver to form numerous hepatic veins that eventually empty into the inferior vena cava..

Question 43

Describe the functions of bile.

Answer 43

Bile salts and lecithin function in the mnechanical digestion of lipids, allowing more efficient chemical digestion of triglycerides.

Question 44

Describe the functions of the gallbladder.

Answer 44

It is a saclike organ that stores, concentrates, and releases bile that the liver produces.

Question 45

Distinguish between pancreatic islets and pancreatic acini in terms of structure and function.

Answer 45

Pancreatic islets contain areterioles and venules, and alpha cells (which secrete glucagon) and beta cells (which secrete insulin). Pancreatic acini are saclike arrangements of acinar cells, that secrete amylase, lipase, proteases and nucleases. THey also contain duct cells that serete HCO3-.

Question 46

Describe the function of pancreatic amylase

Answer 46

Digests starch

Question 47

Describe the function of pancreatic lipase

Answer 47

Digests fats

Question 48

Describe the function of trypsinogen, chymotrypsinogen, and procarboxypeptidase (inactive proteases).

Answer 48

When activated, they digest protein.

Question 49

Describe the function fo nucleases.

Answer 49

They are necessary for the digestion of nucleic acids (DNA and RNA).

Question 50

Describe the structure of the small intestine.

Answer 50

It is a coiled, thin-walled tube. It consists of three specific segments: the duodenum, jejunum, and ileum. 1) duodenum-form the first segment, approx. 10 in. long and originates at the pyloric sphincter. 2) jejunum-approx. 7.5 feet in length, about two-fifths of the small intestine length. it is the primary region within the small intestine for chemical digestion and nutrient absorption; 3) ileum-the last region. anout 10.8 feet, forms approx. 3/5 of the length. The mucosal and submucosal tunics of the sm. intestine are thrown into internal circular folds (also called plicae circulares), that extend inward toward the lumen. They help increase the surface area through which nutrients are absorbed. There are small fingerlike projections, villi. They also increase surface area. There are also microvilli.

Question 51

Explain the function of the goblet cells.

Answer 51

They produce mucin.

Question 52

Describe the digestion of carbohydrates by the brush border enzymes.

Answer 52

The completion of starch breakdown is accomplished by the brush border enzymes. They include dextrinase and glucoamylase. They break the bonds between glucose subunits of oligosaccharides. Maltase breaks the bond between the two glucose molecules that compose maltose.

Question 53

Describe the digestion of proteins by the brush border enzymes.

Answer 53

The brush border enzyme dipeptidase breaks the final bond between the two amino acids of a dipeptide so that both may be absorbed. Aminopeptidase generates free amino acids from the amino end of peptides.

Question 54

Describe how fats are absorbed across the intestinal wall.

Answer 54

1) Bile salts released from the liver and gallbladder emulify lipid droplets to form micelles. 2) Pancreatic lipase functions within micelles to digest each triglyceride into a monoglyceride and two free fatty acids. 3) Monoglycerides and free fatty acids enter an epithelial cell, while bile salts remain in the intestinal lumen to be reabsorbed and reclycled. 4) Triglyceride molecules are reassembled within epithelial cells. Lipids are then wrapped with protein to form chylomicron. Chylomicrons are are packaged within secretory vesicles and then exocytosed from the cells and absorbed into lacteals.

Question 55

Describe the structure and function of intestinal crypts (also known as intestinal glands, or crypts of Lieberkuhn).

Answer 55

They are invaginations of mucosa. These mucosal cells secrete intestinal juice.

Question 56

Describe the structure and function of the duodenal glands (also called the submucosal, or Brunner gland).

Answer 56

THey are housed within the submucosa layer and found found only in the proximal duodenum. THis gland produces a viscous, alkaline, mucus secretion that protects the duodenum from the acidic chyme.

Question 57

Distinguish between peristalsis and segmentation.

Answer 57

1) Segmentation-mixes chyme with accessory gland secretion through a “back-and-forward” motion. 2) peristalsis-propels material within the GI lumen by alternating contraction of the circular and longitudinal muscle layer in small regions.

Question 58

Describe the gastroilial reflex.

Answer 58

It moves the contents from the ileum into the cecum in response to food entering the stomach.

Question 59

Describe the digestion and absorption of carbohydrates.

Answer 59

IN the oral cavity, salivary amylase breaks the chemical bonds between glucose molecules to partially digest the starch molecule. 1)Pancreatic amylase is produced by the pancreas and secreted into the small intestine; 2) pancreatic amylase continues digestion of starch that began in the oral cavity; 3) brush border enzymes complete the breakdown of starch to individual glucose molecules, and are responsible for the digestion of disaccharides.

Question 60

Describe digestion and absorption of proteins.

Answer 60

It begins within the stomach. Pepsin is released and denatures proteins to facilitate their chemical breakdown. 1_Proteolytic enzymes are released from the pancreas; 2) Enteropeptidase activates trypsinogen to trypsin; trypsin then activates other proteolytic enzymes; 3) Activated pancreatic proteolytic enzymes break proteins into peptides and amino acids; 4) brush border peptidases break peptides tinto single amino acids to be absorbed through epithelial cells into blood.

Question 61

Describe the structure and function of the large intestine.

Answer 61

See diagrams fore structure. The large intestine absorbs water and electrolyes from the remaining digested material that enters into it. The large intestine then stores fecal material until it is eliminated through defecation.

Question 62

Describe the types of mevements that occur in the large intestine and the reflexes that control those movements.

Answer 62

1)Peristaltic movements-they are usually weak and sluggish, but otherwise they resemble those that occur in the wall of the small intestine. 2) haustral churning-occurs after a relaxed haustrum fills with digestede or fecal material until distension stimulates reflex contraction in the muscularis. THese contraction increase churnin and move the material to more distal haustra. 3) Mass movements-powerful, peristaltic-like contraction that propel fecal material tward the rectum. It is initiated by the gastocolic reflex (which in turn is initiated by distension in the stomach). A wave of contraction beinins in the middle of the transverse colon, forcing a large amount of fecal matter into the descending colon, sigmoid colon and the rectum. 4) Defectation reflex-filling of the rectum initiates the urge to defecate. This stimulus results in transmission of nerve signals from the receptors to the spinal cord. IN response, parasympathetic output increass to both the sigmoid colon and rectum, while it decreases to the internal anal sphincter.