Chapter 22 Short Answers

What is the difference between peristalsis and segmentation

-Peristalsis is the rhythmic contractions of layers of smooth muscle that move material through a hollow organ. -Segmentation, also known as intestinal churning, involves contractions of only circular layer of smooth muscle; produces a squeezing motion.

Name the three phases of stomach secretion. In general terms what does each phase accomplish?

1.) Cephalic Phase: Initial cephalic phase, mediated by sight, smell, taste, or even thought of food under direction of CNS; prepares stomach to receive food by increasing release of hydrogen ions into it. These stimuli trigger output from vagus nerve, main nerve of parasympathetic nervous system, resulting in four physiological effects: o Direct stimulation of hydrogen ion release: vagus nerves release ACh onto parietal cells; directly stimulates them to release hydrogen ions o Stimulation of gastrin secretion: vagal nerve stimulation triggers release of a peptide that stimulates G cells to release hormone gastrin; triggers hydrogen ion secretion o Stimulation of histamine secretion: vagus nerve stimulates a specific type of enteroendocrine cell to release hormone histamine; triggers hydrogen ion secretion o Inhibition of somatostatin secretion: somatostatin is produced by a type of enteroendocrine cell located in antrum of stomach; inhibits acid secretion; vagal nerve stimulation inhibits somatostatin release; has effect of increasing hydrogen ion secretion 2.) Gastric Phase: Gastric phase begins when food enters stomach; continues stimulation provided during cephalic phase o Two stimuli trigger acid secretion during gastric phase: First, presence of food in stomach distends stomach wall, which stimulates neurons of ENS and sensory receptors involved with vagus nerve reflexes o Both vagus nerve and ENS neurons release ACh, which has same four effects as in cephalic phase; net effect is to increase acid secretion o Second stimulus is presence of partially digested proteins in gastric juice; stimulates enteroendocrine cells to produce and release gastrin; in turn stimulates acid secretion o Positive feedback loop progresses as follows: acid activates pepsin, which catalyzes protein digestion; digested proteins stimulate more gastrin release; triggers further acid secretion and more protein digestion; example of Feedback Loops Core Principle o Positive feedback loop in stomach does not continue indefinitely; once acid concentration reaches a certain level—a pH of about 2.0—a negative feedback mechanism is initiated o Extremely low pH levels trigger somatostatin release, which in turn decreases acid secretion 3.) Intestinal Phase: Final phase of gastric acid secretion that is responsible for only about 10% of remaining acid secretion, after which it inhibits further acid secretion o Triggered by presence of partially digested proteins in fluid entering duodenum; triggers enteroendocrine cells to release intestinal gastrin o This hormone has same effect as gastrin produced by stomach; stimulates hydrogen ion secretion from parietal cells o Stimulatory effect of intestinal phase is brief; as chyme enters duodenum, declining pH and presence of lipids trigger enterogastric reflex; decreases vagal activity and acid secretion o Low pH in duodenum also triggers production of hormones by cells of duodenal mucosa, including secretin and gastric inhibitory peptide (GIP); both hormones reduce acid secretion

List 5 major hormones/paracrines of the intestines and give the effects of each.

1.) Cholecystokinin (CCK): causes gallbladder to contract and release bile; stimulates secretion of pancreatic enzymes from acinar cells; relaxes hepatopancreatic sphincter 2.) Gastric inhibitory peptide: inhibits acid secretion from parietal cells 3.) Intestinal gastrin: stimulates acid secretion from parietal cells 4.) Motilin: stimulates the migrating motor complex of the small intestine 5.) Secretin: inhibits gastric motility and acid secretion; stimulates bicarbonate release from pancreatic duct cells; increases bile production by the liver 6.) Vasoactive intestinal peptide: inhibits acid secretion by parietal cells; stimulates pancreatic secretion; increases intestinal blood flow

What three structural modifications are found in the small intestines? What are their functions?

1.) Duodenum: initial segment; begins at pylorus; shortest of three divisions, at only about 25 cm (10 in.) long Arches into a "C" shape as it curves around pancreas where only proximal portion of duodenum is within peritoneum; remainder sits posterior to peritoneal cavity and so is retroperitoneal Houses major duodenal papilla; where secretions from gallbladder and pancreas enter small intestine Duodenal submucosa contains specialized glands called duodenal (Brunner's) glands; produce alkaline mucus to protect duodenum from acidic chyme 2.) Jejunum: middle; begins at duodenojejunal flexure and sits within peritoneal cavity; measures about 2.5 meters (7.5 feet) in length; most active site for chemical digestion and absorption 3.) Ileum: small intestine's final segment, is also intraperitoneal; measures about 3.6 meters (10.8 feet) in length; terminates at portion of large intestine called cecum Ileocecal valve - sphincter that controls movement of materials from ileum into cecum Also prevents materials in large intestine from flowing backward into ileum; critical for preventing access to bacteria from large intestine

List 4 major hormones/paracrines of the stomach and give the effects of each.

1.) Gastrin: increases acid secretion by parietal cells 2.) Histamine: increases acid secretion by parietal cells 3.) Serotonin: stimulates gastic mobility 4.) Somatostatin: decreases acid secretion by parietal cells

List and define the 6 major processes occurring during digestive system activity.

1.) Ingestion: Food and water are brought into digestive system by ingestion; occurs via mouth under normal conditions 2.)Secretion: 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 3.) Propulsion: Ingested food and liquids pass from one digestive organ to next by process of propulsion. Propulsion is accomplished largely by rhythmic contractions of smooth muscle of alimentary canal called peristalsis; aided by mucus secreted by multiple organs 4.) Digestion: Food breakdown occurs by process of digestion: Mechanical digestion - digestive organs physically break food down into smaller pieces via processes such as chewing and mixing food by muscular movements of alimentary canal Chemical digestion - enzymes secreted by digestive organs catalyze reactions that break chemical bonds within food particles until only small molecules remain 5.) Absorption: Once food particles are digested, they move through wall of alimentary canal into blood or lymphatic vessels by a process called absorption. Water, electrolytes, and vitamins are also absorbed into blood in same manner. 6.) Defecation: Certain ingested materials are not digestible or usable by body; such materials continue their transit through alimentary canal until they exit body as feces through defecation. Defecation also provides body with a way to eliminate certain metabolic wastes; note that defecation is simply a specialized form of propulsion.

List the 4 types of cells in the stomach, their secretions, and the importance of each component in stomach activity

1.) Mucous neck cells: Cells located near top, or "neck," of gland are called mucous neck cells; secrete acidic mucus that prevents neutralization of acid produced by parietal cells 2.) Parietal cells: secrete hydrochloric acid (HCl); responsible for acidic pH of gastric juice. Acid is an important component of gastric juice because it activates pepsinogen, destroys many disease-causing organisms, and stimulates parietal cell production of chemical intrinsic factor; required for intestinal absorption of vitamin B12 3.) Chief cells: secrete inactive precursor enzyme pepsinogen; when pepsinogen encounters an acidic pH, it becomes active enzyme pepsin, which begins protein digestion in stomach 4.) Enteroendocrine cells: Located at bottom of glands are enteroendocrine cells; secrete hormones that influence digestion. Enteroendocrine cells called G cells secrete hormone gastrin; stimulates secretions from other cells known as parietal cells

What two types of cells are responsible for making saliva and how do their secretions differ? Describe the composition and functions of saliva and explain how salivation is regulated?

1.) Serous cells: secrete a water-based fluid with enzymes and other solutes; secretions are involved in digestive processes; generally released just before or during eating 2.) Mucous cells: secrete mucus; primarily involved in keeping oral mucosa moist, and so are released continually Saliva is mostly water but also has electrolytes, is slightly acidic, has proteins and metabolic wastes, and has amylase. The salivary amylase is what is responsible for the enzymatic break down (digestion) of carbohydrates in the oral cavity. Saliva is regulated by the parasympathetic nervous system. When activated by the ingestion of food or the smell, taste, or thought of food, the glands produce more saliva. When the sympathetic system is activated, the glands produce little saliva and that is the dry mouth feeling one gets when nervous or scared.

Explain and give examples of the major functions of the liver

Bile Production: Main digestive function is to produce bile; a liquid that contains multiple components, including water, electrolytes, and organic compounds Bile serves two critical functions - required for digestion and absorption of lipids, and mechanism by which liver excretes wastes and other substances that kidneys cannot excrete Nutrient Metabolism: processes nutrients obtained from diet Detoxification: detoxifies substances produced by body, and substances that we eat or drink, some of which are harmful to body Excretion: directly excretes bilirubin in bile along with other substances liver processes, particularly certain antibiotics; also modifies substances so that they can be excreted by kidneys

List the functions of the tongue

Chewing, swallowing, tasting food; speech

Describe the endocrine and exocrine parts of the pancreas and their secretions

Its endocrine secretions, the hormones insulin and glucagon, are released from pancreatic islets into the blood and affect most cells in the body. Its exocrine secretions are enzymes secreted primarily by clusters of acinar cells. These enzymes are released into ducts of the pancreas that empty into the alimentary canal, where they help with digestion.

Describe the oxygen and nutrient levels in the blood of the hepatic artery and hepatic portal vein

Numerous blood vessels enter and exit the liver at the porta hepatis, including the hepatic artery, which brings oxygen-rich blood to the liver; and the hepatic portal vein, which brings nutrient-rich, deoxygenated blood to the liver from multiple abdominal organs.

What are the enzymes present in pancreatic juice? Explain the function of each

Pancreatic juice consists of water and multiple digestive enzymes and other proteins. In addition, the duct cells secrete bicarbonate ions, a base, which make pancreatic juice alkaline. This helps to neutralize the acidic chyme that enters the duodenum from the stomach and protects the duodenum from damage by the acid. The digestive enzymes, secreted by acinar cells, are crucial in chemical digestion and catalyze the reactions that digest carbohydrates, proteins, lipids, and nucleic acids.

The term "gut brain" does not really mean there is a brain in the digestive system. What does it refer to, what are the two divisions of the "gut brain" and what does each control?

Refers to the enteric nervous system or web of neurons closely associated with the digestive organs.?

What is the role of microorganisms in the colon

The role of microorganisms in the colon is to produce vitamins, metabolize undigested materials, deter the growth of harmful bacteria, and stimulate the immune system.

What is emulsification and how does it aid digestion

When bile is released into the duodenum, the bile salts coat the lipids and physically break them apart into smaller pieces, a process known as emulsification. Although emulsification is mechanical digestion, it is necessary for the chemical digestion and absorption of lipids.