Ch.21 - Digestive System
Name the three parts of the mucosa.
1) epithelium ; 2) the lamina propria 3) muscularis mucosae
Secretion
1) the transepithelial transfer of water and ions from the ECF to the digestive tract lumen (opposite of absorption) 2) the release of substances synthesized by GI epithelial cells (digestive enzymes) to the ECF or lumen
Name the seven regions of the large intestine. What is the main function of the large intestine?
7 regions of large intestine: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus Large intestine: absorbs water from the remaining indigestible food matter and concentrates waste (by absorbing ions, water, minerals, vitamins, small organic molecules), and then passes useless waste material from the body
Name the functions of amylase, lysozyme, and immunoglobulins in the first stage of digestion.
Amylase: breaks starch into maltose after the enzyme is activated by Cl- in the saliva Lysozyme: antibacterial salivary enzyme Immunoglobulins: disable bacteria and viruses
migrating motor complex
Between meals, when the tract is largely empty, a series of contractions begins in the stomach and passes slowly from section to section, each series taking about 90 minutes to reach the large intestine. This pattern, known as the migrating motor complex, is a "housekeeping" function that sweeps food remnants and bacteria out of the upper GI tract and into the large intestine.
How is the acidic chyme neutralized in the small intestine? Name both the facilatory and the inhibitory mechanisms.
Bicarbonate secretion into the small intestine neutralizes the highly acidic chyme that enters from the stomach. Most bicarbonate comes from the pancreas and is released in response to neural stimuli and secretin (a GI peptide in the gastrin category) Facilatory: secretin release stimulates the pancreas to release HCO3-... secretion from duodenum that stimulates production of HCO3- Inhibitory: inhibits acid production and gastric motility
What is the function of crypt cells in the colon?
Crypt cells contain stem cells that divide to produce new epithelium, goblet cells, endocrine cells, and maturing colonocytes
Sections of the small intestine?
DJI: duodenum, jejunum, ileum
Name the four basic processes of the digestive system.
Digestion, absorption, secretion, and motility
What are the roles of endopeptidases and exopeptidases in digestion of proteins?
Endopeptidases (aka proteases) attack peptide bonds in the interior of the amino acid chain and break a long peptide chain into smaller fragments -- proteases are secreted as inactive proenzymes from epithelial cells in the stomach, intestine, and pancreas and are activated in the GI tract lumen --examples of proteases include pepsin secreted in the stomach, trypsin and chymotrypsin secreted by the pancreas Exopeptidases release single amino acids from peptides by chopping them off the ends one at a time -- example of exopeptidases are aminopeptidase, which acts on the amino-terminal end of the protein and carboxypeptidase, which acts at the carboxy-terminal end
What makes up chyme?
Food moving through the GI tract mixes with secretions from secretory epithelium, the liver, and the pancreas and creates a soupy mixture called chyme.
What is the function of the GI tract?
GI tract: esophagus, stomach, small intestine, large intestine The primary function of the GI tract is to move nutrients, water, and electrolytes from the external environment into the body's internal environment.
Why are GIP and CCK peptides of interest to researchers studying diabetes (type II) and obesity respectively?
GIP peptides primary effects include stimulating insulin release in response to glucose in intestine CCK peptides stimulate gallbladder contraction and pancreatic enzyme secretion, affects digestion and appetite respectively
Name the stimuli for release of the following: CCK; GIP; Secretin.
GIP: stimulated for release by glucose, fatty acids, and amino acids in small intestine CCK: stimulated for release by fatty acids and some amino acids Secretin: stimulated for release by acid in small intestine
Name the roles of gastric acid (HCL), pepsin, and gastric lipase in digestion at the stomach. When does carbohydrate digestion cease in the stomach?
Gastric acid: kills bacteria and other ingested microorganisms, activates pepsin (the enzyme that digests proteins), inactivates salivary amylase, and it denatures proteins Pepsin: Endopeptidase, initial digestion of proteins Gastric lipase: digests fats, but less than 10% of fat digestion takes place in the stomach ***Carbohydrate digestion stops when gastric acid inactivates salivary amylase
What is GALT? How does it serve a peripheral function of the digestive system?
Gut-associated lymphoid tissue. Largest collection of lymph tissue (By one estimate, 80% of all lymphocytes in the body are found in the small intestine). Used for protection against bacteria, viruses, other pathogens
What are haustra?
Haustra: bulging pockets caused by contractions of the tenia coli
Although it is a popular myth that excess acid secretion is a common cause of peptic ulcers, the most common cause of peptic ulcers are ___________.
Helicobacter pylori and nonsteroidal anti-inflammatory drugs (NSAIDs) [peptic ulcer: hyperacidity... acid and pepsin destroy the mucosa]
Name the valve between the small and large intestine.
Ileocecal valve
What are Peyer's patches?
In the small intestine, collections of lymphoid tissue adjoining the epithelium form small nodules and larger Peyer's patches that create visible bumps in the mucosa. major part of GALT
_____ use clathrin coated pits with membrane receptors to carry out transcytosis. Once the vesicle crosses to the extracellular compartment, macrophages and lymphocytes rid of the antigens.
M cells M cells provide info about the contents of the lumen to the immune cells of the GALT
Four layers of the GI tract wall?
MSMS: Mucosa, submucosa, muscularis externa, serosa
What are the two purposes of motility?
Moving food from the mouth to the anus and mechanically mixing food to break it into uniformly small particles
What is mucus comprised of, and what is its function? Give the name of the cells which secrete mucus in the stomach vs. the intestine.
Mucus is a viscous secretion composed primarily of glycoproteins called mucins. It forms a protective coating over the GI mucosa and lubricate the contents of the gut Mucous cells in the stomach, goblet cells in the intestines
Give the general outcome of parasympathetic vs. sympathetic stimuli on GI functions.
Parasympathetic: excitatory (rest and DIGEST) Sympathetic: inhibits GI functions
Parietal cells have two important functions in the stomach. Name both, and name the disease state which results from a defect in the second function. What is the end result of that disease?
Parietal cells secrete gastric acid (HCl) and intrinsic factor. Pernicious anemia- occurs when the body lacks intrinsic factor --> leads to vitamin B12 deficiency bc B12 cannot be absorbed, so RBC synthesis is severely diminished
Peristaltic contraction
Peristalsis is progressive waves of contraction that move from one section of the GI tract to the next. In peristalsis, circular muscles contract just behind a mass, or bolus, of food. This contraction pushes the bolus forward into a receiving segment, where the circular muscles are relaxed. The receiving segment then contracts, continuing the forward movement. Peristaltic contractions push a bolus forward at speeds between 2 and 25 cm/sec.
What makes up saliva?
Saliva is made up of water, ions, mucus, and proteins such as enzymes and immunoglobulins
What structure connects the mouth to the stomach? Name the three sections of the stomach.
The esophagus connects the mouth to the stomach three sections of the stomach: fundus, body, and antrum (FBA)
What is the structure and location of the serosa? What is it a continuation of?
The serosa is the outer covering of the entire digestive tract, and it is a connective tissue membrane that is a continuation of the peritoneal membrane (peritoneum) lining the abdominal cavity. The peritoneal membrane also forms sheets of mesentery that hold the intestines in place so that they don't become tangled as they move.
Why is it unnecessary to ingest free amino acids in an attempt to supplement protein anabolism?
They are primary products of protein digestion, all of which can be absorbed.
What are tonic and phasic contractions of GI smooth muscle? When a slow wave potential reaches threshold, what type of voltage-gated channels open?
Tonic contractions are sustained for minutes or hours Occur in some smooth muscle sphincters and anterior portion of the stomach Phasic contractions has contraction-relaxation cycles lasting only a few seconds. Occur in posterior stomach, small intestine Ca2+ channels [Slow wave potentials are cycles of smooth muscle contraction and relaxation are associated with spontaneous cycles of depolarization and depolarization.]
What is emesis?
Vomitting forceful expulsion of gastric and duodenal contents from the mouth, protective reflex that removes toxic materials from the GI tract before they can be absorbed
Because triglycerides in our diet form large clumps in aqueous chyme solution, fat digestion must be aided by _____.
bile salts.
What is the function of amylase in carbohydrate metabolism?
breaks long glucose polymers into smaller glucose chains and into the disaccharide maltose salivary amylase starts starch digestion but then pancreatic amylase resumes digestion of starch into maltose [intestinal carbohydrate absorption is restricted to monosaccharides]
The enteric nerve plexus is referred to as the "little brain". The effect of emotional distress on GI tract function is evidence of the link between this plexus and the _______.
cephalic brain.
Name the three phases of food processing.
cephalic phase, gastric phase, and intestinal phase.
Long reflexes
digestive reflexes integrated in the CNS (no matter where they originate)
What are enterocytes? What is the function of enterocytes in the small intestine?
enterocytes are cells that line intestinal wall function in transporting nutrients and ions into blood
What is the major purpose of bile?
facilitates the digestion of fats
Cephalic phase:
feedforward response caused by reflexes that originate in the cephalic brain stimuli such as sight, smell, sound, or thought of food spark feedforward reflexes and result in responses such as mouth watering and stomach growling
GI peptides are generally divided into three groups. Name the groups, and list the peptides most readily associated with each.
hormones, neuropeptides, cytokines GI hormone families: 1)Gastrin- gastrin, cholecystokinin (CCK); both can bind to and activate the CCKB receptor 2) Secretin- secretin, vasoactive intestinal peptide (VIP), GIP, gastric inhibitory peptide, glucagon-like peptide 1 (GLP-1) 3) Others- motilin
What are interstitial cells of Cajal?
modified smooth muscle cells between smooth muscle layers and intrinsic nerve plexuses, and they may act as an intermediary between the neurons and smooth muscle
Motility
movement of material in the GI tract as a result of muscle contraction (peristalsis)
Describe the structure of the muscularis externa. It contains the second nerve network of the enteric nervous system called the _________.
muscularis externa: contains 2 layers of smooth muscle - inner circular layer and an outer longitudinal layer contraction of circular layer decreases the diameter of the lumen contraction of the longitudinal layer shortens the tube myenteric plexus lies between the two layers
The stomach joins the small intestine at the
pylorus, which is guarded by the pyloric valve.
Short reflexes
reflexes that originate within the enteric nervous system (ENS) and are integrated there without outside input
Crumpled folds of the gut wall are called ______ in the stomach and ______ in the small intestine.
rugae and plicae (RS) and (PSI) (increases surface area)
When fat digestion occurs in the small intestine, an emulsion is broken down to smaller _________ with the action of pancreatic lipase and colipase.
smaller and more stable products
What structure prevents pancreatic fluid and bile from entering the small intestine except during a meal?
sphincter of Oddi
What is the lamina propria?
subepithelial connective tissue that contains nerve fibers and small blood and lymph vessels. Absorbed nutrients pass into the blood and lymph here. This layer also contains wandering immune cells, such as macrophages and lymphocytes, patrolling for invaders that enter through breaks in the epithelium.
The discontinuous layer of longitudinal muscle in the large intestine is concentrated into three bands called the _______.
tenia coli.
Absorption
the active or passive transfer of substances from the lumen of the GI tract to the extracellular fluid.
Digestion
the chemical and mechanical breakdown of foods into smaller units that can be taken across the intestinal epithelium into the body.
In digestion, the intestinal enzymes are aided by exocrine secretions from two accessory glandular organs. Name them.
the pancreas (digestive enzymes, insulin/glucagon, bicarbonate) and the liver (bile).
What part of the enteric nervous system lies in the submucosa and plays a major role in coordinating digestive function?
the submucosal plexus, one of the two major nerve networks of the enteric system. the enteric nervous system helps coordinate digestive function, and the submucosal plexus (aka Meissner's plexus) innervates cells in the epithelial layer as well as smooth muscle of the muscularis mucosae.
Brush border enteropeptidases convert inactive ____ to active ____.
trypsinogen and trypsin
The defecation reflex is aided by forced expiratory movements against a closed glottis. These forced expiratory movements are referred to as __________.
valsalva maneuver.
The plicae project into the lumen in small fingerlike extensions known as______. Each of these structures increase their absorptive surface with ____________.
villi microvilli
Name the three pairs of salivary glands.
Sublingual glands (under the tongue) Submandibular glands (under the mandible (jawbone)) Parotid glands (near the hinge of the jaw)
Although researchers have sequenced over 30 peptides which control GI function, metabolism, and eating behavior, __________ are widely accepted as hormones.
only a small amount
Sodium and bicarbonate ion are both released in response to neural stimuli and the hormone secretin. Leaky junctions between the cells, called the ________ Na+ to move passively, and water follows.
paracellular pathways, allows
The World Health Organization estimates that 4 million people/ year die from dehydration associated with diarrhea. Most of these cases are associated with cholera toxin from the bacteria _____, and enterotoxin from the bacteria _______. Both enhance colonic ______ which is not matched by absorption.
Cholera toxin from vibrio cholerae Enterotoxin from escherichia coli Cl- and fluid secretion
Name the three general functions of the stomach.
1) storage- the stomach stores food and regulates its passage into the small intestine, where most digestion and absorption take place 2) digestion- the stomach chemically and mechanically digests food into the soupy mixture of uniformly small particles called chyme 3) protection- the stomach protects the body by destroying many of the bacteria and other pathogens that are swallowed with food or trapped in airway mucus. At the same time, the stomach must protect itself from being damaged by its own secretions
What is the difference between the location of pancreatic enzymes and brush border enzymes during their active phases?
Brush border enzymes are anchored to the luminal enterocyte membrane and are not swept out of the small intestine as chyme is propelled forward; entire lumen Pancreatic enzymes enter the intestine in a water fluid, most are secreted as zymogens; only on brush border cells
Name the secretions of the following cells: chief cells; D cells; enterochromaffin-like cells; G cells; mucous cells.
Chief cells: secrete inactive pepsinogen --> active pepsin D cells: secrete somatostatin (aka growth hormone inhibiting hormone) Enterchromaffin-like cells: secrete histamine G cells: secrete gastrin Mucous cells: secrete mucus and bicarbonate
What are chylomicrons? How do they enter the blood stream?
Chylomicrons are large droplets of triglycerides joined with cholesterol and proteins --> fat absorption Because chylomicrons are too big to cross the basement membrane to enter capillaries, they are absorbed into lacteals, the lymph vessels of the villi. Chylomicrons then pass through the lymphatic system and finally enter venous blood just before it flows into the heart.
What is the name of the liver cells which secrete bile? Where is the bile stored? How does it reach the duodenum?
Hepatocytes (liver cells) secrete bile. Bile is stored in the gallbladder During a meal, contraction of the gallbladder sends bile into the duodenum through the common bile duct, along with a watery solution of bicarbonate and digestive enzymes from the pancreas.
segmental contractions
In segmental contractions, short segments of intestine alternately contract and relax. In the contracting segments, circular muscles contract while longitudinal muscles relax. These contractions may occur randomly along the intestine or at regular intervals. Alternating segmental contractions churn the intestinal contents, mixing them and keeping them in contact with the absorptive epithelium. (compared to peristalsis which involves one-way motion)
Irritable bowl syndrome
Irritable bowl syndrome is a motility disorder characterized by altered bowel habits and abdominal pain.
What is mastication? What is deglutition?
Mastication: chewing, create a bolus to be swallowed Deglutition: swallowing, reflex, pushes bolus into esophagus
Tubular invaginations are called ___ in the stomach and ___ in the intestine
gastric glands crypts
How are bile salts recycled?
not altered during fat digestion, in ileum (terminal section of small intestine): cells transport them back into hepatic portal circulation, re-enter liver
