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192 Cards in this Set

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With regards to physiological mechanisms, what is our current model on control of food intake based on?
Hypothalamic centers that are influenced by neurotransmitters and hormones.Control of these centers involves many brain neuropeptides and "brain-gut" hormones.
What are the two centers in the hypothalamus that regulated food intake?
1)a feeding center:tonicaly active.
2)a satiety center: stops food intake by inhibiting the feeding center.

Efferent signals from these centers cause changes in eating behavior and created sensations of hunger and fullness.
What are the two classic theories for regulation of food intake? Elaborate.
1)The Glucostatic Theory: glucose utilization by hypothalamic centers regulates food intake. When blood glucose concentrations go down the satiety center is suppressed and the feeding center takes over, and visa versa.

2)The Lipostatic Theory:a signal from the body's fat stores to the brain adjusts eating so that the body keeps a certain weight. The more fat stores, the less eating. Obesity comes from disruption of this pathway.
What served as evidence for signalling b/w adipose tissue and the brain?
The discovery of Leptin, which is a protein hormone synthesized in adipocytes, under the control of the obese(ob) gene. Most people have high lebtin levels, but those that lack the gene or have defective leptin receptors become obese.
Explain the important signal molecule 'neuropeptide Y' (NPY).
It is a brain neurotransmitter that appears to serve as the actual trigger for food intake. Normal-weighted animals have leptin that inhibits NPY in a negative feedback pathway.
What is Ghrelin?
A peptide secreted by the stomach that increases hunger.It also promotes release of growth hormone and the brain peptides called orexins (hypocretins)that play a role in sleep.
Appetite and eating are influenced by sensory input via the nervous system, in addition to chemical signalling. Name some.
Acts of swallowing, chewing, sight, smell, taste, even stress (even though more closely assiciated w/ psychological factors).
Name some Peptides that Modulate food intake.
1)Increase food intake:
NPY(hypothalamus), Orexins(hypothalamus), Galanin(hypothalamus), Melanin-concentrating hormone(MCH)(hypothalamus), Ghrelin(stomach).

2)Decrease food intake:
CCK(small intestine; neurons), Corticotropin-releasing hormone(CRH)(hypothalamus), alpha-MSH(hypothalamus), Cocaine-and-amphetamine-regulated transcript (CART)(hypothalamus), GLP-1 (intestines), PYY3-36(intestines).
Explain the NaCl secretion by colonic crypt cells(fig21-29).
1)Na+ and Cl- enter by cotransport from intersitial fluid into the crypt cell.
2)Cl- enters lumen of colonic crypt through CFTR channel.
3)Na+ is reasborbed into the interstitial fluid(ATP between and K+ going in from interstitial to crypt cell).
4)Negative Cl- in lumen attracts Na+ by paracelular pathway. Water follows.
The digestive system, like the renal system, plays a key role in ____ in the body.
mass balance
Regulation of GI tract function illustrates the complex interactions that take place b/w ___ and ____ systems and the protective function of the ____.
2)neural control
3)immune system
What does the GI tract move from the external to internal environment?
nutrients, water and electrolytes
What are the four processes of the digestive system? Elaborate on each briefly.
1)digestion-chemical and mechanical breakdown of foods into absorbable portions.
2)absorption-transfer of substances from the lumen of the GI tract to the ECF.
3)motility-movement of material through the GI tract.
4)secretion-transfer of fluid and electrolytes from ECF to lumen or the release of substances from cells.
The GI tract contains the largest collection of lymphoid tissue in the body. what is it named?
"gut-associated lymphoid tissue" (GALT)
What is Chyme?
a soupy substance made as ingested food is broken down by mechanical and chemical digestion.
Food entering the digestive system passes through what passages?
mouth, pharynx, esophagus, stomach(fundus, body, antrum), small intestine(duodenum, jejunum,ileum),large intestine, rectum, and anus.
What do the salivary glands, pancreas, and liver add?
exocrine secretions w/ enzymes and mucus to the lumen.
p661, fig21-2a
What four layers does that wall of the GI tract consist of?
1)inner mucosa
2)middle submucosa
3)smooth muscle layer
4)outer serosa
p664-665, fig21-2d
What does the mucosa consist of?
epithelium, lamina propria, lymphoid tissue, and the muscularis mucosa. Small villi increase the surface area, as do microvilli on the cells.
The submucosa contains what?
blood and lymph vessels, and the submucosal plexus of the enteric nervous system.
The outer wall consists of two layers of what? What lies b/w them?
circular and longitudinal muscle layers, the myenteric plexus b/w them.
What does motility do?
moves food from mouth to anus and mechanically mixes food.
How are GI smooth muscles connected? What type of contractions are exhibited by them?
1)electrically connected by gap junctions.
2)some segments are tonically contracted but others show phasic contractions.
Intestinal muscle exhibits what type of potential? what happens in its basic process?
spontaneous slow wave potentials, which fire action potentials and contract when the threshold is reached.
Where do slow waves originate?
In the interstitial cells of Cajal.
What moves remnants from the upper GI tract to the lower regions between meals.
the migrating motor complex.
Explain Peristaltic contractions.
they are progressive waves of contraction, mediated by the enteric nervous system and altered by hormones, paracrines, and neurocrines.
What are Segmental contractions primarly?
mixing contractions.
About 2 liters of fluid perday enter the GI tract through the mouth and another 7L secreted by the body. How much of this volume is reabsorbed?
nearly all
Digestive enzymes are secreted by ...?
exocrine glands or by gastric and intestinal epithelium. some enzymes are secreted as zymogens(inactive proenzymes in the digestive system).
Mucus from mucous cells and goblet cells (single exocrine cells)forms what?
protective coating and lubricates contents of the gut.
The digestive system secretes and absorbs what?
water and ions
Most nutrient absorption takes place where?
in the small intestine
Amylase and Disaccharidases do what?
1)digests starch to maltose
2)digest disaccharides to monosaccharides
Endopeptidases (proteases) and Exopeptidases do what?
1)break proteins into smaller peptides.
2)remove amino acids from peptides.
Fat digestion requires ..?
lipase and colipase
Why is the enteric nervous system called the little brain ?
b/c it can integrate info w/o input from the CNS.
Where are the short reflexes and the long reflexes originated and integrated?
1)within the enteric nercous system, integrated there.
2)within or outside the enteric nervous system, integrated in the CNS.
Digestive hormones are divided into to portions. Name them and give the examples.
1)Gastrin family:gastrin, cholecystokinin
2)secretin family:secretin, vasoactive intestinal peptide, glucose-dependent insulinotropic peptide, and glucagon-like peptide 1
3)hormones that dont fit into either of the 2 families:motilin(increases gastrointestinal motility and stimulates the production of pepsin)
GI peptides do what and stimulates them?
1)excite or inhibit motility and secretion
2)ingestion of food
GI paracrines, like histamine, interact with what?
hormones and neural reflexes.
In the cephalic phase (digestive reflexes triggered by stimuli received in the brain)of digestion, what initiates GI reflexes?
sight, smell, or taste of food
Mechanical digestion begins with what? Elaborate briefly.
with chewing, or mastication. Saliva moistens and lubricates food. Salivary amylase digests carbohydrates.
Swallowing, or deglutition,is a reflex integrated by what?
a medullary center
What does the stomach do in general terms?
digests proteins and fats and regulates movement of chyme into the intestine.
What does the stomach secrete?
mucus and bicarbonated from mucous cells, gastric acid from parietal cells, pepsinogen from chief cells, somatostatin from D cells, histamine from ECL cells, and gastrin from G cells.
Acid in the intestine, CCK, and secretin function to do what?
delay gstric emptying
Intestinal enzymes are part of the ____. Some pancreatic enzymes need to be activated after secretion.
brush border
The pancreas secretes a watery____soln that neutralizes gastric acid.
Epithelial cells secrete ___ using the cystic fibroses transmembrane regulator, or CFTR chloride channel.
Bile from the liver contains what?
bile salts, bilirubin, and cholesterol.

Glucose absorption uses what? and Fructose uses what?
1)Na+ glucose symporter and GLUT2 transporter.
2)GLUT5 transporter
Amino acids are absorbed by?
Di- and tripeptides are absorbed using?
Some larger peptides are absorbed intact by?
1)Na+ dependent cotransport.
2)H+ dependent cotransport
Fat digestion requires what to do what? As enzymatic and mechanical digestion proceed, fat droplets form what?
2)emulsifies fat
Fat absorption occurs primarly by ...?
simple diffusion
What is the process that takes Monoglycerides and fatty acids to the lymph?
they reassemble into triglycerides in the intestinal cell, then combine with cholesterol and proteins into chylomicron, which are absorbed into the lymph.
Eplain how Fat-soluble vitamins, water-soluble vitamins, vitamin B12, and minerals are absorbed.
1)along with fats
2)by mediated transport
3)requires intrinsic factor (protein secreted by gastric parietal cells)secreted by the stomach
4)by active transport
Most volume reabsorption takes place in the?
small intestine. the large intestine concentrates the 1.5L of chyme that enter it daily.
Undigested material in the colon moves forward by ?
mass movement
The defecation reflex,a spinal reflex subjec to higher control, is triggered by what?
sudden distension of the rectum.
Colonic bacteria use fermentation to do what?
digest organic material
Cells of the colon can reabsorb or secrete fluid. If it is not modulated, what can happen?
What do protective mechanism of the GI tract include?
acid, mucus, vomiting and diarrhea, increased salivary flow
M cells do what?
sample gut contents and present antigens to cells of the GALT like macrophages and lymphocyes
Where is vomitting integrated?
in the medulla
The PH at which diff digestive enzymes function best reflects the location where they are most active. Therefore, enzymes that act in the stomach work well at what type of PH?
Most nutrient absorption takes place in the ?
small intestine
Like secretion, absorption of nutrients and ions across the GI epithelium uses many of the same transport proteins that the ____uses.
kidney tubule epithelium
The complex carbohydrates we eat include ...?
cellulose, glycogen, and starch
what enzymes are responsible for the first and last steps of breakdown of carbohydrates before absorption?
Amylase (found in saliva, pancreas)breaks it down to maltose and other disaccharides. Disaccharidases (like maltase, lactase,sucrase-found in intestine)are responsible for the breakdown to monosaccharides.
What enzymes are responsible fore the breakdown of proteins like the Interior peptide bonds, the Terminal peptide bonds, that working at the NH2-terminal end, and that working at the COOH-terminal end?
1)Endopeptidases (in stomach, intestine, pancreas)breaks it into peptides.
2)Exopeptidases (in stomach, intestine,pancreas) breaks it into amino acids.
What enzymes are responsible for the break down of fats like the Triglycerides and the Phospholipids?
1)Lipase(in mouth, stomach, pancreas) breaks it into Monoglyceride and free fatty acids.
2)Phospholipase(in pancreas) breaks it into Lysophospholipid and free fatty acids).
Most proteins are ingested as?
proteins or large polypeptides.
Which proteins are the least digestible? Which are the most?
1)Plant proteins
2)Egg protein (85-90%)
Between 30-60% of the protein found in the intestinal lumen comes not from ingested food but from the ...?
sloughing of dead cells and from protein secretions such as enzymes and mucus.
The enzymes for protein digestion are classified into what two broad groups?
1)Endopeptidases(proteases):attack peptide bonds in the interior of the amino acid chain and make smaller peptide fragments from a long chain. They are secreted as inactive proenzymes from epithelial cells in the stomach, intestine, and pancreas.
2)Exopeptidases: release single amino acids from peptides by chopping them off at the ends(digesting terminal peptide bonds), one at a time.
Nearly 90% of our fat calories come from..?
triglycerides b/c they are the primary from of lipid in both plants and animals.
Most lipids are not water-soluble, making fat digestion complicated. Enzymatic fat digestion must be helped by ?
nonenzymatic secretions that help make fat globules into smaller portions.
Explain the digestion of nucleic acid polymers DNA and RNA, which dont make up a significant part of most diets.
digested by pancreatic and intestinal enzymes: into component nucleotides->bases->monosaccharides. Bases absorbed by facilitated diffusion, monosaccharides absorbed by facilitated diffusion and secondary active transport (like other simple sugars).
What does the "dumping syndrome" often experienced after surgery point out?
The lack of regulation that is usually performed by the stomach, the least obvious aspect of digestion is probably the storage function of the stomach(upper stomach).
Enhanced gastric motility during a meal is mostly under neural control and is stimulated by ?
distension of the stomach.
Chemical digestion in the stomach is done by ?
Enzymes and HCl(gastric acid):
HCl:kills pathogens, denatures proteins(destroys their teriary structure by breaking disulfide and H bonds, to allow enzymes in to do their work on the peptide bonds).
Pepsin:protease that carries out the initial digestion of proteins, esp collagen(in meat).
Gastric Lipase:co-secreted w/ pepsin and begins fat digestion, aided by lingual lipase(~10% fat digestion happens in the stomach).
Carbohydrate digestion that began in the mouth continues in the stomach until ?
mixing exposes the amylase to the gastric acid-salivary amylase is inactivated at low PH.
Elaborate on the Parietal cells.
deep in the gastric glands, secrete HCl, which in the stomach averages 1-3 liters a day and can create a luminal PH of l. Cytoplasmic pH of the parietal cell is roughly 7.2, meaning that the cells are pumping H+ against a tremendous gradient. In the acid secretion process, the H+ comes directly from the water. An H+-K+-ATPase pumps the H+ into the lumen. Cl- moves into the lumen via an open chloride channel. The OH- from water combines with CO2 to form HCO3-, which is absorbed into the blood.

Parietal cells also secrete a protein called "intrinsic factor" which complexes with B12 and is essential for B12 absorption in the intestine. B12 deficiency causes pernicious anemia(erythropoiesis-redbloodcellsynthesis is severely diminished).
The long reflexes that begin in our cephalic brain creat a feedforward response that is known as ..?
the cephalic phase of digestion. neurons in the medulla oblongata are activated. the medulla then sends an efferent signal through autonomic neurons to the salivary glands and through the vagus nerve to the enteric nervous system. In response, the stomach, intestine, and accessory glandular organs begin secretion and increase motility in anticipation of food.
Salivary secretion is under what control and begins chemical digestion with the secretion of what?
1)autonomic control
2)salivary amylase (activated by Cl-)and a bit of lipase.
Small nonsalivary glands in the mouth secrete a second digestive enzyme called?
lingual lipase, not significantly contributive to fat digestion in the mouth but remains active in the stomach.
When is the stimulus for deglutition (swallowing) created?
When the bolus is pushed against the soft palate and back of mouth with the tongue. Sensory input to a swallowing center in the medulla oblongata begins the reflex.
Explain the swallowing movements from the mouth to the stomach.
=>First, the epiglottis folds over the opening of the larynx and simultaneously, respiration is inhibited and the upper esophageal sphincter relaxes as the bolus enters the esophagus. waves of peristaltic contraction and gravity push the bolus down to the stomach. The lower end of the esophagus lies just below the diaphragm and is separated from the stomach by the lower esophageal sphincter(not a true sphincter but a region of higher muscle tension acting as a barrier), which is does not stay contracted, gastric acid and pepsin can irritate the esophagus(hearburn). The walls of the esophagus expand during inspiration, when the intrapleural pressure falls. Expansion creates subatmospheric pressure in the esophageal lumen that can literally suck acidic contents out the of stomach if the sphincter is relaxed.
Digestive activity in the stomach begins with ?
the long vagal reflex of the cephalic phase, before food even arrives. When the stomach receives food, stimuli in the gastric lumen initiate a series of short reflexes (the gastric phase of digestion).
In gastric phase reflexes, what activate endocrine cells and enteric neurons?
the presence of peptides or amino acids in the lumen.
What influence motility and secretion?
Hormones, neurocrines and paracrines.
Carbonic Anhydrase is?
Enzyme that catalyzes the conversion of CO2 and water into carbonic acid.
Chief cells in the gastric glands do what?
secrete the inactive enzyme pepsinogen. Pepsinogen is cleaved to active pepsin in the lumen of the stomach by the action of H+.
D cells, closely associated w/ the parietal cells, secrete..?
the paracrine somoatostatin.
Enterochromaffin-like (ECL) cells secrete the..?
paracrine histamine.
G cells, deep in the gsatric glands, secrete the hormone ..?
gastrin, it's release stimulated by the presence of amino acids and peptides in the stomach, by distension of the stomach, and by nervous reflexes mediated mediated by gastrin-releasing peptide. Coffee is a good stimulant of gastrin release. Gastrin release is inhibited by somatostatin and by a luminal pH below 1.5.
Mucous cells in the neck of the gastric glands secrete ?
both mucus and bicarbonate. they protect from autodigestion.
Parasympathetic neurons from the vagus nerve stimulate G cells to ..?
release gastrin in the blood.
Gastrin promotes ...?
acid release, both directly and indirectly by stimulating histamine release.
Histamine is released how and does what?
1)from ECL cells in response to gastrin and acetylcholine from the enteric nervous system.
2)it diffuses to its target, the parietal cells and stimulates acid secretion by combining w/ histamine H2 receptors on parietal cells.
Acid in the lumen stimulates and does what?
stimulates pepsinogen release from chief cells thru a short reflex. in the lumen, acid converts pepsinogen into pepsin, and protein digestion begins. acid also triggers somatostatin release from D cells, which acts in a neg feedback fashion to inhibit gastric acid, gastrin, and pepsinogen secretion.
Excess acid secretion is an uncommon cause of peptic ulcers. What are the two most common causes?
nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and Helicobacter pylori (creates inflammation of the gastric mucosa).
When does the intestinal phase of digestion begin?
As the chyme enters the small intestine.
The reflexes that regulate the delivery rate from the stomach are?
neural and hormonal
The presence of acidic chyme in the duodenum does what?
Releases the hormone secretin which then inhibits acid production and gastric motility, slowing gastric emptying. It also stimulates production of pancreatic HCO3- to neutralize the acidic chyme that has entered the intestine.
If a meal contains fats, then what happens?
the hormone CCK is secreted into the bloodstream. CCK also slows gastric motility and acid secretion (fat digestion is slower than either protein or carbohydrate).
If a meal contains carbohydrates, then what is released?
the hormone GIP. in large doses GIP inhibits gastric acid secretion.
How does osmolarity play into the intestinal phase of digestion?
The mix of acid, enzymes, and digested food in chyme usually forms a hyperosmotic soln. Osmoreceptors in the the intestine wall are sensitive to the osmolarity of the entering chyme. when stimulated by high osmolarity, they inhibit gastric emptying in a reflex mediated by some unknown blood-borne substance.
what is the net result of all three phases of gastric function?
the digestion of proteins in the stomach by pepsin; the formation of chyme by the action of pepsin, acid, and peristaltic contractions; and the controlled entry of chyme into the small intestine so that further digestion and absorption can occur.
Absorption in the intestine thru...? elaborate.
transporting abosrptive cells (eterocytes). most digested nutrients are absorbed into the blood of the hepatic portal system. the specialized region of the circulation has two capillary beds: one set to pick up absorbed nutrients at the intestine and the other set to deliver the nutrients directly to the liver.
Digested fats, though, go into the lymphatic system instead of the blood, b/c the intestinal capillaries have a basement membrane that most lipids cannot cross.
GIP acts in a feedforward fashion to initiate ..?
insulin release from the endocrine pancreas.
Pancreatic enzymes enter the intestine in a watery fluid and are active in the lumen. How about intestinal enzymes?
They remain bound to the apical membranes of brush border cells, anchored by transmembrane protein "stalks." The brush border enzymes include peptidases, disaccharidases, and a protease called enteropeptidase.
Name a few of the enzymes secreted by exocrine cells in the pancreas. What is the activation process needed by some of them?
1)pancreatic amylase, lipase, and several proteases and exopeptidases.
2)it is a cascade. Brush border enteropeptidase converts inactive trypsinogen to active trypsin, which then converts the other inactive pancreatic enzymes to their active forms.
The pancreatic reflex is mediated both by ..?
the nervous system and the hormone CCK.
In the pancreas, bicarbonate production req's high levels of the enzyme..?
carbonic anhydrase.
With regards to bicarbonate secretion, a basolateral Na+-H+ antiporter transports H+ into the ECF, helping balance the HCO3- put into the blood by parietal cells. Sodium secretion in these tissues is a passive process, driven by, created by...?
electrochemical gradients, created by epithelial cell transport of Cl- from the ECF to the lumen. The basolateral membrane contains a Na+-K+-2Cl- symport protein that brings Cl- into the cell. On the apical membrane, a gated channel known as the CFTR chloride channel, allows Cl- to enter the lumen. The Cl- then re-enters the cell in exchnge for HCO3-. The movement of neg charge from the ECF to the lumen attracts pos Na+, which moves down its electrochemical gradient by the paracellular pathways, which creates an osmotic gradient, so water follows by osmosis. net result is secretion of watery sodium bicarbonate soln from the pancreas.
What is bile?
a non-enzyme soln secreted from hepatocytes, or liver cells. The key components of bile are bile salts for fat digestion, bile pigments (like bilirubin), that are the waste products of hemoglobin degradation, and cholesterol. Bile acids are steroids that combine w/ amino acids to form bile salts. Bile salts are detergents w/ polar side chains that allow them to interact w/ both lipids and water, creating water-soluble droplets. Bile is secreted into hepatic ducts that lead to the gallbladder, to be stored and concentrated. During a meal, contraction of the gallbladder sends bile into the duodenum via the common bile duct, along w/ a water soln of bicarbonate and digestive enzymes from the pancreas.
What is gallbladder contraction stimulated by?
What are the end produts of carbohydrate digestion?
glucose, galactose, and fructose.
If glucose is the major metabolic substrate for aerobic respiration, what does the intestinal epithelial cell structure use for their own metabolism?
the amino acid glutamine as their main energy, letting absorbed glucose to pass unaltered into the bloodstream.
Once inside the epithelial cell, di- and tripeptides have two major possible fates and one minor...?
1)most are digested by cytoplasmic peptidases into amino acids, then transported across the basolateral membrane and into circulation.
2)undigested ones are transported intact across the basolateral membrane.
3)some larger peptides are absorbed by transcytosis after binding to a receptor on the luminal surface of the epithelium. *peptide fragments then may act as antigens! allergies![it has been shown that in newborns, large peptide absorption takes place primarly in intestinal crypt cells]
Define 'enteric'
of, relating to, or being w/in the intestine.
Fats enter the small intestine from the stomach in the form of a coarse emulsion created by ..?
acid lipase digestion and mechanical mixing.
Where and what coat the coarse emusion of fats to help stabilize the droplets?
In the duodenum, by bile salts.
Digestion of triglycerides in the emulsion is carried out by ...? It is unable to penetrate the bile salt coating by itself, and so requires assistance of ..., a protein cofactor secreted from the pancreas to diplace some of the bile salts.
1)pancreatic lipase
Lipase digests triglycerides into?
Monoglycerides and free fatty acids.
As enzymatic and mechanical digestion cont, fatty acids, bile salts, monoglycerides, phospholipids, and cholesterol form ...?
Small cylindrical micelles.
Fats are absorbed mostly by simple diffusion across the apical membrane of the intestinal cells. Explain how fatty acids, monoglycerides, and cholesterol are moved into the epithelial cells.
1)At the brush border surface, fatty acids and monoglycerides move from the micelles and into the epithelial cell.
2)Cholesterol is transported on a specific, energy-dependent membrane transporter.
Where do the monoglycerides and fatty acids go, once in the cytoplasm of the epithelial cell? What happens then?
To the smooth endoplasmic reticulum, resynthesized into triglycerides. These then mix with cholesterol and proteins into large droplets called chylomicrons.
Why do chylomicrons have to be packaged into secratory vesicles and leave the epithelial cell by exocytosis to the EC space?
B/c they are so large.
Why are chylomicrons absorbed from the EC space into the lymph vessels of the villi, the lacteals?
B/c of their large size, they cannot cross the basement membrane surrounding the capillaries.
What happens to the chylomicrons after they are absorbed into the lymph vessels?
They pass through the lymphatic system and then enter the venous blood right b4 it flows into the heart.
What happens to shorter fatty acids (less than 10 carbons) that are not assembled into chylomicrons?
They can cross the capillary basement membrane and go right into the blood.
What happens to the bile salts that get carried along the process of fat digestion?
They are left unaltered, all the way to the terminal section of the small intestine, the ileum, they meet cells that transport them back into the hepatic portal circulation, where they then return to the liver, are taken back up into the hepatocytes, and resecreted.
Recirculation of bile silts is essential to maintain fat digestion b/c of the body's pool of bile salts will cycle how many times for ea meal?
2-5 times.
What happens to the bilirubin and other wastes secreted in bile?
They are left unabsorbed and pass into the large intestine for excretion.
In general, how are the fat-soluble vitamins (A,D,E,and K) absorbed?
Along with the fats.
How are the water-soluble vitamins absorbed? What is the major exception?
1)By mediated transport.
2)vitamin B12.
What is vitamin B12 also known as?
What are our sources for vit. B12? When does the intestinal transporter for it recognize it? Where is the transporter found?
1)made by bacteria but most of our dietary supply comes from seafood, meat, and milk products.
2)only when it is complexed with intrinsic factor.
3)only in the ileum.
Mineral absorption usually occurs by what type of transport?
Active transport.
Iron and calcium (minerals)are two of the few substances whose intestinal absorption is linked to what?
Their concentration in the body.
For both the minerals iron and calcium, a decrease in body concentration leads to ?
Enhanced uptake at the intestine.
Oral rehydration therapy usually includes sugar(sucrose)and table salt. The salt enhances intestinal absorption of glucose. how?
Glucose must be cotransported w/ Na+.
A total of 9 liters enter the lumen of the small intestine each day. What are the sources for this? How much of it is reabsorbed from the small intestine?
1)~5.5L of luid and food enter the small intestine. 3.5L of pancreatic and intestinal secretions are added.
The absorption of organic nutrients takes place in the .... and creates an osmotic gradient for water absorption.
duodenum and jejunum
One apical transporter carries Cl- w/ ... Another is a ....symport that brings all three types of ions into the cell. K+ and Cl- also pass through the the small intestine.
3)cell junctions
By the end of the ileum, ~1.5L of unabsorbed chyme remain, passing into the...? Talk about the ileocecal valve's contraction.
1)large intestine
2)It is a tonically contracted region of muscularis that narrows the opening b/w the ileum and the cecum. The ileocecal valve relaxes ea time a peristaltic wave reaches it and relaxes at the same time that food leaves the stomach as part of the gastroileal reflex.
Name the seven regions of the larger intestine.
1)cecum (a blind end pouch).
appendix (finger-like projection at the end of the ventral end of the cecum)
2)ascending colon
3)transverse colon
4)descending colon
5)sigmoid colon
6)rectum(12cm terminal section
7)anus (separates rectum from external environ.
How is the wall of the colon different from that of the small intestine? What about the mucosa and epithelial cells?
1)Its muscularis has an inner circular layer but a discontinuous longitudinal muscle layer that is concentrated into three bands called the 'tenia coli.'
2)Contractions of the tenia pull the wall into bulging pockets called 'haustra.'
3)The mucosa of the colon has two regions, like the small intestine. The luminal surface lacks villi and appears smooth, composed of transporting epithelial cells called colonocytes and mucus-secreting goblet cells. The crypts contain stem cells that divide to produce new epithelium, goblet cells, endocrine cells, and maturing colonocytes (crypt cells).
Describe the two types of motility in the large intestine.
1)segmental contractions
2)unique coonic contraction-'mass movement.'This contraction tightens the diamter of a pice of the colon and moves bolus forward, which occur 3-4 times a day, and are associated with eating and distension of the stomach through gastrocolic reflex.
What is responsible for the sudden distension of the rectum that triggers defecation?
mass movement, that distends the rectum.
How does defecation resemble urination?
It is a spinal reflex triggered by distenstion of the organ wall.
What reactions push material in the rectum toward the anus? What about the actual act of defecation?
The smooth muscle anal sphincter relaxes and peristaltic contractions in the rectum. If appropriate, the external sphincter, under voluntary control, is consciously relaxed, aided often by conscious abdominal contractions and forced expiratory movements against a closed glottis, the 'valsalva maneuver.'
What is one treatment for constipation?
glycerin suppositories, which attracts water to help soften the feces.
We know that the bacteria in the first part of the colon break down significant amounts of undigested complex carbohydrates and proteins through...? What do the end products include?
2)lactate, short-chain fatty acids like butyric acid, vitamin K, flatus-intestinal gases (like H2S).
What do the colonocytes used as their preferred energy substrate?
The fatty acids from fermentation by bacteria in the colon.
What happens in the colon as far as secretion and absorption of fluid and electrolytes?
The colonocytes on the mucosal surface absorb NaCl via different membrane transporters. Simultaneously, K+ is secreted at the apical membrane, influenced by aldosterone, which promotes Na+ reabsorption and K+ secretion, like it does in the kidney. The reaborption back into the ECF is done by Na+-K+-ATPase (K+ comes in the cell).[fig21-28, p687]

Crypt cells in the colon (and sm intestine) secrete Cl into the lumen. By secondary active transport into the crypt cell(cotransport)with Na+, Cl-then exits into the lumen via the apical CFTR channel. Na+ and water follow by the paracelular pathway b/c of the neg Cl- in the lumen(the Na+ had been reabsorbed out of the crypt cell to the interstitial fluid (by Na+-K+-ATPase), where the water follows it to the lumen via the paracellular pathways). The secreted fluid mixes w/ mucus secreted by goblet cells to help lubricate the contents of the gut.[fig 21-29, p688]
What are some causes of diarrhea?
1)if normal intestinal water absorption mechanisms are disrupted
2)osmotic diarrhea (unabsorbed osmotically active solutes that 'hold' water in lumen)-caused by things like undigested lacose and sorbital, olestra.
3)secretory diarrheas-when bacterial toxins like the cholera toxin enhance colonic Cl- secretion. The excessive fluid secretion and increased motility causes the diarrhea.
What enhances Na+ absorption when it comes to treating diarrhea?
For most nutrients is or isnt absorption regulated ("what you eat is what you get")? Motility and secretion are continuously regulated.
What directs the use and storage of digested nutrient that have been absorbed and have reached the cells?
cellular metabolism, some of the same chem signal molecules are used in both altering digestive motility and secreation, and control of metabolism.
What are the three pairs of salivary glands?
1)sublingual under the tongue
2)submandibular under the mandible or jawbone
3)parotid that lie near the hinge of the jaw
How much food can the stomach hold when fully expanded?
2-3 Liters
what are the three divisions of the stomach?
1)upper fundus
3)lower antrum
What are the three divisions of the small intestine?
1)duodenum (~25 cm)

the last two total ~260cm
What are the two accessory glandular organs that aid by their exocrine secretions the process of digestion (carried out by intestinal enzymes)? Where do their product enter?
Pancreas and liver, through the intial section of the duodenum via a common duct . A tonically contracted sphincter of oddi holds the fluids from entering until meal times.
Geographically, where are the liver and pancreas located?
1)bulk on left side, above the stomach
2)generally centered, under the stomach
What is "plica?" how does it relate to the small intestine?
A fold or ridge, as of skin, membrane, or shell-referring to the villi.
In the stomach, surface area is increased by invaginations called..?
gastric glands
What is the lamina propria?
the highly vascular layer of connective tissue under the basement membrane lining a layer of epithelium, holding them together. It contains nerved fibers and small blood and lymph vessels into which absorbed nutrients can pass. It also contains wandering immune cells like lymphocytes and macrophages.
What is the Muscularis mucosae?
The thin layer of smooth muscle in the GI mucosa, b/w the Mucosa and submucosa. Contraction of this thin layer may somehow alter the effective surface area for absorption by moving the villi back and forth, like the waving tentacles of a sea anemone.
What is the Mucosa?
It lines the villi in the intestinal tract. It is the inner lining of the gastointestinal tract, created from a single layer of epithelial cells, the lamina propria, and the muscularis mucosae. It also holds the Peyer's patches.
What is the submucosa?
the middle layer of the gut wall and stomach, composed of connective tissue with larger blood (artery and vein)and lymph vessels. It also contains the submucosal plexus, one of the two major nerved networks of the enteric nervous system, which is a unique division of the nervous system that helps coordinate digestion function.
What is the circular muscle?
It lies b/w the submucosa and the longitudinal muscle, its contaction responsible for the decreasing of diameter of the lumen.
What is the longitudinal muscle?
It lies b/w the circular muscle (and myenteric plexus) and the serosa. Its contraction is responsible for the shortening of the tube.
What is the serosa?
a connective tissue membrane that surrounds the entire digestive tract. It is a continuation of the peritoneal membrane (peritoneum) that lines the abdominal cavity.
The peritoneum forms sheets of mesentary that do what?
hold the intestines in place so that they dont become tangled as they move.
What is the Myenteric plexus?
It lies b/w the two muscle layers of the digestive tract (circular and longitudinal). It is the second major nerve network of the enteric nervous system.
What does the stomach have b/w the circular muscles and the submucosa?
an incomplete layer of oblique muscle.
What do the villi consist of?
The lamina propria (or really, the mucosa), the crypt lumen, lymph vessels, artery and vein, capillaries. Goblet cells and absorptive cells line the epithelium of the villi and endocrine cells line inside the crypt lumen. The absorptive cell, consists of the brush border on its surface, which are made of microvilli on their surface.
What does the outer section of the gut wall consist of?
the layers of smooth muscle that are covered with the connective tissue serosa.
The wall of the mucosa is crumpled into folds known as the stomach and the sm intestine.
2)plicae (intestinal mucosa also projects into the lumen the villi).
Some of the deepest invaginations of the surface of the mucosa form..?
secretory submucosal glands that open into the lumen through ducts.
What is the most variable feature of the GI tract?
the epithelial cells that line the lumen, changing from section to section.
The epithelial cells include what?
1)transporting epithelial cells-move ions and water into the lumen and absorb ions, water , and nutrients. 2)endocrine and exocrine secretory cells-At the mucosal(apical) surface, secretory cells release enzymes, mucus, and paracrines into the lumen. On their serosal (basolateral) sides, they secrete hormones or paracrines into the interstitial fluid, where they act on neighboring cells.
3)GI stem cells-they are rapidly dividing undifferentiated cells in the crypts or glands that continuously produce new epithelium. Newly formed cells are pushed up toward the luminal surface of the epithelium. The average life-span of a GI epithelial cell is only a few days.
The cell-to-cell junctions that tie the GI epithelial cells together vary how?
In the stomach, the junctions form a tight barrier so that little can pass b/w. In the sm intestine, the junctions are not as tight, are leaky, b/w some water and solutes can be absorbed by going b/w the cells instead of just through them. These junctions have plasticity, regulated to some extent by the nutritional status of the body.
In a living person, the GI tract from mouth to anus is about how long and about how much of this consists of the large and small intestines?
2)~13ft ranging from 1-3in in diameter
Different regions of the GI tract exhibit different types of contraction. what are they?
1)tonic contractions are sustained for minutes or hours. they occur in some smooth muscle sphincters as well as in the anterior portion of the stomach.
2)phasic contractions with contraction-relaxation cycles that last only a few sec, occur in the posterior region of the stomach and the small intestine.

the cycles of smooth muslce contraction are associated w/ spontaneous cycles of depolarization and repolarization known as slow wave potentials.
how do slow wave potentials differ from myocardial pacemaker potentials? How does it work?
they do not reach threshold w/ ea cycle. one that does not will not cause a contraction in the muscle fiber. when one does reach threshold, voltage-gated Ca++ channels in the muscle fiber open, Ca++ enters, and the cell fires one or more action potentials. The depolarization phase is due to Ca++ entry into the cell, and initiation of muslce contraction is too. Contraction of smooth muscle is graded according to how much Ca++ enters the fiber. The higher the amp of the slow wave, the longer the duration of the contraction. Both amp and duration can be modified by neurotransmitters, hormones, or paracrines. Slow wave freq varies by region of the digestive tract, ranging from 3waves/min in the stomach to 12waves/min in the duodenom. Slow waves originate in modified smooth muscle cells called the interstitial cells of Cajal and pass into smooth muscle cells via gap junctions.
In addition to being associated with the slow waves, the interstitial cells of Cajal are related to ..?
enteric neurons and may act as an intermediary b/w the neurons and smooth muscle.