absorption and secretion

Question 1

what controls GI secretions

Answer 1

-hormonal, paracrine and neurocrine actions

Question 2

what is GI secretion like

Answer 2

-> 9L fluid/day, most of which is absorbed in small intestine, remainder in large intestine own lost in faeces
-integration of salt and water balance to regulate ECF volume and maintain pressure (for kidneys)
-regulation of ECF volume by adjustment of total body NaCl
-steady state: Na+ intake by GI tract= Na+ output by renal pathways

Question 3

what are the stomach 3 main functions

Answer 3

-secretion
-motor: regulates food intake/transit,( how much moves into SI) mixing, decrease of food particles- aggressive churning in the stomach
-humoral: gastrin, somatostatin

Question 4

what are the distal secretions in the stomach

Answer 4

-gastrin
-somatostatin
-pepsinogens

Question 5

what are the proximal secretions in the stomach

Answer 5

-HCl
-pepsinogens
-intrinsic factor
-mucins/bicarbonate ions
-many other secretions via endocrine cels

Question 6

stomach secretory cells: what are the oxyntic glands

Answer 6

-body
-epithelial cells- HCO3
-mucus neck cells- mucus
-parietal cells= HCl, intrinsic factor
-enterochromaffin-like cells- histamine
-cheif cells- pepsinogen
-enterochromaffin cells- serotonin, VIP, substance P
-D cells- somatostatin

Question 7

stomach secretory cells: pyloric glands

Answer 7

-antrum
-G cells- gastrin hormone

Question 8

what is the function of the secretion of epithelial cells

Answer 8

-secretion: HCO3-
-this combines with mucus to form a protective barrier and neutralises acid

Question 9

what is the function of the secretion of enterchromaffin-like cells

Answer 9

-secretion: histamine
-increases parietal cell HCl secretion

Question 10

what is the function of the secretion of parietal cells

Answer 10

-secretion: HCl, intrinsic factor
-this HCl activates pepsinogen (by hydrolysis) , this denatures proteins and inhibits pathogens
-intrinsic factor is required from B12 absorption

Question 11

what is the function of the secretion of mucous neck cells

Answer 11

-mucus protects epithelia from other agents harmful to stomach lining

Question 12

what is the function of the secretion of chief cells

Answer 12

-secretion: pepsinogen
-precursor of pepsin- cleavage results in digestion of protein
-these cells are hydrolytic

Question 13

what is the function of the secretion of enterchromaffin cells

Answer 13

-secretion: serotonin, VIP, substance P
-serotonin regulates motility and secretion
-VIP needed for motility, vasodilation and decreases HCl (dont want too much HCl as it causes stomach ulcers
-substance P needed for smooth muscle contraction

Question 14

what is the function of the secretion of D cells

Answer 14

-secretion: somatostatin hormone
-inhibits gastrin release (feedback mechanism)

Question 15

what is the function of the secretion of G cells

Answer 15

-secretion: gastrin hormone
-increases parietal cell HCl secretion, increases motility and increases pepsinogen release

Question 16

what is the structure of a parietal cell

Answer 16

-in resting: cytoplasmic pool of tubulovesicular membrane on apical side, and contains acid secreting H,K-ATPase
-stimulation induces cytoskeletal changes: fusion of tubovesicular and canalicular membranes, 50-100x increase in SA, microvilli appearance, insertion of H,K-ATPase pump and K+ and Cl- channels

Question 17

what happens in parietal cells during gastric acid secretion

Answer 17

-H2CO3 dissociates
-H+ secreted across apical membrane via H+-K+ATPase (Cl- follow via channel)
-HCO3- absorbed into blood via a Cl–HCO3- exchanger
-net secretion of HCl and absorption of HCO3-
-K is recycled into the lumen via the K channels

Question 18

how is HCl secretion regulated

Answer 18

-stimulation iof H+ secretion: histamine (ECL), ACh (vagus), gastrin (G cells)
-inhibition: low pH, somastatin, prostaglandins

Question 19

why does fluid and electrolyte absorption vary along the intestine

Answer 19

-SI +LI have similar structures- crypts of leibemkuhn
-SI + LI absorption fluid and electrolytes Na+, K+, Cl-, HCO3- via villus cells/surface epithelia respectively
-SI also absorbs hydrolysed food after digestion via luminal+ brush border enzymes
-crypt epithelial cells secrete fluid and electrolytes (protective- bacteria/ toxins)
-polar cells have apical and basolateral membranes separated by tint junctions
-transport- trancellular or paracellular

Question 20

why is net absorption in the intestine important across the entire length

Answer 20

-for fluid balance and electrolyte homeostasis

Question 21

what’s transcellular movement like in the intestines

Answer 21

-usually active (across at leats one membrane)

Question 22

what is fluid movement coupled with in the intestines

Answer 22

-solute movement

Question 23

what’s paracellular movement like in the intestines

Answer 23

generally passive , determined by tight junctions

Question 24

what is the process of secretion in the intestines

Answer 24

-Na+K+Cl- cotransporter brings ions into cells from blood
-Cl- ion diffuses into the lumen through Cl- channels in the apical membrane (closed but opens in response to ACh for an example)
-Na+ follows Cl- secretion passively (paracellular) - water drawn/ secreted following the movement/ secretion of NaCl

Question 25

what happens in cellular transport (absorption) in the jejunum

Answer 25

-all Na+ transcellular absorption via Na-K ATPase -low IC Na+- driving force for Na+ entry via Na+ channels or Na+-H+ ion exchanger and Na+ glucose cotransport (secondary active transport0 -luminal HCO3- increases Na absorption by stimulating the apical Na=H ion exchanger (H+ source H2CO3)

Question 26

what happens in cellular transport (absorption) in the ileum

Answer 26

-same transport mechanism as jejunum and the Cl--HCO3- exchanger and Cl- transporter ( no HCO3- movement across basolateral membrane) -H+ and HCO3- generated, H+ secreted into the lumen via the Na-H exchanger -HCO3- secreted into the lumen via Cl--HCO3- exchanger -net effect- movement NaCl into the cel (absorbed into blood) -ileum- net absorption of naCl (jejunum net absorption of NaHCO3)

Question 27

what happens in pancreatic secretion

Answer 27

-acinar cells secrete enzymes (CCK receptors) -centroacinar cells secrete isotonic aqueous portion, which are modified by ductal cells -HCO3- secreted into pancreatic juice by Cl-HCO3 exchnager -H+ transported into blood via NaH exchanger -net effect- secretion of HCO3- and absorption of H+ (acidification of pancreatic venous blood) -ductal cells receptors for CCK, ACh and secretin to up regulate production

Question 28

what happens in absorption in the large intestine (colon)

Answer 28

-synthesis of Na+ channels induced by aldosertone -increased Na+ channels causes an increase in Na+ entry ( apical membrane ) which causes an increase in Na+ across the basolateral membrane via NaK ATPase -this leads to an increase in K+ in cell leading to an increase in K+ secretion across apical membrane

Question 29

where are products of hydrolysis absorbed

Answer 29

-in the jejunum and ileum across brush borders -carbohydroases: alpha-amylase, alpha-dextrinase, maltase, sucrase, trehlase, lactase- products absorbed into villus blood -proteases: pepsin, trypsin, chymotrypsin, elastase, carboxypeptidases- product into villus blood -lipases and bile salts: products into lacteals within villus

Question 30

how does lipid absorption work

Answer 30

-pancreatic lipase and other lipases/ esterase, aided by bile salts complete lipid hydrolysis in duodenum and jejunum -products of lipid digestion: cholesterol, lysophosphoplipids, monoglycerides and free fatty acid solubilised in mixed micelles -micelle exterior lined with amphipathic bile salts -in the mucous gel layer lining the epithelial surface fatty acids become protonated and cross luminal surface of enterocyte by 3 methods: diffusion, incorporated into enterocyte membrane, carrier mediated transport -products re-esterified with free fatty acids in SER -packackged into chylomicrons -cross basolateral membrane and absorbed into lacteals (glycerol enters blood supply) -bile salts recycled (enterohepatic)

Question 31

how does carbohydrate absorption work

Answer 31

-3 end products of carbohydrate digestion are glucose, galactose and fructose -glucose and galactose enter epithelial cells by Na+ cotransport (SGLT1) against an electrochemical gradient , fructose uses facilitated difusion of GLUT5, through apical membrane -low concentration of Na+ inside cell, hence glucose and galactose and be cotransported, low conc due to Na+ leaving cell via Na+K+ ATPase through the basolateral membrane

Question 32

how does protein absorption work

Answer 32

-similar to carbohydrates as there is a Na+,K+ ATPase on the basolateral membrane, keeping Na+ levels inside the cell very low -this means AA can move in down the Na+ gradient through the apical membrane= this is called secondary active transport -dipeptides and tripeptides can move through apical membrane into cell using a H+ cotransporter, low H+ conc inside cell as H+ is being pumped out by the Na+H+ exchanger -peptidase will turn the dipeptides and tripeptides into AA