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physiology and pharmacology > absorption and secretion > Flashcards

absorption and secretion Flashcards

1
Q

what controls GI secretions

A

-hormonal, paracrine and neurocrine actions

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2
Q

what is GI secretion like

A

-> 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

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3
Q

what are the stomach 3 main functions

A

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

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4
Q

what are the distal secretions in the stomach

A

-gastrin
-somatostatin
-pepsinogens

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5
Q

what are the proximal secretions in the stomach

A

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

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6
Q

stomach secretory cells: what are the oxyntic glands

A

-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

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7
Q

stomach secretory cells: pyloric glands

A

-antrum
-G cells- gastrin hormone

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8
Q

what is the function of the secretion of epithelial cells

A

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

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9
Q

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

A

-secretion: histamine
-increases parietal cell HCl secretion

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10
Q

what is the function of the secretion of parietal cells

A

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

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11
Q

what is the function of the secretion of mucous neck cells

A

-mucus protects epithelia from other agents harmful to stomach lining

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12
Q

what is the function of the secretion of chief cells

A

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

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13
Q

what is the function of the secretion of enterchromaffin cells

A

-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

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14
Q

what is the function of the secretion of D cells

A

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

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15
Q

what is the function of the secretion of G cells

A

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

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16
Q

what is the structure of a parietal cell

A

-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

17
Q

what happens in parietal cells during gastric acid secretion

A

-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

18
Q

how is HCl secretion regulated

A

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

19
Q

why does fluid and electrolyte absorption vary along the intestine

A

-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

20
Q

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

A

-for fluid balance and electrolyte homeostasis

21
Q

what’s transcellular movement like in the intestines

A

-usually active (across at leats one membrane)

22
Q

what is fluid movement coupled with in the intestines

A

-solute movement

23
Q

what’s paracellular movement like in the intestines

A

generally passive , determined by tight junctions

24
Q

what is the process of secretion in the intestines

A

-Na+K+Cl- cotransporter brings ions into cells from blood
-Cl- ion diffuses into the omen 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

25
Q

what happens in cellular transport (absorption) in the jejunum

A

-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)

26
Q

what happens in cellular transport (absorption) in the ileum

A

-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 men via Cl–HCO3- exchanger
-net effect- movement NaCl into the cel (absorbed into blood)
-ileum- net absorption of naCl (jejunum net absorption of NaHCO3)

27
Q

what happens in pancreatic secretion

A

-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

28
Q

what happens in absorption in the large intestine (colon)

A

-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

29
Q

where are products of hydrolysis absorbed

A

-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

30
Q

how does lipid absorption work

A

-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)

31
Q

how does carbohydrate absorption work

A

-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

32
Q

how does protein absorption work

A

-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

physiology and pharmacology (24 decks)

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