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module 104 - theme 1 > salt & water transport & their control > Flashcards

salt & water transport & their control Flashcards

1
Q

what does water & electrolytes provide medium for?

A
  • digestive processes within the GI tract
  • metabolic processes within the body absorption
  • replace daily loss of body fluids on sweat, urine, lungs and faeces
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2
Q

what is secretion/absorption of water and electrolytes regulated by?

A
  • gut luminal contents in absence of food electrolytes are primarily responsible for creation of osmotic in gut lumen
  • enteric and autonomic signals
  • endocrine hormones
  • immunogenic signals
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3
Q

how is the conservation of water and salts efficient?

A
  • bidirectional secretion and absorption occur across GI epithelium daily
  • secretions dominate the upper GI tract: saliva, bile, gastric and pancreatic juice, intestinal juice
  • facilitate movement along the GI tract, mixing with digestive enzymes, chemical reactions, nutrient absorption
  • absorption dominates overall: 98% ingested and secreted water and electrolytes are absorbed
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4
Q

why is the small intestine the primary site for absorption?

A
  • ingested and secreted water and electrolytes are predominantly absorbed in the small intestine
  • distinct secretion and absorptive profiles depend on variations in epithelial membrane transport proteins and permeability along the GI tract
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5
Q

what are the adaptations of the small intestine?

A
  • surface area: folds, villi, microvilli
  • villi lymphatic and blood vessels
  • enterocyte actin microfilaments rhythmically contract to move microvilli for maximum exposure to lumen contents
  • rapid response to chyme: hypertonic (osmosis of water into lumen to form isotonic chyme) and acidic (rapid increase in HCO3- rich secretions)
  • epithelium is more permeable than the large intestine
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6
Q

what are the adaptations of the large intestine?

A
  • no villi but surface is covered with crypts/intestinal glands
  • smaller role in transport of water and salts, bacterial microbiome role in protein digestion/vitamin synthesis
  • smaller role in digestion: nutrient absorption is limited
  • epithelium is less permeable than small intestine
  • has additional absorptive capacity for water and NaCl in exchange for K+ loss
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7
Q

how do non-polar hydrophobic molecules move across membranes?

A
  • molecules are small and uncharged polar molecules

- they diffuse down a concentration gradient

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8
Q

how do large and charged polar molecules move across membranes?

A

facilitated diffusion: required carrier or channel membrane proteins

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9
Q

how does primary active transport work?

A
  • hydrolysis of ATP provides energy to move ions against their electrochemical gradient
  • Na+/K+ ATPase pump
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10
Q

how does secondary active transport work?

A
  • uses gradient created by primary pump to move substances against its electrochemical gradient
  • symport
  • antiport
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11
Q

what does symport mean?

A

2 molecules in same direction

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12
Q

what does antiport mean?

A

2 molecules in opposite direction

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13
Q

how does the sodium potassium ATPase pump work?

A
  • 3 Na+ exported and 2 K+ imported against electrochemical gradient using energy from ATP hydrolysis by ATPase
  • keeps Na+ cytosol concentration low
  • important mechanism driving gut absorption
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14
Q

what are the principles of enterocyte transport?

A
  • polarised with an apical and basolateral memrbane
  • tight junctions provide a barrier to free flow of gut lumen contents
  • tight junctions more permeable in proximal small intestine
  • tonicity of chyme entering duodenum affects bidirectional fluid flux
  • occurs by trans cellular and paracellular routes
  • transcellular absorption may be against concentration gradient and require ATP
  • paracellular routes do not require energy
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15
Q

what are the 3 types of electrolyte transport?

A
  • passive
  • solvent drag
  • active
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16
Q

what is passive electrolyte transport?

A

down electrochemical gradient through ion channels or carriers or permeable tight junctions

17
Q

what is solvent drag electrolyte transport?

A

water follows Na+ gradient via osmosis taking other ions

18
Q

what is active electrolyte transport?

A

requires ATP, Na+/K+ ATPase pump depletes cellular Na+ and draws Na+ across apical membrane from gut lumen via channel or cotransporter

19
Q

how are water and electrolytes transported in the small intestine?

A
  • chyme contains water and key electrolytes Na+, K+, Cl0 and HCO3- from injected food and secretions of the GI tract
  • rapid osmotic equilibrium in the duodenum to form isotonic chyme (secretion of H2O into hypotonic chyme and absorption of H2O from hypotonic chyme)
  • jejunum absorbs Na+, K+, Cl- and H2O
  • ileum secretes HCO3-
  • Na+ and Cl- are considered, HCO3- secretion
20
Q

how are water and electrolytes transported in the large intestine?

A
  • smaller volume of chyme enters large intestine, 100-200ml is excreted
  • water and electrolytes are absorbed in proximal colon
  • tight junctions prevent back diffusion of ions into lumen allowing more compete NaCl absorption, limited loss in faeces
  • secretion of HCO2- and K+ occurs
21
Q

how is Na+ absorbed in the small intestine?

A
  • drives absorption of ions, organics, H2O
  • driven by basolateral active transport of Na+ into the interstitial space via Na+/K+ ATPase pump
  • dominance of mechanisms vary along the small intestine
22
Q

how doe mechanisms vary along the small intestine?

A
  • via passive diffusion
  • bia apical membrane carrier proteins
  • organic substrate Na/glucose and Na/amino acid co-transporters
  • Na/H antiport
  • Na/Cl symport (ileum)
23
Q

how is Cl- absorbed in the small intestine?

A
  • Cl- absorption occurs from electrical gradient
  • Na+ absorbed with nutrient is electrogenic (less to net negative charge in lumen and net positive charge in paracellular spaces) which drives an electrochemical gradient for Cl- absorption
  • cotransport with Na+: dominant mechanism in proximal ileum
  • counter transport in exchange for HCO3-: dominant in distal ilia as Na+ decreased, large intestine
  • carbonic anhydrase mediated production of HCO3- in the cell occurs for Cl- exchange
24
Q

how is Na+ absorbed in the large intestine?

A
  • driven by Na+/K+ ATPase in the basolateral membrane
  • sodium entry by: Na+ channels, Na+/H+ transporter and no glucose/amino acid carrier activity
  • K+ becomes concentrated in lumen as water is absorbed from gut; may be absorbed down a concentration gradient or secreted when lumen concentration is low (net secretion)
25
Q

how is Cl- absorbed in the large intestine?

A
  • Cl- ions are exchanged for HCO3-
  • net secretion of HCO3- provides buffer for acid produced by bacteria with absorption of Cl- ions
  • tight junctions ensure no ion back flow into lumen
  • Na+ and Cl- movement creates osmotic gradient for transcellular water movement
26
Q

how does aldosterone help to regulate absorption?

A
  • dehydration causes aldosterone release from adrenal cortex
  • up regulates Na absorption by stimulation of Na+/K+ ATPase pump and Na channels
  • increased NaCl and water absorption occurs from the lumen at the expense of K+ secretion into the lumen
27
Q

how do glucocorticoids and somatostatin help to regulate absorption?

A

increase water and NaCl absorption by up regulation of Na+/K+ ATPase pump

28
Q

how does the enteric nervous system help to regulate absorption?

A

parasympathetic promotes secretion; sympathetic promotes absorption

29
Q

how is water absorbed in the GI tract?

A
  • all water absorption in the GI tract is via osmosis from gut lumen via enterocytes into extracellular spaces and then into blood
  • process is absolutely dependent on absorption of nutrients and electrolytes, particularly sodium which creates and osmotic gradient causing water movement from the gut epithelium
30
Q

what are the paracellular and transcellular routes of water absorption?

A
  • basolateral Na+/K+ ATPase pump lead to a build up of Na+ in the paracellular spaces between enterocytes and keeps cell Na+ concentration low
  • Na+ moves into enterocytes and keeps cell Na+ concentration low
  • Na+ moves into enterocytes down concentration gradient and is pumped out into interstitial fluid
  • H2O follows by osmosis and enterocytes and the paracellular spaces
  • hydrostatic pressure in interstitial fluid causes water and solute movement into blood vessels
31
Q

how are water and ions secreted in the GI tract?

A
  • enterocytes in crypts and villi express different combinations of transport proteins indicating different roles
  • enterocytes on the SI villi are absorptive and dominate nutrient transport
  • enterocytes in the crypt are secretory with minimal nutrient transport
  • water and electrolyte secretion in the small and large intestine is via crypt enterocytes
32
Q

how is the secretion of water and NaCl in the GI tract driven by Cl-?

A
  • via crypt enterocytes to maintain lumen liquid content
  • Na+/K+ ATPase pumps establish a Na+ electrochemical gradient across the basolateral membrane
  • this is used to drive Na+, Cl- and K+ ions through Na+/K+/2Cl- cotransporters into crypt cells
  • Cl- ions leave the cells via apical Cl- channels including CFTR into the intestinal lumen
  • Cl- ions provide electronegativity in the intestinal lumen to draw Na+ into lumen
  • creates an osmotic gradient for water movement into the gut lumen via paracellular routes
33
Q

how is water secreted in the GI tract?

A
  • via osmosis down an osmotic gradient created by solutes of ions
  • in the duodenum hypertonic chyme causes water to move into gut lumen via osmosis to form isotonic chyme (in addition to Cl- drive mechanism)
  • in the distal small intestine solutes are absorbed and water follows by osmosis maintaining a smaller volume of isotonic chyme
  • factors that disrupt tonicity of gut lumen contents (osmotic) or increase enterocyte secretion (secretory) may cause diarrhoea
34
Q

what are the mechanisms in the body that cause cystic fibrosis?

A
  • disruption of secretory mechanisms
  • congenital autosomal recessive disease
  • deletion in gene for CFTR channel
  • CFTR is main Cl- channel in apical membrane of gut, pancreatic and airway epithelium
  • secretion of stick mucus and high viscosity of luminal contents occurs
  • present with intestinal obstruction and meconium ileum in newborns
35
Q

how does the cholera toxin permanently switch on enterocyte secretion?

A
  • cholera toxin secreted by bacteria vibrio cholera
  • binds to cell receptor on apical membrane of crypt cells to irreversible up regulate adenylate cyclase generating excess cAMP which stimulate Cl- secretion via CFTR Cl- channels
  • depends on Na+/K+ ATPase pump
  • leads to massive Cl- efflux, Na+ and water (via osmosis) into gut lumen particularly in jejunum
  • profuse, water, secretory diarrhoea, circulatory shock caused by dehydration, life threatening
  • permanent: effects only reduced following enterocyte turnover
  • treatment via rehydration therapy
36
Q

how does oral rehydration therapy work

A
  • oral rehydration solutions (ORS) promote fluid reabsorption by coupling sodium with glucose in solution
  • the membrane carrier protein specific for Na+ - glucose transport (SGLT-1) is preserved in most diarrhoea disease
  • SFLT-1 binds 2 Na+ to one glucose, transporting them into cell, Cl- follows for electrochemical balance
  • replaces salt and water loss from secretory diarrhoea
37
Q

how does lactose intolerance work?

A
  • caused by a deficiency in the enzyme lactase
  • lactose is not digested and remains in the lumen
  • creates and osmotic gradient to cause osmotic diarrhoea

module 104 - theme 1 (17 decks)

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