Salt and water transport and their control Flashcards
what do water and electrolytes do in the GI tract
provide medium for digestive processes in GI tract, metabolic processes within body on absorption, replace daily loss of body fluids in sweat, urine, lungs and faeces
what happens when regulatory processes of water and electrolytes in the GI tract fail
it can lead to life threatening diarrhoea and electrolyte imbalance
how are water and electrolytes tightly regulated
Net absorption and minimal loss in faeces
Secretion/absorption regulated by osmolarity of gut lumen contents in absence of food electrolytes creates osmotic pressure, enteric and autonomic signals, endocrine hormones, immunogenic signals
how are water and salts efficiently conserved
Bidirectional secretion and absorption occurs across GI epithelium daily
Secretion dominates upper GI tract but absorption dominates overall (98% ingested and secreted water and electrolytes absorbed)
what is the SI the primary site for
absorption
Ingested and secreted water and electrolytes are predominantly absorbed in SI
Distinct secretion and absorptive profiles depend on variation in epithelial membrane transport proteins and permeability along GI tract
how is the SI adapted for absorption
Large SA – folds, villi, microvilli
Villi lymphatic and blood vessels
Enterocyte actin microfilaments rhythmically contract to move microvilli for max exposure to contents
Rapid response to chyme – hypertonic (osmosis of water into lumen to form isotonic chyme) and acidic (rapid increase in HCO3- rich secretions)
Epithelium more permeable than large intestine
how is the large intestine adapted
No villi but has crypts/intestinal glands
Smaller role in transport of water and salts, bacterial microbiome role in protein digestion/vitamins synthesis, digestion (nutrient absorption limed, lack of enzymes)
Epithelium less permeable than SI
Has additional absorptive capacity for water, NaCl in exchange for K+ loss
what is an electrolyte
Electrolytes dissociate in solution into ion
Conc varies in intracellular fluid and cytosol but osmotic balance maintained
Electrochemical gradient across membrane – diffuse down it
what does an increase in osmotically active particles lead to
will create a hypertonic environment
what are types of active transport
Primary active transport – hydrolyse ATP (eg Na/K pump)
Secondary active transport – gradient created by primary pump to move substance against conc gradient eg symport or antiport
what is the Na/K pump
3 Na+ out and 2 K+ in
Keeps Na low
Important mechanism for absorption and secretion
Na+ can be used for symport/antiport down its conc gradient to move other molecules against theirs
Eg symport of glucose or amino acids or antiport of Ca2+ or H+
wha are the principles of electrolyte transport
Polarised with apical and basolateral membrane
Tight junctions to provide a barrier to free flow of gut lumen contents, more permeable in proximal SI
Tonicity of chyme entering duodenum affects bidirectional fluid flux
how does electrolyte transport occur
Occurs by transcellular (against conc gradient and requires ATP, via transport proteins – variations in mechanism along GI tract) or paracellular (between cells, no ATP) routes
what are transcellular transport protein examples
Pumps eg Na/K, H/K
Channels eg Na or Ca
Carriers eg Na/H exchange, Na/Glucose exchange
what are the types of electrolyte transfer
Passive
Solvent drag – water follows Na+ gradient via osmosis taking other ions with it
Active
what are the key variations in water and electrolytes in SI
Chyme contains water and key electrolytes (Na, K, Cl, HCO3) from ingested food and secretions – rapid osmotic equilibration in duodenum to form isotonic chyme (secrete H2O into hypertonic chyme and vice versa)
Jejunum absorbs Na+, K+, Cl-, H2O and ileum secretes HCO3-
Na and Cl conserved
what are the key variations in water and electrolytes in LI
Smaller vol of chyme enters
Water and electrolytes primarily absorbed in proximal colon
Tight junctions stop back diffusion so NaCl more completely absorbed and less lost in faeces
Secrete HCO3 and K occurs
how is Na absorbed in the the SI
Drives absorption of ions, organics, water
Driven by basolateral active transport of Na+ by Na/K pump
Dominance of mechanism vary along SI
Via passive diffusion or apical membrane carrier proteins (organic substrate transporters, Na/H antiport, Na/Cl symport in ileum)
how is Cl- absorbed in the SI
Down electrical gradient
Na+ absorption with nutrients is electrogenic (net positive charge in paracellular spaces and positive in lumen so gradient for Cl- absorption)
Cotransport with Na+ dominant mechanism in proximal ileum
Counter-transport in exchange for HCO3- dominant in distal ileum as Na+ decreased
Carbonic anhydrase mediated production of HCO3- occurs for Cl- exchange
how does Cl- link to HCO3-
Cl= exchanged for HCO3-
Net secretion of HCO3- = buffer for acid produced by bacteria with absorption of Cl-
Na and Cl movement creates osmotic gradient for transcellular water movement
how is absorption regulated
Aldosterone – dehydration = release from adrenal cortex to upregulate Na absorption by stimulating Na+/K+ and Na channels, increased NaCl and water absorption occurs from lumen at expense of K+
Glucocorticoids and somatostatin increase water and NaCl absorption by upregulation of Na/K pump
Enteric NS – PS promotes secretin, sympathetic promotes absorption
how is water absorbed
All absorption in GI tract via osmosis from gut lumen via enterocytes into extracellular spaces and into blood
Process dependant on absorption of nutrients and electrolytes, particularly Na (hydrostatic pressure inc in interstitial fluid so watered solute into BVs)
Paracellular and transcellular
how does secretion vary between enterocytes in crypts and villi
Enterocytes in crypts and villi express different combos of transport proteins
In SI villi are absorptive and dominate nutrient transport
In crypts are secretory with minimal nutrient transport
Water and electrolyte secretion in S and L I is via crypt enterocytes
how is water and NaCl secreted
Secretion of water and NaCl is driven by Cl- via crypt enterocytes to maintain lumen liquid content
Na/K pump establish Na electrochemical gradient across basolateral membrane
Used to drive Na, K, Cl ions through Na/K/2Cl cotransporters into crypt cells
Cl- leave via apical Cl- channels inc CFTR
Electronegative in lumen to draw Na+
Osmotic gradient for water via paracellular route
what does the cholera toxin (secreted by vibrio cholera) do
Cholera toxin secreted by bacteria vibrio cholera
Bings to cell receptor on apical membrane of crypt cells to irreversibly upregulate adenylate cyclase generating excess cAMP which stimulates Cl- secretion via CFTR channels
Depends on Na/K pump
Massive Cl- efflux, Na and water follow into gut lumen (particularly jejunum)
Profuse watery secretory diarrhoea, circulatory shock caused by dehydration, life threatening
how is cholera treated
Permanent – only reduced by enterocyte turnover
Treated by oral rehydration therapy
what is ORS
Promote fluid absorption by coupling Na with glucose in solution
Membrane carrier protein for Na/Glucose preserved in most diarrhoea (binds 2Na to a glucose, transport into cell with Cl- following for charge balance)
Replace water loss
what results from lactose intolerance
Deficiency in lactase
Lactose (disaccharide) is not digested and remains in lumen to create an osmotic gradient leading to osmotic diarrhoea
