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
