Physiology- GI salt and water transport Flashcards
8.5L/day enters gut, how much is lost?
Where do electrolytes from this fluid come from?
How much is reabsorbed in the small intestines: large intestines? Where exactly are ions reabsorbed?
100ml
8.4L reabsorbed
GI secretions and food
6.5L in SI: 2L in LI
Jejunum- Na+, K+, Cli- and H2O absorption. Higher Na+ coupled with nutrient absorption
Ileum- secretes HCO3- and absorbs H2O. Highest absorption of NaCl
Colon- reabsorbs Na+, Cl- and H2O and secretes K+ and HCO3- (Aldosterone increases synthesis of Na+ channels -> increased K+ secretion-> hypokalaemia
How does reabsorption occur?
Tight junctions in SI more permeable than colon
Na+ transportation coupled with glucose and amino acids
Anion passively follow electrical potential established by Na+
k+ diffuse by osmotic gradients
Describe the structural properties of enterocytes
Polarizes
Gatekeepers for ion and fluid movement
Apical and basolateral membranes with tight junctions between cells
There are two types of epithelial transport. Describe the transcellular route
Employs membrane transporters; activity drives ion flux and establishes conc grandients dictating passive transport . Can work against electrochemical gradients
There are two types of epithelial transport. Describe the paracellular route
occurs through tight junctions, primarily by electrochemical gradients established by transcellular transport
Define and give an example for secondary active transport
Define and give an example for facilitated diffusion
Cotransport of molecule with ATP driven ion transport
e.g. Na-Gluc cotransporter
Facilitate passive transport across epithelial layer
e.g. glut-5 (fructase)
Define a channel, carrier and pump
Channel- fluid-filled pores built from protein aggregates; ion specific and based on ECG
Carrier- facilitate movement through conformational change; energy dependent; transport of single molecule, cotransport or exchange
Pumps- against ECG uses ATP; exchange
What is relationship between water and solute transport
Water transport follow solute transport in a linear relationship through tight junctions
Describe the transport of Na+ across membranes
- Na+/K+ ATPase creates an electrochemical gradient between enterocyte and lumen. It is found on the basolateral membrane
- Na+ coupled transport via secondary active transport, electrogenic (lumen becomes more negative and drives parallel abosrtion of Cl-)
How does oral rehydration work?
The solution uses glucose coupled sodium absorption. SGLT-1 binds 2 Na+ to 1 glucose molecule transporting then into cell
Cl- secretion occurs in conjunction by basolateral Na+/K+/Cl- transport. Describe it
Primarily in crypts
As Cl- conc increases, Cl- secreted via apical channels increases
Expression of CFTR higher in ileum and colon
Na-K ATPase drives Na+ gradient, further Cl- secretion through apical CFTR channel
Water follows NaCl . 95% of water absorbed in SI by osmosis
How is net loss of 35g/day of Na prevented in intestines
Driven Na-K ATPase in basolateral membrane
Net -ve intracellular and +ve in lumen
Produces Na+ gradient across apical membrance and used to reabsorb Na+ in jejunum
Describe in detail reabsorption in Jejunum
Na+ contransport with AA/glucose
Na+/H+ exchange though apical membrane Na+ ion channels
ELECTROGENIC
Electrical gradient for Cl- reabsorption (passively through apical Cl- channels and Cl-/HCO3- exchange)
In distal ileum and colon Cl- reabsorption occurs via Cl-/HCO3-
Describe paracellular transport of water/ in gut
The activity of basolateral membrane Na/K ATPase pump
Leads to build up of NaCl in paracellular spaces
Provides osmotic gradient that draws water into enterocytes and paracellular spaces
Describe reabsorption in the large colon
Driven by Na+/K+ ATPase
Na+ entry by channels and Na+/H+ antiport
Diffusion under aldosterone control (increases channels)
Cl-/HCO3- exchanger provides buffer for acid produced by bacteria in lumen and moves Cl-
Tight junctions ensure no ion backflow
Na+ Cl- ions create osmotic gradient for transcellular water movement
