Renal: Renal Tubules 1 Flashcards
how much of the glomerular filtrate fluid is reabsorbed
what does the effectiveness of reabsorption reflect on
what are these the key parameters for
> 99% of Glomerular Filtrate Fluid is Reabsorbed
The effectiveness of reabsorption is ultimately reflected in the VOLUME and ION CONCENTRATION (SPECIFIC GRAVITY) of the urine.
These are key parameters of KIDNEY FUNCTION
what functions take place in the proximal tubule, loop of henle, distal tubule and collecting duct
proximal = filtration, reabsorption, secretion
loop of henle = reabsorption
distal tubule = reabsorption, secretion
collecting duct = reabsorption, secretion, excretion
what is absorbed in the proximal tubule and what %
what is secreted in the proximal tubule
ABSORPTION
water = 60%
NaCl = 60%
glucose and AA = 100%
bicarb = 60-85%
SECRETION
creatinine = into lumen by tubular epithelial cells
pathways for reabsorption in the proximal tubule
which one is most important
-transcellular pathway (most important, active mechanism to selectively choose more molecules?)
-paracellular pathway
features of paracellular vs transcellular transport routes
paracellular
-1% of surface area
-5-10% of water absorption
-for ions, non-polar solutes, passive diffusion and solvent drag are only reabsorption mechanisms for this route
-requires favourable electrochemical gradient
transcellular
-99% of surface area
-90-95% of water absorption
-passive or active transport
-all active transport occurs by this route
primary active transport
direct utilization of energy (ATP) to transport molecules across a membrane
generates bot concentration and charge (early PT) gradients favouring Na movement from the tubular lumen into the PT cells into the peritubular capillary
secondary active transport
Use of an existing electochemical potential difference (eg. created by 1° active transport) to transport molecules across a membrane.
1) Counter transport (antiport) - 2 different ions “pumped” in opposite directions, one with gradient, one against. (one in one out)
2) Co-transport (symport) - 2 different ions “pumped” in the same direction, one with gradient, one against (pulls one in along with another one going in)
what are all of the tubular reabsorption mechanisms (4) and their details
Key Primary Active Transport Mechanism:
* Na +/K +-ATPase activity
* Generates both concentration and charge (early PT) gradients favoring Na + movement from the tubular lumen into the PT cells into the
peritubular capillary
Key Secondary Active Transport Proteins:
* Na +/H + antiporter in luminal membrane
* Na +/X cotransporters in luminal membrane
Water:
* Active absorption of Na+ results in water movement through (aquaporins) and between (paracellular) cells.
Solvent drag:
* Na + reabsorption fuels passive reabsorption of many other substances
(eg. Na + moves, water follows and takes dissolved ions like Mg+, Ca ++
Phosphorus with it).
primary and secondary active transport in the proximal tubules
Symporters and Antiporters use 2° Active Transport to move in molecules against their concentration
gradients.
1° Active Transport establishes electrochemical gradient in cells
why are different things absorbed at different parts of the renal system
These differences are due to different proteins present in the tubular cells, and different membrane charges that
allow movement of Na+ and other solutes.
sodium reabsorption within the proximal tubule - entry and exit, what happens to relative tubular fluid concentration of Cl, where it happens
Luminal entry into cells from GF
-1. Co-transport (symported) in with
glucose, amino acids, phosphorus
(facilitates absorption of these essential molecules also).
-2. Na+/H + antiporter (H + secreted into fluid, Na + transported into cells )
Basolateral exit from cells into blood
-Na +/K + ATPase pumps Na + out and K+ into cells.
Note: Relative tubular fluid Cl - concentration initially increases (due to absorption of other anions)
happens in the early proximal tubule
chloride reabsorption in the proximal tubules and where it happens
1) Paracellular movement of Na + and Cl -
2) Transcellular: Cl - /Anion exchange at tubular membrane, transport Cl - into cells. Cl - channels at basolateral membrane
in the late proximal tubule
bicarbonate reabsorption in the proximal tubule; steps
- Bicarbonate in lumen reacts with H+ (actively transported out of cell) to form H2 CO3
- Carbonic anhydrase (CA) dissociates H2 CO3 into CO2 and H2O.
- CO2 diffuses into PCT cell and undergoes the reverse reaction to form H2 CO3 (catalyzed by CA)
- H 2 CO3 dissociates into H+ and HCO3-
- HCO3- is co-transported out of the cell with Na+ into interstitium and ultimately, bloodstream
glucose reabsorption in the proximal tubules
Two step process occurring only in proximal tubule
1) Secondary active transport (cotransport) of glucose across luminal membrane (Sodium Glucose Co- transporter)
2) Carrier mediated diffusion (passive) on basolateral membrane via GLUT1, GLUT2 transport proteins
The process can be saturated: When too much glucose is present in the filtrate (eg. diabetes) the tubular transport maximum is exceeded, glucose remains in the filtrate and
passes out in the urine
what happens if glucose transport is saturated
-PT glucose transport can be saturated - Glucosuria “Spillover” - When too much glucose is present in the filtrate (eg. diabetes mellitus, D5W fluids) the tubular transport maximum is exceeded, glucose remains in the filtrate and passes out in the urine. Glucose “Threshold” ~10 mmol/L
-Glucosuria of renal origin (rare) ~Fanconi syndrome - multiple problems (glucose, AA, phosphates) –
Basenji dogs.
~Renal Failure - Sometimes seen as a component of renal failure – glucose levels higher in lumen of poorly functional tubules. This is not diagnostic for kidney disease but sometimes is seen
