Renal- Transport Processes Flashcards
Where is the bulk of reabsorption done?
And what makes this possible?
- Proximal tubule
- first segment of PT has lots of mitochondria to facilitate active transport
- the later segments (S2 and S3) have deeper brush border to increase the surface area
Na driven transport
- FL of Na is high and 99% of it gets reabsorbed back into blood
- Na/K ATPase pumps allow the Na to be absorbed into the cells by managing the Na gradient within the cell
Proximal convoluted tubule
S1, S2
- Active Na cotransport brings glucose, amino acids, phosphate, and organic acids back into the cell.
- Na/H antiporters allow H secretion into the proximal renal tubular fluid while the Na is being reabsorbed
Proximal straight tubule
S3
- Na/H antiporters continue to reabsorb Na and secrete H into the tubular fluid
- reabsorbtion of Na causes fluid to follow which creates electrochemical gradient that facilitates Cl reabsorption
- Cl concentration increases along the PT segments as water is reabsorbed
- Cl- going into the cell causes other anions to leave the cell back into the filtrate (HCO3-, SO4-, OH-, oxalate)
Thin descending tube of Henle
- this segment is impermeable to Na and most other solutes but permeable to H2O
- The interstitial osmotic gradient brings water back into the cell through aquaporins
- this causes the tubular fluid remaining to be more concentrated
Thick ascending limb of Henle
- this segment is impermeable to water
- special Na+-K+-2Cl- cotransporters allow for reabsorption of electrolytes
- after the electrolytes get reabsorbed, the tubular fluid entering the distal tubule is more dilute.
- these cotransporters are the where loop diuretics target.
- There is a backleak of K+ out of the cells into the tubule lumen, creating a + difference compared to interstitial fluid
- this allows movement of cations out of the tubular lumen
- There are Na/H antiporters that reabsorb Na and secrete H into the tubule
Distal Tubule
- early DT has Na+-Cl- cotransporters (can be inhibited by thiazide diuretics)
- has Na/K channels that are increased by aldosterone
- resulting in greater Na and water reabsorption and K excretion
Collecting Duct
- also has Na/K channels affected by aldosterone
transport maximum
- usually in reference to glucose
- Normally very high transport maximum and all of the FL of glucose is reabsorbed
- If the plasma level of glucose is high, the FL will be more than the Na/glucose transporters can bring back in, causing urine in the glucose.
HCO3-
How much filtered/absorbed?
freely filtered and 100% reabsorbed
How is HCO3- reabsorbed?
- reabsorbed indirectly
- in tubular lumen, filtered HCO3- and secreted H+ form Carbonic acid, carbonic anhydrase (CA) breaks Carbonic acid into CO2 and H2O (catalyzed by brush border and CA)
- CO2 and H2O easily diffuse into the cell
- In the cell, the CO2 and H2O are converted back into carbonic acid(H2CO3) (by intracellular CA)
- Carbonic acid easily breaks up into HCO3-
- HCO3- is transported out of the cell via HCO3-/Cl- exchangers or Na/HCO3 cotransporters
What happens to K in the nephron?
- Proximal tubule- K reabsorption happends between cells, not into the cells. (70% of K reabsorption)
- Thick ascending Limb on Henle- transporters use the Na and Cl gradients to facilitate transport of K (20% of K reabsorption)
- Late distal tubules- an increase in aldosterone will increase number of Na/K ATPases which will put K back into the tubule. (to keep Na in?)
- Collecting ducts- an increase in aldosterone will cause K to go out through K channels. An H+/K+ atpase facilitates K reabsorption in exchange for H secretion into tubular fluid
***normally a net secretion of K into the urine unless there is a dietary K depletion.
Potassium handling during high K intake
- High plasma K stimulates secretion of aldosterone which stimulates secretion of K from the Distal tubule and collecting duct into the tubular fluid.
How does aldosterone respond to increased plasma K concentrations?
High K causes secretion of aldosterone, which causes Na absorption and K to be secreted into UF at the late distal tubule and the collecting ducts.
What do the H+/K+ ATPases do in the collecting ducts?
maintain system acid/base balance by buffering and excreting the H+ into the tubular fluid where it is buffered and excreted
The K enters the cells.
60% of Ca2+ is freely filtered at the glomeruli and 99% of that is reabsorbed. How and where is Ca2+ reabsorbed?
- Proximal tubule- 70% of Ca is reabsorbed between cells (paracellular) via solvent drag caused by the movement of Na.
- Thick ascending limb of Henle- paracellular with Na again. 20%
- Distal tubule- in response to decrease in plasma Ca, PTH is released which increases Ca channels and Na/Ca exchangers and Ca ATPase pumps which transport Ca out of cell and into interstitium.
