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HF B4 Renal Potassium Handling
So named for the folder which holds this info in its camtasia file
Terms in this set (50)
What 2 intrinsic factors work to keep fractional reabsorption of Na and water relatively stable when GFR changes?
flow-dependent Na reabsorption in the distal part of the nephron
What effect do increases in GFR have on salt and water reabsorption and excretion?
increased GFR increases both salt and water reabsorption and excretion
As GFR increases, what happens to fractional reabsorption of salt and water?
As GFR increases, what happens to fractional excretion of salt and water?
When are the changes of fractional reasborption and fractional excretion seen to a greater extent?
during diuresis, when reabsorption is inhibited
Overall, how drastic are the changes that affect FR and FE with changing GFR?
not that drastic (relatively stable FR and FE with changing GFR)
intrinsic phenomenon independent of hormones and nerves whereby the proximal tubule REABSORBS A CONSTANT FRACTION OF THE FILTERED LOAD OF NA AND WATER when the GFR changes
What must increase with GFR, due to glomerulotubular balance?
absolute reabsorption of Na
What effects result in the increaed absolute reabsorption of Na seen with increasing GFR?
peritubular and luminal tubule effects
As the Na filtered load increases, what part of the nephron will account for the fact that the absolute reabsorption of Na must also increase?
What 2 luminal factors contribute to glomerulotubular balance?
1. increased organic solutes for reabsorption (na-linked)
2. increased humoral factors promoting reabsoprtion
How do increased organic solute for reabsorption (Na-linked) affect glomerulotubular balance?
increased flow resulting from increased GFR results in greater delivery of organic solutes, which are reabsorbed by Na-cotransport.
Increased GFR therefore results in increased Na-coupled solute transport with water transport following isosmotically
How do increased humoral factors promoting reabsorption contribute to glomerulotubular balance?
increased volume of filtration increases results in the increased presentation of humoral factors promoting Na reabsorption (like angiotensin II) to the luminal membrane of proximal tubular cells.
How do peritubular physical factors contribute to glomerulotubular balance?
changing forces for peritubular capillary reabsorption from the peritubular interstitium occur as a result of changes in filtration fraction (GFR/RPF)/ If the filtration fraction increases as a result of increase in efferent resistance, a greater COP will facilitate capillary reabsorption of interstitial fluid, and the reduced flow exiting the glomerulus (because of increased FF) will result in a lower hydrostatic pressure, opposing reabsorption
How does the proximal tubule accomodate for high interstitial pressures in the peritubular capillaries?
it is leaky to water (water can flow back into the lumen)
What 2 factors enhance water reabsorption from the interstitial fluid of the peritubular capillaries?
decreasing hydrostatic pressure and increasing COP
Under normal conditions, how do hydrostatic pressure and COP interact to enable reabsorption?
In the glomerular capillary, hydrostatic pressure doesn't change, and the COP slowly rises due to filtration.
A drop in hydrostatic pressure across the efferent arteriole results in a large net force for reabsorption
How does increasing resistance in the efferent arteriole increase reabsorption?
increasing efferent resistance will:
decrease afferent resistance, increasing fractional filtration and efferent COP, and
decreasing efferent plasma flow and peritubular hydrostatic pressure, creating a greater net force for reabsorption.
Does the distal nephron increase Na reabsorption as Na load increases?
yes; greater Na delivery to Na transport sites in the distal nephron segments results in increased Na reabsorption.
How much K do we excrete a day?
about 100mMol/day (about 14% of filtered load
How much K is usually excreted by the GI tract? When does this change?
during renal failure, 30-50%
serum K >/= mEq/l; results in muscle weakness and paralysis, cardiac conduction abnormalities, and cardiac arrhythmias
Why is regulation of plasma K critical?
it has important influences on neural and cardiac function. both decreases and increases in plasma K have significant impact on ECG
What are the 4 sites of K reabsorption in the nephron?
80% in proximal tubule
10% in the thick ascending limb
2% in the distal tubule
6-40% in the collecting ducts
How much of the filtered load of K is normally excreted?
What is K handling in the distal portion of the nephron dependent upon?
K in the diet
Is Renal K or Na handling more efficient?
Proximal tubule handling of K
K transport largely passive
paracellular, driven by the positive Transepithelial potential difference in the later proximal tubule
Thick ascending limb handling of K
occurs approximately equally between the paracellular and transcellular pathways.
paracellular driven by the positive transepithelial PD
transcellular through the Na/K/2Cl cotransporter and through basolateral K channels
Proximal and thick ascending limb K reabsorption are largely _____ by K status
What determined K excretion?
distal K handling
alpha-intercalated cells in K transport
actively transport K into the cell, across the apical membrane through the H-K countertransport ATPase; exit from the cell across the basolateral membrane occurs via K channels
K secretion by late distal tubule and cortical collecting tubule (principal cells)
1. uptake of K across basolatera Na/K ATPase
2. high (and variable) apical K permiability through K channels.
3. Favorable electrochemical gradient favors K exiting across the apical membrane
What are the effects of aldosterone on the principal cells of the late distal and coritcal collecting tubules?
increases ATPase, basolateral membrane, and ATP
What 5 factors affect K secretion by principal cells?
distal nephron flow rate
luminal Na concentration
High K diet
How does distal nephron flow rate affect K excretion by principal cells?
dilutes and moves K from the lumen around the principal cells, enhancing the concentration gradient (increases secretion of K)
How does luminal Na concentration affect K secretion by principal cells?
apical Na entry depolarizes cell membrane and enhances electrochemical gradient for K exit
(more Na to enter distal tubule drives more K out)
How does alkalosis affect K secretion by principal cells?
causes hypokalemia due to cell uptake and DECREASES K secretion
How does acidosis affect K secretion by principal cells?
decreases K secretion
Why does acidosis decrease K secretion?
increased H+ in the cell inhibits the Na/ atpase, decreasing intracellular K; therefore there is less K in the cell to be excreted
How does aldosterone affect K secretion by principal cells?
increases K secretion by
1.) increasing Na/K atpase and amplification of basolateral membrane surface area
2.) stimulating apical ENaC channels
3. increasing K conductance of the apical membrane
How does a high K diet affect K secretion from principal cells?
causes amplification of basolateral membrane surface area; increased plasma K stimulates aldosterone secretion; inhibits proximal Na and fluid reabsorption, increasing distal flow and Na delivery (high K increases secretion)
What is stimulated by hyperkalemia? What channels do they stimulate? What effect does this have on K secretion or reabsorption?
insulin, epinephrine, and aldosterone release
stimulates the Na/K ATPase to enhance K uptake into cells (results in increased secretion of K from principal cells)
Basic kinetics of handling the K load of a meal
1. ingestion of K load is quickly absorbed by the GI tract and increases plasma K
2. absorbed K is rapidly taken up into the large intracellular K pool, effectively buffering plasma K sustained increases
3. The ingested K load is then slowly excreted by the kidneys over the next several hours.
What is an example of a K-sparing diuretic? where does it act?
amiloride; distal tubule
What are 3 examples of K wasting diuretics?
carbonic anhydrase inhibitor
loop diruetic (lasix)
Where does carbonic anhydrase inhibitor act?
Where does lasix act?
thick ascending limb
Where does thiazide act?
What type of diuretics are K wasting? WHy?
diuretics taht act upstream of principal cells
they cause increased NaCl and flow past the principal cells and increase K loss in unregulated manner.
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