Proximal Tubule Flashcards
What is the constitutive function of the nephron?
Occurs with little regulation and mediates a lesser renal response to changes in solute or fluid balance
How much reabsorption of NaCl and water occurs at the proximal tubule? Is this constitutive or regulatory? How does PT fluid reabsorption occur? How would one describe the PT in terms of epi?
Approximately 67%, most of which is constitutive; isoosmotically; leaky, meaning rapid equilibration of solutes and water across epi
What mediates most reabsorption of most organic solutes? Where does most reabsorption of organic solutes occur? What else is mediated by the proximal tubule and what is moved? How is reabsorption described in the PT?
Proximal tubule; proximal tubule; secretion of organic anions and organic cations, both of which remain in urine and are excreted;
constitutive and saturable
What does an increase in TF/P for inulin indicate? What does a TF/P of close to one mean for Na? Why does Cl TF/P increase? What has decreased TF/P ratios?
Reabsorption of water from the tubular fluid; equivalent reabsorption of water and Na from PT fluid;
preferential reabsorption of HCO3 over Cl in early PT;
bicarb, aa’s, glucose, which help drive reabsorption of water
Why does the transepi voltage change from -3 to +3 mV down the PT?
Exit of more pos cationic charged solutes early on, with negative anionic charge leaving from tubular fluid later on (later 75%)
For proximal tubule Na and Cl reabsorption, how can this occur (2)? Why is there backleak of Na into the lumenal fluid? How much of the Na is leaked transcellularly? What drives Na reabsorption paracellularly in late PT?
Can occur transcellularly, or paracellularly; through the tight junctions, Na can leak into tubular fluid due to transepithelial membrane voltage; 33%;
Lumen pos transepithelial voltage difference
How is Na taken in transcellularly? How does Na pass across the cellular membrane?
Symporters and exchangers/antiporters (Na-glucose and Na-H); Na-K ATPase and Na-bicarb symporter
For Cl reabsorption, where does transcellular transport occur? What drives the paracellular transport early in PT? What does this efflux mean for Na in the late PT? In the late proximal straight tubule, what drives transcellular transport? How does Cl move across the basolateral membrane?
Late proximal straight tubule; the negative transepi voltage difference; drives Na paracellular efflux in late PT;
Cl-anion antiport;
Passive efflux through Cl channel or K-Cl symporter
What primarily drives paracellular and transcellular water reabsorption? What helps move water transcellularly and paracellularly through the PT? How does water move from peritubular space to peritubular capillaries? When water moves paracellularly, what can this do to Na and Cl reabsorption?
Small osmotic gradient resulting from active solute reabsorption across the PT;
High water permeability of PT and aquaporin channels in apical and basolateral membranes of PT cells;
net difference in Starling forces favoring absorption;
Solvent drag
How much bicarb is reabsorbed in PT? What is the ECF bicarb? Is this reabsorption constitutive or regulated? What does PT help secrete? What is generated once this product is secreted?
85%; 24 mM; constitutive;
H+ (aa metabolism, lactic acid and ketone bodies made, bicarb loss through diarrhea);
generates new bicarb
For bicarb reabsorption at PT, how is it broken down? How is Na transported? Is transport of bicarb constitutive or regulated? How does OH arise in the cell? How is Na transported out? How is bicarb? How is H move across the luminal membrane? What kinetics are involved with HCO3? What else can stimualte bicarb reabsorption?
Carbonic anhydrase IV;
Na-H exchanger; constitutive;
water breakdown; Na-K ATPase and Na-bicarb symporter;
Recycled because of water breaking down to H+ and Na-H antiporter; saturable;
ECF volume contraction that changes the Starling forces
How can PT excrete H? What participates in excretion of H+ as NH4? How can H+ be excreted as titratable acid? In PT, how can NH3 arise?
Titratable acid, NH4+;
prox tubule, thick ascending loop of Henle, collecting duct;
dibasic phosphoric acid to monobasic phosphoric acid;
intracellular glutamine metabolism (uptake across membrane)
When H+ isn’t being used for Na-H exchanger or H-ATPase, what can it be used for? What is generated as each H+ is secreted?
Titratable acid;
new bicarb to replace bicarb ion lost in buffering of metabolic acid
How can NH4+ be moved into the lumen? How do the protons arise in the lumen? What is made with each H+ lost? What is the concept at play here in terms of binding this H to NH3?
You can use luminal membrane Na/H antiporter to transport ammonium; NH3 can freely diffuse;
Na/H exchanger or H-ATPase;
diffusion trapping
What is the renal compensatory response to respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis?
Primary increase in ECF PCO2, primary decrease in ECF PCO2, primary decrease in ECF bicarb concentration, primary increase in ECF bicarb concentration; all serve to maintain bicarb/CO2 ratio at 20 and ECF pH close to 7.4
