Renal Flashcards
Where in the renal vasculature are the major sites of resistance?
the afferent and efferent arterioles
What is the effect of a pre-renal problem on GFR? A post-renal problem?
- both will lower GFR, but by different mechanisms
- a pre-renal problem lowers the hydrostatic pressure of plasma
- a post-renal problem increases the hydrostatic pressure of bowman’s space
What is the ultrafiltration coefficient? What is it’s utility?
- the ultrafiltration coefficient, K(uf), reflects the surface area and permeability of the glomerular membrane
- in conjunction with the filtration forces, it defines GFR
- GFR = K(uf) x P(uf)
Describe the layers of the glomerular filtration membrane.
- fenestrated endothelium
- atop a basement membrane
- under which podocyte foot processes, called pedicels, form the epithelial layer
What component of the glomerular filtration membrane forms the charge barrier? What sorts of ions pass through most easily?
negatively charged glycoproteins in the basement membrane allow small cations to pass through more readily than anions of the same size
What transport drives nearly all absorption and secretion in the renal tubules? Describe it’s action.
Na/K-ATPases pump three sodium ions out of the basal surface of the epithelial cells and two potassium ions in across that surface
What electrical gradient exists across the tubular epithelium?
-50mV, with the tubular lumen more negative (makes sense because the gradient is set up by the Na/K-ATPase which pumps three cations out of the cell and two into it)
Where is most calcium reabsorbed in the kidney? Through what mechanism?
most is reabsorbed in the proximal tubule via the paracellular pathway
What substance is used to estimate GFR? Why is it a good marker of GFR?
- creatinine is used because it is freely filtered, not reabsorbed, and minimally secreted
- furthermore, unlike inulin, it requires no infusion or emptying of the bladder beforehand
What is a normal value for GFR?
roughly 100-125 mL/min
What equation is used to calculate GFR from creatinine?
GFR = (urine creatinine)(rate of flow of urine)/(plasma creatinine)
How is fractional excretion of a solute calculated?
FE = (solute excreted)/(solute filtered) = (urine solute)(urine flow rate)/(GFR x plasma solute) = (plasma creatinine x urine solute)/(urine creatinine x plasma solute)
What is the normal value for fraction excretion of sodium?
1-3 percent
What substance is used to calculate renal plasma flow? What characteristics make this a good marker?
para-amniohippuric acid (PAH) is used because between filtration and secretion, there is nearly 100% excretion of all PAH that enters the kidney
What equation is used to calculate renal plasma flow? How about renal blood flow? What are normal values for these two measures?
- RPF =(urine PAH)(urine flow rate)/(plasma PAH) = 660 mL/min
- RBF = RPF/(1-Hct) = 1.2 L/min
What is the normal BUN/Cr ratio? What does it represent?
- it is an important indicator of both glomerular and tubular function (creatinine is mainly filtered while urea is filtered and reabsorbed)
- the normal value is roughly 15
How does the BUN/Cr ratio and FE(sodium) change in someone with prerenal, intrarenal, or post-renal failure? In which cases is the kidney’s ability to concentrate urine affected?
- BUN/Cr > 15, and FE is normal in someone with pre-renal failure
- BUN/Cr < 15, and FE > 3% in someone with intra-renal failure
- BUN/Cr is not a particularly good indicator for post-renal failure, but the FE > 3%
What is filtration fraction? How is it calculated? What is a normal value?
FF = GFR/RPF = 0.2
How do control of ECF osmolarity and ECF volume differ?
- for volume, the system changes urinary excretion of sodium
- for osmolarity, the system changes the urinary excretion of water
What causes the release of ADH? What are it’s overall effects? Through what mechanisms does it elicit these changes?
- a rise in ECF osmolarity is detected by osmoreceptors in the hypothalamus (supraoptic and paraventricular nuclei)
- these sensors trigger the thirst mechanism and release of ADH from the pituitary
- secondarily, ADH can be released in response to low volume to a lesser degree
- ADH has a high affinity for V2 receptors on the basolateral surface of principal cells in the epithelium of the collecting tubules
- binding to V2 activates Gs and rising cAMP levels trigger expression of aquaporin2 molecules in the apical surface
- these aquaporin2 molecules promote reabsorption of water from the hypotonic fluid passing the the collecting tubule
- ADH has a lower affinity for V1 receptors found in the periphery on endothelial cells
- when it binds V1 receptors, it works through a Gq signal transduction mechanism to induce vasoconstriction
Describe the countercurrent mechanism of the LoH.
- the descending limb is permeable to water and the medulla becomes more hypertonic as you descend, drawing fluid out of the tubular lumen
- the result is an extremely hypertonic solution at the bottom of the LoH
- the ascending limb is permeable only to salt, so as the fluid rises, solutes are drawn out
- the resulting fluid is hypotonic as it enters the distal tubule
What is the purpose of the vasa recta countercurrent exchange? Describe the characteristics of the vasa recta that allow it to carry out this purpose.
- it is essential for conserving the composition of the medullary interstitium, which provides the foundation for the Loop of Henle’s countercurrent mechanism
- the hairpin structure and slow rate of blood flow allow minimal disruption of the medulla’s gradient while still allowing the vessel to supply nutrients to cells
How does the rate of blood flow in the vasa recta or the length of the loop of hence affect the ability of the kidney’s to concentrate urine?
- slow blood flow in the vasa recta prevents it from disturbing the medullary salt gradient and diminishing the countercurrent effect of the LoH
- the length of the LoH lowers or extends the vertical osmotic gradient to which the fluid is subjected, changing the degree to which it is concentrated
What triggers the release of ANP and BNP? What are it’s effects? What mechanisms elicit these effects?
- ANP and BNP are released from the atrial and ventricular walls in response to increased blood volume and distension of those walls
- in the adrenal cortex, ANP reduces aldosterone production
- in the hypothalamus, it reduces ADH secretion
- in the tubules it promotes closure of ENaC sodium channels, inhibiting Na reabsorption, thus promoting elimination of water
- it reduces sympathetic input to the kidneys, causing vasodilation of the afferent arteriole while also inducing vasoconstriction of the efferent arteriole
- the net effect of this is increased GFR, which increases sodium delivery to the macula densa and impairs renin release