Elements of renal function Flashcards by Stephen Schaffner

What other body systems are the kidneys integrated with? In what way?

Ion balance: endocrine, gastrointestinal
Water balance: CNS
Blood pressure, Na+, K+: autonomic NS
Acid-base balance: respiratory, CNS
Blood pressure: cardiovascular
Elimination of wastes, toxins: liver

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What percent of CO is dedicated to renal supply?

20%

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High pressure in the _____________________ causes filtration of blood. Lower pressure in the _____________________ permits fluid reabsorption

How can pressure in both capillary beds be regulated?

Glomerular capillaries

peritubular capillaries

can be regulated by resistance changes in afferent and efferent arterioles

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What are the two types of nephrons?

Juxtamedullary and superficial cortical

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What are the loops of henle like in cortical nephrons?

Short loops of henle, surrounded by peritubular capillaries

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What are the juxtamedullary loops of henle like? What are their long efferent arterioles divided into? What do these function to do?

long loops of Henle
long efferent arterioles are divided into specialized peritubular capillaries (vasa recta)
functions to concentrate urine

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Detail the steps of renal microcirculation

Afferent arteriole
Glomerular capillaries
Efferent arteriole
Peritubular capillaries
Interlobular vein
Arcuate vein
Interlobar vein
Renal vein

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Describe the first and second capillary networks

First capillary network (glomerular capillaries): high hydrostatic pressure; large fluid volume filtered into Bowman’s capsule
Second capillary network (peritubular capillaries): low hydrostatic pressure; large amounts of water and solute are reabsorbed

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How does a slow blood flow rate through the medulla enable us to concentrate our urine?

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•Does renal fraction (of CO) change during exercise?

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In afferent arterioles, sympathetic neurons synapse on:

Causing?

•Smooth muscle causing arteriolar constriction
–Protective during increased BP
•Granular cells causing renin secretion

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Sympathetic stimulation causes powerful constriction of afferent and efferent arterioles (afferent > efferent). this leads to?

‒Decreases renal blood flow
‒Diverts the renal fraction to vital organs

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Apart from constriction of arterioles, what other effects does sympathetic stimulation have?

Stimulates renin release from granular cells.

•Stimulates Na+ reabsorption in proximal tubule, thick ascending limb of Henle’s loop, distal convoluted tubule, collecting duct

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What is glomerular filtration?

•Filtration of plasma from glomerular capillaries into Bowman’s capsule

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What is tubular reabsorption?

•Transfer of substances from tubular lumen to peritubular capillaries

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What is tubular secretion?

•Transfer of substances from peritubular capillaries to tubular lumen

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Define excretion

•Voiding of substances in the urine

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What are the basic processes of urine formation?

Glomerular filtration

tubular reabsorption

tubular secretion

excretion

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•Urinary excretion = ?

amount filtered – amount reabsorbed + amount secreted

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•Tubular reabsorption = ?

glomerular filtration - urinary excretion

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•For most substances, the rate of their filtration and reabsorption are?

large relative to rate of excretion

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–Small changes in filtration or reabsorption can lead to?

large changes in excretion

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–Increasing plasma Na+ increases its filtration rate, and a smaller fraction of the filtrate is reabsorbed, leading to an?

increased excretion

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What is the glomerular filtration rate?

•volume of plasma filtered into the combined nephrons of both kidneys per unit time (e.g. ml/min)

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GFR is an index of?

functioning renal mass

–GRF is determined by?

Starling forces in the glomerulus and glomerular capillary permeability

•Reductions in GFR in disease states are most often due to?

a decrease in net permeability resulting from a loss of filtration surface area induced by glomerular injury

•In normal subjects, GFR is primarily regulated by?

alterations in glomerular hydrostatic pressure that are mediated by changes in arteriolar resistance

•Filtration rate of any freely filtered substance =

•Filtration rate of any freely filtered substance = GFR x plasma concentration of substance

What is filtration fraction? What is NL?

•Filtration fraction is the part of the renal plasma flow (RPF) that is filtered into the tubules –Normally about 20% (GFR/RPF)

What does FF change with? (filtration fraction) What happens with an increased FF?

ultrafiltration pressure –With an increased FF, the oncotic pressure of the efferent arterioles increases, aiding reabsorption of tubular fluid

ØNet rate of reabsorption or secretion of a substance =

ØNet rate of reabsorption or secretion of a substance = difference between glomerular filtration and urinary excretion (assuming substance is not produced or metabolized by the kidneys)

–If excretion \< filtration,

net reabsorption occurred

–If excretion \> filtration,

net secretion occurred

What is the urinary excretion rate?

•Product of urine flow rate x concentration of substance “a” in the urine (i.e., Ua x V)

Define renal clearance

•the volume of plasma from which a substance is completely removed (cleared) by the kidneys in a given time period –Units are volume/time, e.g. ml/min, l/hr, etc.

Renal clearance describes how effectively the kidneys...

remove a substance from the bloodstream and excrete it in the urine; different substances have different clearances.

What is the formula for clearance?

Clearance = (Urine concentration of x ×Urine flow rate)/(Plasma concentration of x) = (U×V)/P

Calculate the clearance of urea... ## Footnote P_urea = 20 mM U_urea = 400 mM V = 1 ml/min

Curea = (400 mM · 1 ml/min) / 20 mM = 20 ml/min

What compound is the gold standard for measuring GFR? How do you administer?

Inulin I.V.

What is another method of estimating GFR that is not so invasive?

measure creatinine

Where is creatinine produced?

•endogenously from metabolism of creatine by skeletal muscle.

•Normally, creatinine excretion = ?

creatinine production

•P_creatinine inversely proportional to?

GFR

•Theoretically, if GFR falls to 25% of normal, Pcreatinine should?

increase 4x over a few days

* Theoretically, if GFR falls to 25% of normal, Pcreatinine should increase 4x over a few days * In reality, this inverse relationship isn’t perfect, due to?

–Differences in lean muscle mass among patients, meat intake –Compensatory increased proximal tubule secretion

What is creatinine useful for?

Long-term monitoring of renal fx

What is important to keep in mind regardin creatinine monitoring?

•Significant disease progression can occur with little or no elevation in plasma creatinine, especially in patients with GFR \> 60 mL/min

–In glomerular disease (drop in glomerular permeability due to decreased surface area available for filtration), a drop in GFR is counteracted by?

tubuloglomerular feedback to maintain GFR at near-normal levels

–Nephron loss may be compensated for by?

remaining nephrons (25-30% loss can still appear normal)

–Once GFR does fall, the rise in plasma creatinine will be minimized?

by increased creatinine secretion in proximal tubule

If BUN/Cr is \>20/1 then? Describe the mechanism

Problem is Prerenal. ## Footnote BUN reabsorption is increased. BUN is disproportionately elevated relative to creatinine in serum. Reduced renal perfusion due to hypovolemia

If BUN/Cr is 10-20/1? Describe mechanism

Normal range or postrenal Normal range. Can also be postrenal disease (obstruction) BUN reabsorption is within normal limits

If BUN/Cr is \<10/1? Describe mechanism

Intrarenal ## Footnote Renal damage causes reduced reabsorption of BUN and a lower BUN:Cr ratio.

The principle behind the BUN/Cr ratio is the fact that both urea (BUN) and creatinine are freely filtered by the glomerulus, however...

urea reabsorbed by the tubules can be regulated (increased or decreased) whereas creatinine reabsorption remains the same (minimal reabsorption).

What is Cystatin C?

•Cystatin C is a 13 kilodalton protein that is continuously produced by all nucleated cells in the body, and is freely filtered by the glomerulus. Another marker for GFR

•Similar to creatinine, serum levels of cystatin C become elevated when...

GFR declines.

What is an advantage of Cystatin C for measuring GFR?

•Advantage of Cystatin C is that levels not affected by muscle mass, age, or gender

What is used to estimate RPF? Why?

•Para-aminohippuric acid (PAH) –Freely filtered –Avidly secreted in proximal tubule

•PAH: completely cleared from plasma of \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_when plasma PAH concentraion is low

peritubular capillaries

•RPF ~ CPAH = ?

•RPF ~ CPAH = (UPAH· V)/PPAH