Elements of Renal Function Flashcards
Identify the different segments of the nephron and specify their locations within the renal cortex and/or medulla.
Glomerulus - cortex Bowman's Capsule - cortex PCT - cortex Descending LOH - cortex into medulla Ascending LOH - medulla into cortex DCT - cortex Collecting duct - cortex
Describe the tubular segments through which ultrafiltrate flows after it is formed at Bowman’s capsule to when it enters the renal pelvis.
Filtrate flows through: Bowman's capsule PCT straight proximal tubule descending thin limb of LOH Ascending thin limb of LOH Ascending thick limb of LOH Macula densa DCT Cortical collecting duct Medullary connecting duct
List in sequence the arteriolar and capillary elements of the renal microcirculation. Describe the anatomical relationships of these vessels to the nephron segments.
Arterial: afferent arteriole - affects GFR glomerular capillaries - does filtration efferent arteriole - affects GFR peritubular capillaries - full of blood sludge, helps maintain osmotic gradient. Usually has low hydrostatic pressure.
Venous: Interlobular vein arcuate vein interlobar vein renal vein
Explain the clearance principle and given values for all variables, use the clearance equation for calculating renal clearance of a substance to estimate GFR with accurate units.
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.
Describes how effectively the kidneys remove a substance from the bloodstream and excrete it in the urine; different substances have different clearances.
Denoted by C=UV/P
Measurement of GFR relies on clearance!
How is excretion calculated?
Denoted by UV
Or, urinary excretion = amt filtered -amt reabsorbed + amt secreted
Why is GFR important?
GFR is an index of functioning renal mass
- determined by starling forces in the glomerulus and glomerular capillary permeability
Typically moderated by changes in glomerular hydrostatic pressure mediated by changes in arteriolar resistance
In pathology, usu due to lost permeability due to lost surface area
What is filtration fraction?
Filtration fraction is the part of the renal plasma flow (RPF) that is filtered into the tubules
Normally about 20% (GFR/RPF)
FF changes with ultrafiltration pressure
- increased FF –> increased oncotic pressure/blood-sludginess –> increased tubular fluid resorption
What are the properies of inulin?
Achieves stable plasma concentration
Freely filtered at glomerulus
Not resorbed, secreted, synthesized or metabolized by the kidney
Gold standard for measuring GFR
- must be infused via IV
What are the properties of creatinine?
Produced endogenously
Secreted in proximal tubule
Ucreatinine overestimated due to secretion in PCT
Blood substances can overestimate Pcreatinine
Good for long term monitoring of GFR
- Pcreatinine is inversely proportional to GFR
- some differences occur, YMMV
Predict how changes in filtration, reabsorption, or secretion will affect renal excretion of each compound.
less filtration = less reabsorption and secretion
more filtration = more reabsorption and secretion, depending on mechanism
excretion< filtration = reabsorption
excretion> filtration = secretion
What is reabsorption?
Reabsorption: net rate of reabsorption or secretion of a substance = difference between glomerular filtration and urinary excretion
-assuming no metabolism or production by kidneys
What are the limitations of inulin and creatine clearances as measures of GFR?
Creatine: endogenous substance, basal rates will vary depending on individual
Overestimation can occur:
creatinine secretion in PCT - overestimate Ucreatinine
substances in blood cause overestimation of Pcreatinine
Inulin is not endogenous, and needs IV infusion
Describe and be able to interpret a graph explaining the relationship between GFR and urinary excretion of BUN and creatinine vs. their serum concentration.
Urea: produced in liver, filtered by kidney
- it is reabsorbed> secreted
- ~40% of filtered urea is excreted
Prerenal >20:1
BUN reabsorption is increased.
BUN is disproportionately elevated relative to creatinine in serum.
Reduced renal perfusion due to hypovolemia
Normal
Can be postrenal/obstruction
BUN reabsorption in normal limits
Intrarenal <10:1
Renal damage = reduced reabsorption of BUN
Lower BUN/Cr ratio
What is PAH and what is it’s effect?
Para-aminohippuric acid
- freely filtered
- secreted in PCT
- completed cleared from plasma of peritubular capillaries when plasma PAH conc. is low
What should creatine clearance reflect?
As GFR falls, Cr clearance should fall with it
- serum Cr should rise as a consequence
But!
- IRL, serum Cr climbs precipitously/exponentially as renal fxn drops below 40%
- fairly stable otherwise
Plasma Cr doesn’t always reflect glomerular disease or GFR
