Elements of Renal Function Flashcards
What are the primary driving forces for increases or decreases in filtration?
Changes in glomerular hydrostatic pressure OR pressures in Bowman’s space (disease state)
What is the primary driving force for increased or decreased reabsorption into peritubular capillaries?
Changes in capillary hydrostatic pressure due to changes in RBF
How does exercise affect RBF?
Is this good?
Decreases, since it’s shunted away to the muscles
Can be dangerous to kidneys if too much
How does blood flow change in the different regions of the kidney?
Why?
Decreases the deeper you get
Medulla needs slower flow so that concentrated ions, etc. are not washed away
A patient has consistent increased blood flow to the medulla of the kidneys. How will this affect its ability to concentrate urine?
Decreased - the gradient in the medulla will be washed out due to increased blood flow to the medulla
A patient drinks a lot of hypotonic water. How will this affect:
GFR?
Peritubular Starling forces?
Reabsorption?
GFR constant (autoregulation)
Cap. hydro pressure UP (more fluid in capillary)
Reabsorption DOWN (Starling ∆)
What are neurohormonal reflexes?
Changes in sympathetic discharge and hormone release based on changes in blood volume and osmolarity
2 main sensors that cause downstream regulation?
Where are each?
Volume receptors (R. atrium)
Osmoreceptors (brain)
An increase in blood volume will cause what sympathetic changes?
Decreased discharge - increased urination, decreased ADH, decreased RAAS, decreased thirst, decreased reabsorption
Sympathetic discharge on the kidney will have what 4 main effects?
Decreased GFR
Increased PCT reabsorption
Increased DCR reabsorption
Increased thirst (Ang 2)
The overall effect of sympathetic discharge on the kidney is what?
Due to what 3 things?
Fluid and electrolyte retention
Decreased urination
Decreased Na excretion
Increased water intake
If sympathetic discharge causes afferent constriction, what will be the effects on RBF and GFR?
Compensation?
What about reabsorption? Why?
RBF will decrease, so GFR will decrease
Ang2 causes efferent constriction, returning GFR to normal
Sympathetics cause renin release (granular cells), stimulating Na (and thus H2O) reabsorption (aldosterone) in the PCT, TAL, DCT, and CD - INCREASED B.V.
How are sympathetics stimulated in the kidney?
What are the downstream affects on reabsorption?
Drop in volume receptor activation
Vagus n. to vasomotor center
Sympathetic stimulated there
Increased RAAS, so increased aldosterone, so increased reabsorption
How to calculate urinary excretion volume?
F + S - R
Urinary excretion = amount filtered – amount reabsorbed
+ amount secreted
Definition of GFR
How does this relate to RBF?
Volume (mL) of plasma filtered into the kidneys per minute
Normally 20% of RBF, but only partially dependent on RBF
Definition of filtration rate
Calculate?
Amount of a given substance filtered per unit time
GFR x plasma [ ] Filtration rate (Filtered load) of any freely filtered substance = GFR x plasma concentration of substance
Urinary excretion rate
Calculate?
Amount of a substance excreted per unit time
Urine [ ] x urine volume (V)
Renal clearance
Calculate?
Used how?
Volume of substance from which a given substance is completely cleared in a given time
Urinary excretion rate / plasma [ ]
Measures renal function and GFR
How can GFR be related to clearance?
The clearance of substances freely filtered and neither metabolized, secreted, or reabsorbed = GFR
Why is creatinine better than inulin?
Creatinine is endogenous and consistently made by skeletal muscle
How is creatinine concentration related to GFR?
Inversely related
If GFR goes up, clearance of it is going up, so plasma concentration is down
Why is the inverse relationship between creatinine [ ] and GFR not perfect?
Some creatinine will undergo compensatory secretion in the PCT
What is the idea behind the BUN/Cr ratio?
Both are freely filtered, but only BUN is reabsorbed and can be regulated, making its concentration much higher and variable in the blood
A ratio of >20:1 means what?
There is more BUN in the blood than normal, thus the patient is dehydrated OR blood is not getting to the kidney for filtration
PRE-RENAL
A ratio of
BUN is not being reabsorbed properly OR is being overly filtered due to a glomerular issue
INTRA-RENAL
A ratio between 10:1 and 20:1 means what?
Either the patient is normal…
OR urine is not being excreted due to a downstream block
POST-RENAL / NORMAL
What is the use of PAH?
Freely filtered but AVIDLY secreted in PCT
PAH clearance = RPF since ALL is excreted, especially in smaller [ ]’s
Filtration Fraction
A higher FF means what?
GFR/RPF
Less fluid in efferent arteriole, so higher oncotic pressure, so more reabsorption
