The kidney
(Pseudo) Practical questions…
for questions… Ella ([email protected])
• The kidneys are all about keeping the body’s
homeostasis
Ingestion
Product of
metabolism
H2O
Ca++
K+
Na+
H2O
ClH2O
Mg++
Excretion
metabolism
∑
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Ingested +
Product of metabolism
=∑
Excreted + Metabolized
Glomerular Filtration Rate (GFR)
GFR is the volume of fluid filtered out of the plasma through
glomerular capillary walls into Bowman's capsules per unit of
time.
GFR
It is also the best estimate of kidney function for diagnosis
of kidney disease.
The normal range of GFR for males and females is: (depending on age,
race, etc.)
Males: 97 to 137 ml/min. (139 – 197 L/day!)
Females: 88 to 128 ml/min.
Kidney function assessment
• Assessing GFR?
• GFR is best assessed by computing the
Renal clearance (C)
• Renal clearance - The volume of plasma
completely cleared of a specific compound
per unit time (units – ml/min)
• Most accurate assessment is by computing
Inulin clearance – it is are freely filtered but
not secreted or reabsorbed
• second most accurate is computing
creatinine clearance
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Kidney function assessment
• You wish to evaluate kidney function in a 65 year
old male and ask him to collect his urine over a 24
hours period.
He returns to you 4.32 L of urine, collected over the
preceding 24 hours. The clinical lab returns the
following results from analysis of his urine and
plasma samples:
Plasma creatinine:
Urine creatinine:
Plasma potassium:
Urine potassium:
0.03 mg/ml
0.3 mg/ml
0.005 mmol/ml
mmol/ml
0.01 mmol/ml
mmol/ml
What is this man’
man’s GFR?
Plasma creatinine:
Urine creatinine:
Plasma potassium:
Urine potassium:
0.03 mg/ml
0.3 mg/ml
0.005 mmol/ml
mmol/ml
0.01 mmol/ml
mmol/ml
Cs =
Us
V
Ps
Us V
Ps
= the substance’s concentration in the urine
= the volume of urine collected per time unit
= the substance’s concentration in the plasma
Ccr = (0.3 mg/ml * 4320 ml/day)
0.03 mg/ml
= 43200 ml/day = 30 ml/min
So his GFR is lower than normal (97 to 137 ml/min)…
implying kidney disease
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Plasma creatinine:
Urine creatinine:
Plasma potassium:
Urine potassium:
0.03 mg/ml
0.3 mg/ml
0.005 mmol/ml
mmol/ml
0.01 mmol/ml
mmol/ml
• What is the Renal clearance of K?
• CK = (0.01 mmol/ml * 4320 ml/day)
0.005 mmol/ml
= 8640 ml/day = 6 ml/min
• As CK < GFR, K is reabsorbed (NET reabsorbed!)
• What is the renal reabsorption rate of potassium (K) in
this patient?
K+ excretion = K+ filtered – K+ reabsorbed + K+ secreted
• Computing the difference between how much is filtered
and how much is excreted gives total reabsorption rate.
Plasma creatinine:
Urine creatinine:
Plasma potassium:
Urine potassium:
0.03 mg/ml
0.3 mg/ml
0.005 mmol/ml
mmol/ml
0.01 mmol/ml
mmol/ml
• Assuming that K is freely filtered, it would be filtered at a rate
of GFR * plasma concentration (this is the Filtered load)
• FLK = GFR x PK = 30 ml/min * 0.005 mmol/ml
= 0.15 mmol/min
• the difference, or reabsorption rate, is therefore:
FLK – UKV = 0.15 mmol/min – 0.01 mmol/ml * 4320 ml/day
= 0.15 mmol/min – 0.01 mmol/ml * 3 ml/min
= 0.12 mmol/min
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• Therefore most of the K is reabsorbed…
• How is this achieved?
• K is freely filtered, it is mostly reabsorbed
and later secreted.
• The regulation is only for secretion
K+ Reabsorption
Not regulated!
In the proximal tube and thick
ascending limb of the loop of Henle
Absorption of ~90% of K+
K+ Secretion
Coupled to Na+
reabsorption
Regulation of K+
secretion is by
aldosterone that drive
increased uptake by the
Na+/K+ ATPase
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Hydrogen ion regulation
PH=7.4
pH
pH
[H+]
[H+]
Acidosis
Alkalosis
Carbonic
anhydrase
Lung
regulation
CO2 + H2O
H2CO3
Carbonic
acid
H+ + HCO3-
Kidney
regulation
Bicoarbonate
• The clinical laboratory returns the following arterial blood
values for a patient:
pH: 7.31 (normal - pH 7. 36-7.44)
plasma HCO3- : 32 mmol/L (normal - 24-32 mmol/L)
plasma PCO2 : 65 mmHg (normal - 35-45 mmHg)
what is this patient’s acid-base disorder?
acidosis/alkalosis?
respiratory or metabolic based?
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pH: 7.31 (normal - pH 7. 3636-7.44)
plasma HCO3- : 32 mmole/L
mmole/L
(normal - 2424-32 mmol/L)
mmol/L)
plasma PCO2 : 65 mmHg (normal - 3535-45
mmHg)
• pH is lower than normal, therefore:
Acidosis
• What is the origin of the acidosis?
• Increased PCO2 implies that it is
respiratory based (not clearing out enough
CO2)
• How do the kidneys compensate for
acidosis?
Carbonic
anhydrase
CO2 + H2O
H2CO3
Carbonic
acid
HCO3- + H+
Bicoarbonate
Bicarbonate “reabsorption”
H+ ATPase pumps
Na+/H+ countertransporters
Carbonic
anhydrase
CO2 + H2O
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H2CO3
Carbonic
acid
HCO3- + H+
Bicoarbonate
Bicarbonate creation
Carbonic
anhydrase
CO2 + H2O
H2CO3
Carbonic
acid
HCO3- + H+
Bicoarbonate
Bicarbonate creation
NH3 + H+ -> NH4+
Carbonic
anhydrase
CO2 + H2O
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H2CO3
Carbonic
acid
HCO3- + H+
Bicoarbonate
pH: 7.31 (normal - pH 7. 3636-7.44)
plasma HCO3- : 32 mmole/L
mmole/L
(normal - 2424-32 mmol/L)
mmol/L)
plasma PCO2 : 65 mmHg (normal - 3535-45
mmHg)
• Therefore we expect to find in this patient
with respiratory acidosis:
• Higher plasma HCO3- (we don’t see it!
Perhaps the kidneys are not handling this
acidosis well)
• elevated urine NH4+
• lowered urine pH
• reduced urine excretion of HCO3-
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