9. Renal Physiology III - Measurement of Renal Function Flashcards
FUNCTIONS of KIDNEY
- EXCRETION of metabolic waste products and foreign chemicals
- REGULATION
(water and electrolytes balance, arterial pressure, acid-base, erythrocyte production, D3 production..) - SECRETE, EXCRETE or METABOLISE HORMONES
- Gluconeogenesis (contributes)
what is RENAL CLEARANCE
the VOLUME OF PLASMA that is completely CLEARED of the SUBSTANCE by kidneys per unit of time
(provide useful way of quantitating effectiveness with which kidneys excrete various substances)
we can use RENAL CLEARANCE to quantify:
- RENAL BLOOD FLOW
- GFR
- TUBULAR REABSORPTION
- TUBULAR SECRETION
how much of the CARDIAC OUTPUT (BLOOD) flows through KIDNEYS
about 20%
RENAL PLASMA FLOW (RPF) is equal to
Renal Blood Flow x (1-Hematocrit)
(hematocrit : volume of red blood cells in blood)
usually 55% of rbf
GFR averages how much of RENAL PLASMA FLOW (filtration fraction = GFR/RPF)
20%
(20% of plasma that goes through glomerulus is filtered)
what is the GFR in an average adult human
125 ml/min
or 180 L / day
(RPF = 625 ml/min
RBF = 1200 ml/min)
(FF = 125/625 x 100 = 20%)
how much of the FLUID FILTERED is normally REABSORBED
OVER 99%
(Urine flow rate is less than 1% of GFR)
CALCULATION to measure RENAL CLEARANCE (clearance rate of a substance) C
C = U V / P
U = conc. of substance in urine
V = urine flow rate / volume
P = conc. of substance in plasma
what substance do we use to measure exact GFR where C = GFR
INULIN (polysaccharide
(freely filtered, NOT reabsorbed, NOT secreted, not broken down, not toxic)
(clearance rate would be 125 ml/min)
Creatine can be used but will be secreted so overestimate (140 ml/min)
how much of RENAL PLASMA FLOW is actually filtered (GFR)
20%
what substance can we use to calculate RENAL PLASMA FLOW where C = RPF
PAH (PARA-AMINOHIPPURIC ACID)
- freely filtered and almost completely SECRETED
(average of 90% is cleared by kidneys from renal blood stream in single circulation)
C = UV/P = RPF
U= PAH conc in urine
V= urine flow rate
P = PAH conc in plasma
how to calculate RBF when you have RPF
RBF = RPF X (1-HEMATOCRIT)
hematocrit = volume rbc in blood
how to calculate EXCRETION RATE
U x V
how to calculate Reabsorption Rate
(GFR x P) - (U x V)
Clearance rate of Glucose
0
(all reabsorbed)
(unless diabetes)
Clearance rate of Sodium
0.9 - LOW
MOST SODIUM REABSORBED
Clearance rate of Sodium
0.9 - LOW
MOST SODIUM REABSORBED
what would happen to creatinine if GFR levels declined
secrete more creatinine from blood so stay same
therefore not accurate marker
another marker for GFR
SERUM CYSTATIN C concentration
cystatin c - small protein
expressed in all nucleated cells
Freely Filtered
NOT secreted
reabsorbed but catabolised (so not returned to plasma)
markers for GFR
- Inulin
- Creatine (standard surrogate marker)
- cystatin c
3 LAYERS in the FILTRATION BED of the GLOMERULUS
- ENDOTHELIUM
- BASEMENT MEMBRANE
- EPITHELIUM - PODOCYTES
(have foot like-processes which form slit pores)
what is PROTEINURIA
large amounts of PROTEIN in the URINE
urine dip stick to test
what is HAEMATURIA
BLOOD in URINE
Urine dip stick to test
