conditions with decreased afferent pressure (renal artery stenosis, aortic valve
stenosis, aortic coarctation, hypoperfusion states) a compensatory constriction of
the efferent arteriole would provide enough pressure to allow ultrafiltration. The
glomerular arterioles tone is mainly regulated by angiotensin.
The renal function of infants approaches adult state at the end of the second year
of life. Useful parameters to assess the renal function in children are the
glomerular filtration rate (GFR), urine output, urine concentrating capacity,
sodium balance, acid/base balance and plasma renin activity.
Glomerular Filtration rate
Glomerular filtration rate (GFR) refers to the amount of serum filtrated across the
glomerulus each minute. A GFR of 50 mL/min per 1.73m^2 is considered normal in
the first week of life, increasing to 86 mL/min per 1.73m^2 at the end of the first
year. Adult levels of GFR are reached around two years of age, as the lean body
mass increases and steady creatinine production is achieved (90-140 mL/min).
[25]
Inulin clearance is the gold standard method to determine the GFR because it is
excreted 100% by the kidney after its administration. The test measures inulin
concentration in plasma and urine after 90 minutes of IV infusion of inulin diluted
in mannitol.
Clearance of inulin is calculated using the following formula:
C= UV/PC=clearance, U=urinary concentration, V=volume, P=plasma concentration
Values are corrected according to the surface body area applying the following
formula: