PreClinical Insight: Renal Physiology

In the glomerulus, the primary urine is filtered from the blood, which flows through the capillaries of the glomerulus. With the exception of proteins over a molecular mass of 10 kDa, all components of the plasma are filtered into the primary urine.The renal blood flow is approximately 20% of the cardiac output at rest (1–1.2 l/min). About 10% of renal blood flow is filtered and make up the primary urine. The filtration speed of the primary urine is called glomerular filtration rate (GFR) and is approximately 120 ml/min.The following variables increase the GFR: increased hydraulic pressure in the glomerular capillaries and the area (amount) of glomerular capillaries. The following variables decrease the GFR: increased hydraulic pressure in the Bowmann's capsule, increased colloid osmotic pressure in the glomerular capillaries, decreased permeability of the glomerular capillaries.The glomerular filtration rate cannot be measured directly. For approximation, the elimination of a substance is measured, which is

• completely filtered with the primary urine

• not reabsorbed in the renal tubules

• not secreted in the renal tubules

The concentration of creatinine (UKrea in mg/dl) and the urine volume (Uvol in ml) of a 24 hour urine collection is measured. In addition, the serum concentration of creatinine (PKrea in mg/dl) is measured. The time span of the urine collection has to calculated in minutes (usually 24 h = 1440 min).Substances with exclusive glomerular filtration (without tubular secretion or reabsorption) as creatinine have serum concentrations in direct dependence of the glomerular filtration rate (GFR). A halving of GFR leads to a doubling of the serum creatinine concentration. This means, that a reduction of the GFR by 75% leads to a significant fourfold increase of the serum creatinine concentration. Further (only slight) reductions in GFR lead to dramatic increases creatinine concentrations. Your goal is to ascertain how GFR is impacted by Renal Failure. Hint: Cover the basics to understand pathology!