Physiology of the kidney (4/7): Glomerular filtration rate

Anatomy of the kidney (1/7): Gross anatomy
Anatomy of the kidney (2/7): Histology of the glomerulus and nephron

Anatomy of the kidney (3/7): Histology of renal tubules
Anatomy of the kidney (4/7): Physiology of the glomerular filtration rate
Anatomy of the kidney (5/7): Physiology of the tubular reabsorption
Anatomy of the kidney (6/7): Physiology of the renin-angiotensin-aldosterone system
Anatomy of the kidney (7/7): Physiology of erythropoetin, endothelins and vitamin D

Review literature: (Benninghoff, 1993) (Schmidt und Thews, 1995).

Glomerular Filtration Rate

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. Subsequently, the active and passive reabsorption of important substances in the renal tubules from the primary urine is of elementary importance (see tubular reabsorption .

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.

Renal Clearance

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

Definition of the Renal Clearance (Cl)

Renal clearance is the virtual plasma volume per minute, from which a substance is completely eliminated. If a substance is chosen, which is not secreted or reabsorbed in the renal tubules, the measured clearance corresponds to the glomerular filtration rate (GFR).

Calculation of the creatinine clearance

The concentration of creatinine (U_Krea in mg/dl) and the urine volume (U_vol in ml) of a 24 hour urine collection is measured. In addition, the serum concentration of creatinine (P_Krea in mg/dl) is measured. The time span of the urine collection has to calculated in minutes (usually 24 h = 1440 min). The creatinine clearance (Cl_Krea) is calculated with the following formula:

Calculation of the creatinine clearance

An estimation of the creatinine clearance can be obtained using the formula of Cockroft and Gault using the serum concentration of creatinine (S_Krea in mg/dl), gender (factor F=72 for men and F=85 for women), weight (kgKG in kg) and age (Alter in years):

Estimation of the glomerular filtration rate using the formula of Cockroft and Gault

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 [fig. creatinine concentration and GFR].

fig. creatinine concentration in dependence of the glomerular filtration rate (GFR). Only a pronounced reduction of the GFR leads to a significant rise of the serum creatinine concentration.

Kidney, distal nephron

Index

Kidney, reabsorption

Index: 1–9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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References

Benninghoff 1993 BENNINGHOFF, A.:: Makroskopische Anatomie, Embryologie und Histologie des Menschen.
15. Auflage.
München; Wien; Baltimore : Urban und Schwarzenberg, 1993
Schmidt, R. F. & Thews, G. T. (ed.): Physiologie des Menschen
26. Auflage
Berlin; Heidelberg; New York: Springer, 1995

Deutsche Version: Physiologie der Nieren: Glomeruläre Filtrationsrate und Kreatininclearance.

Last update:

Dr. med. Dirk Manski (E-Mail)