Calculate the glomerular filtration rate (GFR): the forces that cause the filtration process

2021-04-30 09:40 PM

Normal glomerular filtration levels are not directly measured in humans, they are estimated in animals such as dogs or mice. Based on the results on animals, we can believe that they are approximate in humans.

The glomerular filtration rate is determined by (1) the sum of the hydrostatic pressure and the colloidal pressure across the glomerular membrane, creating the sieve pressure and (2) the filter factor Kf. Mathematically, the glomerular filtration rate is equal to the product between Kf and the sieve pressure:

GFR = Kf x Actual filtration pressure

The net filtration pressure is the result of the sum between hydrostatic pressure and colloidal pressure, which can aid or prevent glomerular filtration through capillaries. This pressure includes (1) hydrostatic pressure in glomerular capillaries (glomerular hydrostatic pressure, PG), promoting filtration; (2) hydrostatic pressure in the Bowman cavity (PB) outside the capillary, preventing leaching; (3) colloidal pressure created by plasma proteins in glomerular capillaries (πG), preventing leaching; and (4) the colloidal pressure of proteins in the Bowman's compartment (πB), promoting filtration (under normal conditions, the protein concentration in the glomerulus is very low so colloid pressure in Bowman's cavity is considered to be zero).

Figure. Synthesis of forces induces filtration by glomerular capillaries. The values ​​shown are estimates for healthy individuals.

The glomerular filtration rate can be calculated as follows:

GFR = Kf x (PG − PB − πG + πB)

Normal glomerular filtration levels are not directly measured in humans, they are estimated in animals such as dogs or mice. Based on the results from animals, we can believe that they are approximate in humans:

Filtration force (mm Hg)

Glomerular hydrostatic pressure 60.

Bowman cavity adhesive pressure 0.

Filter limiting force (mm Hg)

Bowman chamber hydrostatic pressure 18.

Glomerular glue pressure 32.

Sieve pressure = 60 - 18 - 32 = +10 mm Hg

Some values ​​are subject to change under different conditions, which will be exchanged below.

 

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