Glomerular filtration rate

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       Glomerular filtration rate (GFR) is a blood test used that checks how well your kidneys are working. Your kidneys have tiny filters called glomeruli. These filters help remove waste and excess fluid from the blood. A GFR test and excess fluid from the blood passes through these filters each minute.




     The amount of the filtrate formed by the kidneys per minute is called the glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/minute, i.e., 180 litres per day. The kidneys have built in mechanisms for the regulation of glomerular filtration rate.

     * The average per minute is: 125ml/min (20% of renal plasma flow)
   
     * The average per day is: 180 L/day (gallons)

     * 99% will reabsorbed (178.55 L/day) from filtrate and only 1% will excreted (1.5-2 L/day)

  So, most filtered water must be reabsorbed or death would ensure from water lost through urination.

     * These values various with: kidney size, lean body weight and number of functional nephrons.

     * The relation between GFR an Net filtration pressure;

                                   ↑ NFP → ↑GFR
  
                                   ↓ NFP → ↓ GFR

  Normally changes in GFR is a result from change of blood pressure.

    If GFR is too high:

            Fluid flows through tubules too rapidly to be absorbed (will not absorbed very well). It will lead to:
        
                   * Urine output rises

                   * Creates threat of dehydration and electrolyte depletion.

    If GFR is too low:

              Fluid flows sluggishly through tubules. It will lead to:

                   * Tubules reabsorbs wastes that should be eliminated

                   * Azotemia develops (high levels of nitrogen-containing substances in the blood).


      Glomerular filtration depend on:


              1. Pressure gradient across the filtration barrier:

                   * Glomerular hydrostatic pressure (=60 mmHg ). It promotes filtration.    

                   * Hydrostatic pressure in bowman's capsule (=18mmHg). It opposes filtration.

                   * Colloid osmotic pressure of glomerular plasma proteins (=32mmHg). It opposes filtration.

 So, GFR net filtration pressure (NFR)= 60-18-32 =10 mmHg


               2. Blood circulation throughout the kidneys

                    * Renal blood flow through kidney is 20% of cardiac output (1200 ml/min)

                    * Renal plasma flow through kidney is about (650 ml/min)

So, GFR is about 16-20% of renal plasma flow (650 * 0.19 =125 ml/min or 180 L/day)


                3. Permeability of the filtration barrier (through 3 layers):

                    * The endothelium of capillary

                    * Filtration slits of epithelial lining of bowman capsule (podocytes)

                    * Basement membrane (high negatively due to presence of proteoglycans)

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