GLOMERULAR FILTRATION RATE

 

                   2. GFR (GLOMERULAR FILTRATION RATE) & ITS REGULATION

Definition: The amount of filtrate that is formed by both the kidneys per minute is called

                   glomerular filtration rate

Normal value:

                 125 ml / minute

                 180 lt / minute

Factors regulating GFR:

                    GFR = Kf X EFP ( Kf – Filtration coefficient, EFP – Effective Filtration Pressure)

1.      Kf = Filtration coefficient which denotes the efficiency of the filtering membrane to filter the  plasmaThis depends upon the following factors:

A. Thickness of filtering membrane: Inversely proportional to Kf . Increase in thickness reduces Kf and thereby reduces GFR also

B. Surface area  of the membrane: Directly proportional to Kf & GFR

      – the normal surface area is about 0.8 m²

      – Contraction of the messangial cells compresses the glomerulus & reduces the area

         of the membrane

            C. Permeability of the membrane:

      – Glomerular capillaries are 50 times more permeable than capillaries of skeletal

         muscle.

      – Permeability of the membrane is influenced by

                  – size of the particles filtered

      – charge of the particles filtered

      – Charge of the pores in the filtering membrane

            -  Particles below 4 nm size (both negative & positive charged) are filtered easily

            -  Particles above 8 nm size ( both negative & positive charged) are not filtered

            -  Particles of 4-8 nm size are filtered with difficulty (Positively charged particles

                      are filtered in this size range where as negatively charged particles are not

                      filtered)

          Reason: The negative charge of the pores in the filtering membrane repel the

                         negatively charged particles

          E.g – Albumin which is about 6 nm size is not filtered as it is negatively charged

                    particle

1.      Effective Filtration Pressure: It is the net outward pressure which determined by forces acting across the filtering membrane. These forces are called Starling forces.

   Starling Forces acting across the filtering membrane:

          1. Glomerular capillary hydrostatic pressure (PGC) = 45 mmHg (favors filtration)

          2. Glomerular capillary osmotic pressure (πGC) = 25 mmHg (oppose filtration)

          3. Bowmans capsular hydrostatic pressure (PBS) = 10 mmHg (oppose filtration)

          4.  Bowmans capsular osmotic pressure (πBS) = 0 (no effect on filtration)

                  EFP = Forces favoring filtration – Forces opposing filtration

                  EFP = P_GC + π_BS – P_BS + π_GC

                                      = (45+0) – (10 + 25)

                                      = 10 mmHg

                            Thus GFR = Kf  X  EFP

                                              = 12.5 X 10

                                              = 125 ml / minute

 Conditions which alter Starling’s forces:

          Glomerular Hydrostatic pressure (P_GC):

i)                    Change in systemic blood pressure – drectly proportional to P_GC

_ii)                    Constriction of afferent arteriole – decreases P_GC

iii)                Constriction of efferent arteriole  - increases P_GC

           Glomerular osmotic pressure (π_GC):

i)                    Dehydration - increases  π_GC

ii)                  Malnutrition - decreases  π_GC

) Renal blood flow:

                     - directly proportional to GFR. Renal bood flow is influenced by nerves, hormones like

                        catecholamines, angiotensin, dopamine & ANP

 

D) Sympathetic stmulation:

                       -strong acute sympathetic stimulation à constriction of both afferent and efferent 

                         arterioles àdecrease in renal blood flow à decrease in GFR