Renal Physiology: Factors Affecting GFR (PDF)

 Factors affecting the GFR:- 1. Changes in the PGC:  Changes in blood pressure does not effect the GFR unless the mean pressure is above 160mmHg or below 80mmHg.  Drop of ABP below 80mmHg will stop the GFR leading to anuria.  Marked ↑in ABP above 160mmHg ↑the GFR → ↑urine output (pre...

 Factors affecting the GFR:- 1. Changes in the PGC:  Changes in blood pressure does not effect the GFR unless the mean pressure is above 160mmHg or below 80mmHg.  Drop of ABP below 80mmHg will stop the GFR leading to anuria.  Marked ↑in ABP above 160mmHg ↑the GFR → ↑urine output (pressure diuresis)  Afferent arteriolar constriction: ↓the GFR by decreasing PGC.  Efferent arteriolar constriction: Mild constriction ↑ the GFR by ↑ the PGC. Severe constriction ↓the GFR by decreasing PGC. 2. Changes in πGC:-  Hypoproteinemia: ↑ the GFR by ↓ the πGC.  Dehydration: ↓ the GFR by ↑ the πGC. 3. Changes in the PT:-  Edema of the kidney:  Distension of the kidney with in the renal capsule ↑ hydrostatic pressure in the tubules & ↓the GFR.  Obstruction of the urinary tract:  ↑ PT ↓the GFR. 4. Changes in the surface area:  If the glomerular surface area available for filtration ↓ the GFR will ↓.  This occur due to:- A. A decrease in the number of functional nephrons as that occur in chronic renal failure or after nephrectomy. B. Contraction of mesangial cells. 5. Changes in permeability:  The GFR is directly proportional to the glomerular capillary permeability.  In nephritis the permeability increase which ↑ the GFR. 6. Sympathetic stimulation:-  Mild stimulation:  Produces no effect due to auto-regulation.  Strong stimulation:  Causes marked vasoconstriction in glomerular arterioles leading to reduction of both RBF & GFR. 7. Tubulo-glomerular feed back:-  When the tubular fluid flow rate ↑ at the distal part of the nephron (ascending LH & DCT).  GFR in the same nephron will ↓ and vice versa.  Mechanism:-  ↑ the tubular flow rate → large amount of fluid, sodium and chloride enter the distal tubules.  Na+ & CL- enter the macula densa cells.  ↑ Na+ concentration increases Na+ -K+ pump activity.  As a result of ↑ATP hydrolsis more adenosine is formed.  Adenosine act via adenosine A1 receptors on→↑ calcium release.  Calcium causes contraction of the vascular smooth muscle of the afferent arterioles leading to their constriction & ↓ the GFR. Figure 9 Using inulin or creatinine clearance. 1. Freely filtered in the glomeruli, then the filtered amount is excreted. 2. It is neither reabsorbed nor secreted in the renal tubules i.e. the amount of inulin excreted/min = the amount filtered/min.  This means that the volume of plasma that is cleared from inulin/min (inulin clearance) is that volume filtered in the glomeruli/min (the GFR).