Glomerular Filtration Rate (GFR) – Physiology Notes by Dr. Preeti Tyagi

1. GFR (Glomerular Filtration Rate) By Dr. Preeti Tyagi (MBBS, MD), Professor of Physiology VMMC & Sufdarjung Hospital New Delhi

2. INTRODUCTION Kidney is the main excretory organ. kidneys perform important function by filtering the plasma and removing substances from the filtrate at variable rates, depending on the needs of body. Ultimately, kidneys ‘clear’ unwanted substance from the filtrate. their most

3. URINE FORMATION The rate at which different substances are excreted in the urine represents the sum of three renal process :- 1. Glomerular filtration. 2. Reabsorption of substances from renal tubules into blood. 3. Secretion of substances from the blood into renal tubule.

4. Urinary excretion rate= filtration rate- reabsorption rate+secretion rate Urine formation begins when a large amount of fluid that is virtually free of proteins is filtered from the glomerular capillaries into bowmen`s capsule

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6. GLOMERULAR FILTRATION Glomerular filtration is the initial step in urine formation, the plasma that traverses the glomerular capillaries filtered by the highly permeable glomerular membrane and the resultant fluid, the glomerular filtrate is passed into bowman`s capsule Normal renal blood flow=1.22 to 1.3 L/min Approx. 1/10 of RBF is filtered off into bowman`s capsules.Blood passes through the glomeruli, the filtrate is identical with plasma in respect of following 1.Osmolality, ph 2.Electrical conductivity 3.Concentration of electrolytes and of smaller organic molecules E.g. glucose, urea, Creatinine

7. GFR Blood flow to the kidney in resting adult is 1.1 to 1.3 L/min when this blood flows through glomerular capillaries ,filtration takes place through glomerular membrane. Glomerular filtration rate is the rate of formation of ultra filtrate from blood in the bowman`s capsular space when it’s passing through the glomerular capillaries.

8. Normal GFR In an average size adult man, GFR is 125ml/ min or 180 litres/day. As the normal urine value is 1 liter , 99% or more of the filtrate is reabsorbed and only 1% or less is coming out in the form of urine. In case of woman value of GFR is abt. 10% less than that of man

9. MECHANISM OF GFR : Factors governing filtration across glomerular capillaries are the same as all other body capillaries. It depends on: 1.Hydrostatic pressure gradient across capillary wall 2.Osmotic pressure gradient across capillary wall 3.Permeability of capillaries 4.Size of capillary bed

10. GFR = Kf × net filtration pressure Where, Kf = Glomerular filtration coefficient (Gfr at 1mmHg Nfp in both kidney) Normal value of Kf= 12.5 ml/min/mm Hg Net filtration pressure= (Pglo – Pbow) + (πglo – πbow) forces favouring filtration (mmHg) glomerular capillary hydrostatic pressure 60 Bowman’s capsule colloidal osmotic pressure 0 Forces opposing filtration (mmHg) Bowman’s capsule hydrostatic pressure 18 Glomerular capillary colloidal osmotic pressure 32 Source: Guyton & Hall textook of medical physiology 2ndSouth-Asia edition

11. Net effective pressure(10 mmHg) = Glomerular hydrostaticpressure(60 mmHg) – Bowman’s capsule hydrostatic pressure (18 mmHg) – COP glomerular capillary (32 mmHg)+COP bowman’s capsule (0mmHg) Net filtration=(60-18-32+0)mmHg =10mmHg GFR= Kf × NFP = 12.5 × 10 = 125 ml/min

12. Substances used to measure GFR Characteristics of substances used to measure GFR 1.Freely filtered by glomerulus 2.Not secreted by tubules 3.Not reabsorbed by tubules 4.Non-toxic 5.Not metabolised in kidney 6.Doesn’t affect GFR 7.Conc. In plasma as well urine should be measurable Examples: Inulin creatinine Dahlia,source of inulin

13. Inulin clearance Inulin 1.Polysaccharide of fructose 2.Mol. Wt.- 5200 Dalton 3.Extracted from roots of Dahlia WHY INULIN ? Freely filtered Not reabsorbed or secreted Not synthesized, destroyed or stored in kidneys Concentration in plasma and urine can be easily measured Inulin clearance: Highest standard Highly accurate Not commonly used in the clinical practice

14. Endogenous creatinine used to measure GFR Creatinine is 1.By-product of muscle metabolism 2.Almost entirely cleared by glomerular filtration 3.Doesn’t require intravenous infusion 4.Widely used to measure GFR Small amount of creatinine is secreted by tubules Which increases urinary conc. of creatinine i.e Uc × V Plasma creatinine determination are inaccurate at low concentrations because method for determination measures small amount of other constituents Plasma creatinine conc. Is increased due to some non-specific chromogen Error tends to cancel For precise GFR measurement inulin clearance is used

15. FACTORS AFFECTNG GFR Age Renal Blood Flow Afferent Arteriolar Resistance Efferent Arteriolar Resistance Glomerular Hydrostatic Pressure Hydrostatic Pressure in Bowman’s Capsule Colloidal Osmotic Pressure / Oncotic Pressure Glomerular Capillary Filteration Coefficient , Kf

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