Renal Plasma Clearance Flashcards
What is GFR?
→ How much filtrate is removed from the blood each minute
What is the first step in urine formation?
→ Begins with glomerular filtration
What is the composition of glomerular filtrate like?
→ same as plasma
→ no cells
→ protein-free
What is proteinuria a sign of?
→ renal/ urinary tract disease
What is the primary force favoring filtration?
→ Glomerular capillary pressure = 60mmHg
What is the force opposing filtration?
→ Hydrostatic pressure in Bowman’s space = 15mmHg
→ Osmotic force of plasma proteins = 29mmHg
What is the urinary excretion rate?
→ GFR - reabsorption rate + secretion rate
What does GFR contribute to?
→ rapid removal of waste product
How much is the GFR per day?
→ 180L per day
How much is the body plasma volume?
→ 3L
What 2 factors is GFR determined by?
→ hydrostatic and oncotic pressures across capillary membranes
→ permeability of capillary filtration barrier & surface area available
How is GFR measured?
→ not directly but by the measurement of the excretion of a marker
→ a substance that is freely filtered but neither reabsorbed or secreted e.g creatinine
What is inulin?
→ inert polysaccharide
→ filters freely through the glomerular membrane
When does plasma inulin concentration become stable?
→ When inulin fusion rate = inulin excretion rate
What is the GFR equation for the inulin method?
→ GFR = Uin x V / Pin
→ V = urine vol/collection time
→ Pin = plasma inulin concentration
→ Uin = urine inulin concentration
What is the definition of renal clearance?
→ The volume of plasma that is completely cleared of the substance by the kidney per unit time (excreted in urine each minute)
What is the clearance of inulin?
→ same as the GFR
What are the drawbacks of the inulin method?
→prolonged infusion
→ repeated plasma samples
→ difficult routine clinical use
What is used clinically to measure GFR?
→ creatinine
What are the advantages of using creatinine ?
→intrinsic inert substance → released at a steady level in plasma from skeletal muscle → no infusion needed → freely filtered → not reabsorbed
What are the disadvantages of creatinine?
→ some is secreted into the tubule
What is trimethoprim?
→ competitive inhibitor of creatinine
→ given during kidney infections etc
How is creatine made?
→ Taken in the diet
→ found in the liver
What is creatine metabolized by?
→ metabolized to phosphocreatine by the muscle