Renal Flow and Glomerular Filtration Flashcards
What proportion of cardiac output normally perfuses the kidney?
Kidneys recieve 20% of cardiac output which is approx 5L/min so renal blood flow is around 1L/min
What does the glomerular filtration rate depend on?
Glomerular capillary pressure
Plasma oncotic pressure
Tubular pressure
Glomerular capillary surface area
What does freely filtered mean?
Equal concentration in filtrate and plasma
What are the factors affecting filtration of substances in the glomerulus?
Severe haemorrhage - lower hydrostatic pressure GFR down
Obstruction in nephron tubule - more tubular pressure GFR down
Reduced plasma protein concentration - less osmotic pressure GFR up
Small increase in BP - compensated for, no change in GFR
What is the glomerular filtration rate?
Amount of fluid filtered from the glomeruli into the bowmans capsule in ml/min
What is the filtration fraction?
ratio between renal blood flow and amount of filtrate filtered by glomerulus
What is the normal value for glomerular filtration pressures
10-20mmHg
What is glomerular filtration?
Passive process with fluid driven through the fenestrated walls of the glomerular caillaries by heart hydrostatic pressure
What is primary urine?
Clear fluid free of blood and proteins that contains electrolytes and small solutes at the same concentration as in the blood
What is the ultrafiltration coefficient?
Kf, accounts for membrane permeability and surface area available for filtration
What is the normal filtration fraction rate?
normally 20%
What is the normal glomerular filtration rate?
approx 120ml/min
What is the approximate value of renal blood flow and renal plasma flow?
Renal blood flow 1L/min
Renal plasma flow 0.6L/min
What is renal clearance?
Number of litres of plasma that is completely cleared of substance x per unit time
What is the use of renal clearance to asses renal function?
GFR can be calculated by measuring a molecule that is freely filtered and not absorbed eg inulin or creatine
How to calculate renal clearance?
(conc substance in urine x rate of urine production) / conc of substance in plasma
in ml/min
What is inulin?
Plant polysaccharide that is not found in mammals so is transfused
What is special about creatine?
Amount released from body is constant, so clearance stable. Low clearance or high plasma creatine could be renal failure
What is renal failure?
Abrupt fall in glomerular filtration
How is renal plasma flow measured?
PAH - paraaminohippurate
Clearance 625ml/min
Filtered and actively secreted in one pass of the kidney so can be used to measure RPF
Why must drug dose be altered when giving drugs to patients with renal failure?
Excretion of many drugs is impaired in patients with renal failure
Difference in composition of glomerular filtrate and plasma?
Glomerular filtrate has everything in the plasma that is less than 50,000 Da in size
