Flow and filtration Flashcards
What proportion of CO goes to the kidneys?
Receives 20-25% CO in order to generate high pressure for ultrafiltration - achieved with direct aortic branching
What is the filtration fraction?
Ratio of renal plasma flow to amount of filtrate filtered by the glomerulus - normally 0.2
What is normal Renal Blood Flow (RBF)?
approx. 1L/min
What is normal Renal Plasma Flow (RPF)?
approx. 0.6L min-1 (renal blood flow * (1-haematocrit))
What is the equation for Glomerular filtration rate?
GFR = RPF x FF
Describe the process of Glomerular Filtration:
Passively occurs as fluid driven through semipermeable walls of glomerular capillaries into Bowman’s capsule by cardiac hydrostatic pressures
What is the filtration barrier?
Fenestrated capillary endothelia and semipermeable Bowman’s capsule highly permeable to fluids and small solutes but not cells, proteins and drugs carried by proteins
What is “freely filtered?”
Solute found at same conc in blood and filtrate
What is Primary urine?
Clear ultrafiltrate fluid free from blood and proteins
What is the glomerular filtration rate?
Amount of fluid filtered from the glomeruli into the Bowman’s capsule per unit time (ml/min) - sum of rate of all functioning nephrons (loss of nephrons = \/ Kf = \/ GFR)
What is Kf?
Ultrafiltration coefficient and represents membrane permeability and surface area available for filtration - if changed then creates GFR imbalance, and kidney disease may reduce number of glomeruli and hence surface area and Kf
What is a normal GFR?
120ml/min
What will be the effect of drugs or hormones that dilate glomerular arterioles on Kf and hence GFR?
Increase Kf
Increase GFR
What does myogenic auto-regulation in glomerular arterioles allow?
Constant GFR;
VSMCs contract when smooth muscle stretched, so when afferent arterial pressures rise, vessel resistance does too, reducing the blood flow to keep the GFR constant
Describe four events that effect GFR:
Severe haemorrhage: decreases Pgc so will decrease GFR
Nephron tubule obstruction: increases Pt/decreases Kf so decreases GFR
Reduced [plasma proteins]: decreases πgc so increases GFR
Small BP increase: no effect due to myogenic autoregulation
What is the role of GFR in renal diagnostics?
Fall in GFR is cardinal feature of renal disease, and excretory proteins will build up in plasma; raised plasma [creatinine] diagnostic of renal disease, and excretion of other substances will be impaired, so may need to adjust drug dosage
What is the driving force of glomerular filtration?
Hydrostatic pressure in glomerular capillaries (Pgc)
What opposes hydrostatic pressure of glomerular capillaries?
Hydrostatic pressure of tubule (Pt) and Osmotic pressure of glomerular plasma proteins (πgc)
What equation describes net ultrafiltration?
Puf = Pgc - Pt - πgc
What factors effect GFR?
Pgc, πgc, Pt, Kf - can be hormonally or neurally controlled
What will happen to pressures in the kidney if there is an increase in arterial blood pressure?
Increased Pgc hence Puf and GFR increase
What happens if there is an increase in plasma protein concentration?
Increase in πgc hence Puf and GFR decrease
What will happen to Puf and GFR if there is a Ureteral obstruction?
Increases Pt therefore Puf and GFR decrease
What is renal clearance?
The extent to which substances passing through the kidneys are removed from the blood - no. Litres plasma that are completely cleared of the substance per unit time
