lecture 32 - glomerular function Flashcards
Is filtration relatively selective or unselective at the golmerulus?
Unselective
What 3 factors determine glomerular filtration?
Renal blood flow, filtration barrier, driving forces
What proportion of cardiac output goes to renal blood flow?
20%
What are the 4 forces/pressures driving or opposing glomerular filtration?
Glomerular hydrostatic pressure (GHP), blood colloid osmotic pressure (BCOP), Capsular hydrostatic pressure (CsHP) and Capsular colloid osmotic pressure (CsHOP)
What relative pressures favour filtration in the glomerulus (positive or negative)?
Positive pressures
What relative pressures oppose filtration in the glomerulus (positive or negative)?
negative pressures
What are the 2 types of pressures that favour or oppose glomerular filtration?
Hydrostatic pressure, colloid osmotic pressure
What is hydrostatic pressure?
Pressure due to the volume of fluid in a space
What is colloid osmotic pressure?
Osmotic pressure due to protein in a fluid
What are the positive pressures that favour filtration in the glomerulus?
Glomerular hydrostatic pressure
What are the negative pressures that oppose filtration in the glomerulus?
Blood colloid osmotic pressure and capsular hydrostatic pressure
What is the source of glomerular hydrostatic pressure?
Blood pressure - blood in glomerular capillaries tries to force water into the capsular space
What is the source of the blood colloid osmotic pressure?
Protein, specifically albumin, in the blood tries to pull fluid from the capsular space into the glomerular capillary
What is the source of the capsular hydrostatic pressure?
Pressure of filtrate and fluid already present in the capsular fluid opposes filtration
Why is the value of the capsular colloid osmotic pressure 0mmHg?
Because no protein in the the capsular space as it is too big to be filtered
What is the net filtration pressure in glomerular filtration?
+10mmHg (favours filtration)
What is the value of RBF - renal blood flow?
1200mL/min
What is the renal plasma flow?
As 55% of blood is plasma, 55% of RBF = 625mL/min
What is the glomerular filtration rate?
20% of the renal plasma flow = 125mL/min
What is the glomerular filtration rate (GFR) per day?
180L/day
What is the filtered load?
Amount of a particular substance/solute filtered per minute
What is the equation for filtered load?
Filtered load = GFR * solute plasma conc.
What is renal clearance?
The volume (mL) of plasma that is cleared of a substance by the kidneys per unit time (minute)
What can renal clearance be used to estimate?
Glomerular filtration rate (GFR)
What is the equation for renal clearance (Cx)?
Cx = (Ux * V)/Px, where Ux= concentration of x in urine, V= volume of urine produced per unit of time, Px= concentration of X in plasma
What 2 substances can be used to directly measure GFR?
inulin, creatinine
What are the requirements for a substance to be used to measure GFR?
Be freely filtered by the filtration barrier, not be reabsorbed from the tubule, not be secreted into the tubule.
What is inulin?
A polysaccharide that is not metabolised by the body and is excreted in urine. Can be used to measure GFR
What is creatinine?
A waste product/metabolite produced by muscles that can be used to measure GFR
How is clearance used to calculate GFR clinically?
The renal clearance of creatinine is measured by taking the concentration of plasma and urine as well as the volume of urine per unit of time. The renal clearance is approximately equal to the GFR
Why is plasma creatinine concentration a good indicator of kidney function?
If both kidneys are working, then the clearance of creatinine is normal, meaning that the GFR is also normal (~125mL/min)
What is the renal clearance of PAH, and why?
625mL - all of PAH is excreted in urine, so clearance is equal to the renal plasma flow of 625mL
What does the glomerular filtration rate measure?
The amount of fluid filtered per unit of time in the kidneys
What does the renal plasma flow measure?
The amount of plasma that passes through the kidneys per unit time - 55% of the renal blood flow, as only plasma is filtered by the kidneys
