Lecture 32 Physio Glomerular function Flashcards
whats filtration (nephron process)?
specific form of ultrafiltration
– creates a plasma-like filtrate of the blood
where does filtration occur?
at the glomeruli is not particularly selective
whats the filtration rate and volume of urine?
rate of 125mL/min (180L/day)
– produces only 1.5L of urine/day
What defines renal filtration?
- renal blood flow
- filtration barrier
- driving forces
what supplies the glomerulus with blood for filtration?
glomerulus has afferent and efferent arterioles (entering/leaving)
what is the renal blood supply?
• 20-25% of cardiac output (CO)
(brain 13%, heart 4 %)
• 1-1.2L/min (~ 400mL/100g/min)
(brain ~50mL/100g/min, heart ~80mL/100g/min)
• high flow for filtration rather than metabolism
what is the filtration membrane composed of?
fenestrated endothelium, basement membrane and
podocytes with foot processes and filtration slits
whats freely filtered?
small substances (low molecular mass)
eg K+, Na+, Cl-, H2O, Urea, Glucose
whats NOT filtered?
large substances (high molecular mass)
eg hemoglobin, serum albumin
what is glomerular filtration determined by?
- forces / pressure gradients between glomerular capillary / Bowman’s space
- permeability of glomerular capillary
- SA of glomerular capillary
what are the 4 forces (pressures) determining net glomerular filtration pressure (10 mmHg)?
- glomerular hydrostatic (GHP)
- blood colloid osmotic pressure (BCOP)
- capsular hydrostatic pressure (CsHP)
- capsular colloid osmotic pressure (CCOP)
whats glomerular hydrostatic (GHP)?
blood pressure (+ 50 mmHg)
whats blood colloid osmotic pressure (BCOP)?
albumin (- 25 mmHg)
whats capsular hydrostatic pressure (CsHP)?
corpuscule embedded in tissue (- 15 mmHg)
whats capsular colloid osmotic pressure (CCOP)?
no protein in capsular space (+ 0 mmHg)
Filtration in the kidney
A. is dependent on the osmotic force in the Bowman’s space.
B. results in high protein load of the primary urine.
C. results in an isotonic primary filtrate.
D. results in an absence of glucose in the primary urine.
E. is independent of serum albumin.
C. results in an isotonic primary filtrate.
How can we define renal filtration?
Renal Clearance
whats Renal Clearance equation?
Clearance (Cs) = Us x V / Ps (in mL/min)
Clearance (Cs) = Us x V / Ps (in mL/min)
Cs = Clearance of substance Us = concentration of S in urine (mg/L or mol/L) V = volume of urine produced (mL/min or L/hour) Ps= concentration of S in plasma (mg/L or mol/L)
what does the equation describe?
Clearance (Cs) = Us x V / Ps (in mL/min)
rate at which substance S is cleared by the kidneys per unit time.
clearance via the kidneys for all substances that can be detected in plasma and urine!!!
whats glomerular filtration rate (GFR)?
• amount of fluid filtered per unit time
- tightly regulated
- variation from person to person
- declines slowly from age 30
whats glomerular filtration rate (GFR) normally?
~180 L/day or 125 mL/minute
what must a substance be to be used as a measure of GFR?
- not be reabsorbed from the tubule
- not be secreted into the tubule
- not be metabolised
what are the two main substances that can be used as a measure of GFR?
inulin
creatinine
whats inulin?
- polysaccharide not metabolised by body
* not found in body, must be injected
whats creatinine?
- waste product produced by muscles
* already in the body so most commonly used clinically
is creatine filtered freely?
yes at the glomerulus
what cant creatinine do?
be reabsorbed, secreted or metabolised
whats the ideal measurement for clinically estimating GFR?
measuring the renal clearance of creatinine
what is plasma creatinine?
mg/dL
indicator for kidney function
what happens if plasma creatinine is low?
both kidneys are working (125mL/min)
what happens if only one kidney is working (60mL/min)
plasma creatinine is normal
How much of the blood reaching the kidney is filtered?
Filtration fraction
whats RBF?
Renal Blood Flow
= 25 % C.O. /min = 1.25L blood/min
blood is about half plasma / half cells
whats RPF?
Renal plasma flow
625 ml plasma/min
but only 125 ml/min filtered (GFR)
where does rest flow through?
into efferent arteriole
whats the Filtration Fraction?
GFR/RPF
whats the filtration fraction?
(RBF) = 1.25L blood /min (RPF) = 625 ml plasma/min (GFR) = 125 ml/min filtered
only 20 % (125/625) of plasma is filtered
whats filtered load?
amount of a particular substance (solute) filtered per minute
whats the equation for filtered load?
GFR x solute plasma conc.
whats the filtered load?
GFR 125 ml/min = 0.125 L/min
plasma glucose = 5 mmol/L
filtered load = 0.125 L/min x 5 mmol/L
= 0.625 mmol/min
The glomerular filtration rate (GFR) is defined
A. as the amount of substance filtered per time.
B. as the volume of urine produced per day.
C. as the amount of water both kidneys reabsorb per day.
D. as the volume of plasma filtered per time.
E. the amount of sodium filtered per time.
D. as the volume of plasma filtered per time.
summary
whats renal filtration?
renal blood flow, the
filtration barrier and the driving forces;
summary
what can renal clearance of a substance be determined by?
CS = US x V/PS (in mL/min)
what can GFR be estimated by?
clearance of creatinine as creatinine is only filtered
what does the filtration fraction define?
ratio between perfusion and filtration in the kidney (GFR/RPF);
whats filtered load units?
g/min or mol/min
whats filtered load?
amount of a substance filtered per time (GFR x PS)
