Renal Clearance Flashcards
What is flow rate
Amt of combined rate of fluid leaving the renal pelvis of both kidneys and emptying into the bladder per unit time and can vary from 0.5-15 ml/min
It is NOT the amt voided
GFR
Normal GFR
What is dissolved in the filtrate
Formula for GFR
Amt of fluid going into Bowman’s Capsule per unit time
Normal GFR = 100-130 ml/min
Electrolytes, glucose and AAs are dissolved in filtrate
Formula is used to estimste the filtration rate of any molecule assuming it has a relative permeability of 1
What is the filtered/tubular load
Formula
Amt of a specific solute (electrolyte, glucose or AA) entering Bowman’s capsule
Ps is the plasma conc of solute
Becomes important when we consider the capacity of the nephron to reabsorb solutes - reabsorption capacity of the nephron is not infinite
What is Transport Maximum
Each individual transport process has its own transport max
There is a finite amt of transporters and a limit to the rate at which they transport
Like a bus, but fluid is constantly moving so passengers left behind get moved along thr nephron and may get excreted
=> Tm = point at which increases in the conc of a substance beyond this level result in no further increase in transport across nephron walls
What is threshold
Plasma conc below which all available molecules will be reabsorbed in the nephron
i.e. plasma conc at which a substance begins to appear in the urine
Explain this graph
Recall FL = GFR x plasma [glucose]
YELLOW
Glucose has a relative permeability of 1 - as plasma [glucose] increases, the filtered load (amt filtered)
This will be the same for most molecules with a GFR - will increase linearly up to a certain point
RED
Amt reabsorbed
At low plasma levels all the filter load is reabsorbed up to approx 3 mg/ml
=> mechanism of reabsorption exactly matched the filtered load so initially all the filtered load is reabsorbed
As the [glucose] increases we reach the transport mac of the glucose reabsorption machinery and so the rate of reabsorption levels out - reached max rate
GREEN
Amt of glucose excreted
Initially all the filtered glucose is reabsorbed so no glucose is excreted but with the plasma level of glucose exceeds Tmax, some glucose misses the bus, doesn’t get reabsorbed and gets excreted
(plasma conc at which Tmax is exceeded is termed the renal threshold)
What is renal clearance
What does it give info on
Limited by
what values do we need to know
Amt filtered vs amt excreted
Gives info on reabsorption and secretion
As we still treat both kidneys as a single entitiy and we cannot tell which part of what nephron is making a significant contribution to the process
i. e. removing a substance from plasma, putting it in urine and measuring its rate of appearance in urine
- V = vol of urine
- U = conc of substance mg/ml of urine
- P = conc of substance mg/ml of plasma
Measuring renal clearance as a %
C
All of substance was reabsorbed so we can say with certainty that none of it would appear in the urine => 0% clearance
D
All of the substance left in the urine => 100% clearance
We can’t be sure with A and B
What can renal clearance be used for
- Measuring GFR
- Measuring renal plasma flow rate and from there we can calculate the renal blood flow rate
- Determining the renal handling of different substances - whether or not the substance is reabsorbed or secreted by renal tubules
What principle is renal clearance based on
Principle of mass balance - the kidney creates and consumes nothing
1 WAY IN
Plasma and its rate of flow
2 WAYS OUT
Venous blood and its rate of flow
Urine and its rate of flow
What is clearance concerned with
What goes in from arterial supply and what leaves in the urine
Formula for clearance
Clearance = Urine conc x urine flow rate (rate of excretion) DIVIDED BY plasma conc
What is minimum clearance
e.g. all glucose filtered is reabsorbed, no glucose in urine so the clearance is 0 ml/min
What is max clearance
Kidney receives 20% of CO
Max rate of delivery is 600 ml/min
so if all of a substance is removed frrom plasma => max clearance is 600 ml/min
Clearance of PAH
PAH = para-amino hippurate
20% on av of all PAH that enters the glomerulus is filtered - filtration fraction
PAH filtered moves along the nephron like all other substances
No mechanism for reabsorption thus the amt of PAH in the nephron heading for the bladder and excretion is not decreasing
PAH in peritubular capillaries is secreted thus the amt of PAH is the nephron heading for the bladder and excretion (being cleared) is increasing
No PAH leaves the kidney via the renal vein
THUS
Rate of PAH excretion is equal to the rate of delivery to the kidney
The clearance of PAH = plasma flow rate
ONLY TRUE FOR PAH
Freely filtered, not reabsorbed and totally secreted
Rate of excretion = rate of delivery
Secretory mechanism for PAH has a transport max of approx 80/dl
SO when plasma PAH is below this value we can say with some confidence that clearance of PAH = renal plasma flow rate
Clearance of Inulin
Inulin is a small sugar that is freely filtered
20% on av of all inulin that enters the glomerulus is filtered - filtration fraction
No mechanism for reabsorption thus the amt of inulin in the nephron heading for the bladder and excretion is not decreasing
No mechanism for secretion so the amt of inulin in nephron heading for bladder and excretion is not increasing
THUS
Rate of inulin excretion (amt cleared) is = to rate of filtration
Clearance of inulin = GFR
Inulin is special
- Freely filtered
- Not metabolised
- Not reabsorbed
- Not secreted
What does clearance mean
C < 125 ml/min indicates reabsorption
C > 125 ml/min indicates secretion
