Renal: Plasma Clearance Flashcards
What is renal plasma clearance?
The volume of plasma from which a substance is completely removed by the kidney per min. It depends on:
- GFR
- Tubular reabsorption and secretion
- Tubular metabolism
Renal clearance of inulin and creatinine is the same of GFR
Describe glomerular filtration
First step in urine production, forms plasma ultra-filtrate. This= same composition as plasma minus proteins and cells
GFR is how much filtrate is removed from the blood per min, not how much blood passes through the glomerulus each min
Glomerular filtration is governed by Starling’s forces
- Capillary hydrostatic P forces plasma into the urinary space
- Oncotic P and hydrostatic P in the urinary space oppose this and drive reabsorption
How much plasma is actually filtered through the glomerulus?
- ~625mL/min of plasma enters the kidney = renal plasma flow.Only 125mL/min of this 625ml is filtered = GFR
- This means for a freely filtered substance that is not reabsorbed/secreted, all the filtered substance in the plasma will be excreted
- The 500mL/min that’s not filtered will keeps its original composition- nothing from it will enter the tubule and be excreted
What does the rate of urine excretion depend on?
Urine produced in 4 broad steps
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
- Excretion
Rate of excretion depends on the other first 3 steps
Urine excretion rate= GFR – reabsorption rate + secretion rate
Describe the process by which inulin is used to calculate GFR
Inulin is freely filtered but not reabsorbed or secreted. So, inulin excretion rate = GFR.
- must be IV infused until Pin (plasma [inulin]) becomes stable, this is when infusion rate = excretion rate.
- Timed urine sample must be collected whilst this happens
- Urine sample obtains UIn (urine [inulin]) and V* (urine flow rate)
- Pin, Uin and urine volume are recorded for each period to calculate GFR
How do you calculate GFR from the inulin meausrements?
What are the units for the GFR eqn?
Why is inulin clearance not used to measure GFR?
Inulin clearance is not used clinically as it’s a pain in the ass:
- Requires prolonged inulin infusion as it’s not made endogenously
- Repeated blood samples until [Pin] is stable wastes time
Clinically we use creatinine which is released into plasma by skeletal muscle. Doesn’t need to be infused. Inert substance
Outline the conventional clinical method to measure GFR
- Blood sample for PCrn (plasma creatine)
- 24 hr urine collection (usually via catheter)
- Urine volume and V* calculated
- UCrn calculated
- Data used to calculate CCrn
CCrn = UCrn x V* / PCrn
Describe the fate of electrolytes, glucose, aa and organic acids and bases after filtration
Electrolytes: freely filtered but partly reabsorbed from tubules back into blood.
Glucose and aa: freely filtered but NOT excreted into urine because all is reabsorbed from tubule back into blood
Organic acids and bases: freely filtered but NOT reabsorbed and extra is secreted from peritubular capillaries into renal tubules
What is eGFR and how is it calculated?
GFR refers to the rate of blood flow through the kidneys, while eGFR refers to the number calculated to estimate GFR:
What are the limitations of eGFR?
Extreme body types can lead to errors (malnourished, amputees etc)
Not valid in pregnant women, >70s and children
Some ethnic groups don’t fit the equation
How is eGFR used to stage kidney disease?
- Stage 1 CKD: eGFR 90 or Greater
- Stage 2 CKD: eGFR Between 60 and 89
- Stage 3 CKD: eGFR Between 30 and 59
- Stage 4 CKD: eGFR Between 15 and 29
- Stage 5 CKD: eGFR Less than 15
What happens as plasma glucose rises?
- As Pglucose rises, so does Cglucose
- At Pglucose <20uM, SGLT-2 isn’t saturated.
- Once Tm is reached (transport maximum), SGLT-2 gets saturated. No more glucose can be reabsorbed so some stays in tubular fluid and is excreted
What is the renal plasma threshold?
The renal plasma threshold is the minimum plasma concentration of a substance that results in its excretion in urine
