32 :D Flashcards
What is filtration and what is its selectivity and what does it create?
Movement of plasma from the glomerular capillaries (blood) into the glomerular capsule
Most substances in plasma are freely- filtered
- Not very selective at the glomerulus
- Creates a plasma-like filtrate of the blood
What determines glomerular filtration?
- Filtration barrier
- Renal blood flow
- Driving forces
What’s filtered and what’s not filtered?
- small substances (low molecular mass) are freely
- large substances (high molecular mass) are NOT filtered
Albanian can’t pass through
Yep
Glomerular filtration: Renal blood flow
- how much CO do they get?
- why?
Renal blood flow (RBF) ≈ 1/5th or 20% of CO per min
RBF ≈ 11 0 0 -1200mL blood/min
high flow for filtration, rather than metabolism
Two types of driving forces
Hydrostatic pressures
- Pressure due to the volume of fluid
- “Pushes” fluid away
Colloid osmotic pressures
- Osmotic pressure due to protein
- “Pulls” fluid towards
- Positive pressures favour filtration
- Negative pressures oppose filtration
4 pressures / driving forces
Glomerular hydrostatic pressure (GHP)
= blood pressure (+ 50 mmHg)
Blood colloid osmotic pressure (BCOP)
= albumin (- 25 mmHg)
Capsular hydrostatic pressure (CsHP)
= pressure of filtrate already present (- 15 mmHg)
Capsular colloid osmotic pressure (CsCOP)
= no protein in capsular space (+ 0 mmHg)
(If u have a kidney disorder there many be proteins try a pull more fluid from blood)
Together the 4 pressures determine the:
Net filtration pressure
(+50) + (-25) + (-15) + (0) = 10 mmHg
How much of the kidneys plasma (blood) flow is filtered?
Filtration Fraction
Renal Blood Flow, Renal Plasma Flow and Filtration Fraction
Filtration Fraction =
Glomerular filtration rate (GFR)
———————————————
Renal plasma flow (RPF)
Filtration Fraction calculation =
GFR = 125 mL/min
RPF = 625 mL/min
Filtration Fraction =
125 mL/min
—————— = 1/5th or 20%
625 mL/min
≈ 20% of the RPF is filtered
≈ 80% remains in the glomerular capillaries -> the efferent arteriole -> peritubular capillaries
How much plasma is filtered per minute?
Glomerular Filtration Rate (GFR)
Glomerular filtration rate - KNOW THESE NUMBERS
Normally:
- 180 L/day
- 125 mL/minute
But produces only 1.5L of urine perday (we reabsortion 99%)
- Tightly regulated
- Variation from person to person
- Declines slowly from age 30
How much of a substance is filtered per minute?
Renal Filtered Load
What is the renal filtered load?
amount of a particular substance (solute) filtered per unit of time (per minute/hour/day)
How to calculate the filtered load
GFR x solute plasma conc.
How to calculate the filtered load - glucose example
e.g. glucose
GFR (125 ml/min) = 0.125 L/min
Plasma glucose conc = 5 mmol/L
Renal filtered load
= 0.125 L/min x 5 mmol/L
= 0.625 mmol/min
How much plasma is cleared of a substance per minute?
Renal Clearance
What is renal clearance - LEARN
Clearance is the volume (mL) of plasma that is cleared of a substance by the kidneys per unit time (per minute)
Renal Clearance can be used to:
- Quantify how a substance is handled by the kidneys
- Estimate glomerular filtration rate (GFR)
Renal clearncae - creatin and insulin example - how do u calculate it
- 99% of the plasma which was filtered was reabsorbed
- clearance of creatine = volume of plasma filtered per minute = GFR
Renal clearance - PAH - how to calculate it?
- clearance of PAH = volume of plasma flowing through the kidneys per minute = RPF
Renal clearance - glucose - how to calculate it?
Clearance of glucose - NO plasma is cleared of glucose
Renal clearance - sodium - how do you calculate it?
- clearance of sodium = very small volume of plasma per minute
Actual calculation for clearance - dont memorise ist understand
The equation describes the clearance (via the kidneys) for all substances that can be detected in plasma and urine!
Estimating GFR using Renal Clearance
- requirements
To be used to measure GFR a substance must:
- Be freely filtered
- NOT be reabsorbed from the tubule
- NOT be secreted into the tubule
Estimating GFR using Renal Clearance
- two main substances that meet the requirements
Inulin (not to be confused with insulin!!)
• Polysaccharide, not metabolised by body
• Not found in body, must be injected
Creatinine
• Waste product produced by muscles
• Already in the body, so most commonly used clinically
Food with high inulin
Estimating GFR using Renal Clearance
- creatine example
Relationship between GFR and plasma creatinine
Plasma creatinine conc is an important mark to apply drugs!!
indicator of kidney function:
- If both kidneys are working (GFR = 125 mL/min) plasma creatinine is low
- Even if only one kidney is still working (GFR = 60 mL/min), plasma creatinine is fairly normal
- When GFR < 25 mL/min plasma creatinine conc increases as the kidneys ability to clear waste products from the blood is reduced
GFR, RPF and Renal Clearance
GFR
?
The amount of plasma filtered per unit time by the kidneys
