Glomerular Function Flashcards
what determines glomerular filtration?
- filtration barrier
- renal blood flow
- driving forces
describe the factor, filtration barrier, that effects glomerular filtration
- small substances (low molecular mass) are freely filtered (eg. sodium, potassium, chloride, water, urea, glucose)
- large substances (high molecular mass) are NOT filtered (eg. haemoglobin and albumin)
describe the factor, renal blood flow, that effects glomerular filtration
renal blood flow (RBF) is roughly equal to 1/5 or 20% of cardiac output per minute.
- RBF roughly 1100-1200mL blood/min
- high flow for filtration, rather than metabolism
what are the two types of forces (pressures) that can act as driving forces for glomerular filtration?
There are two types of forces (pressures):
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
describe the factor, driving forces, that effects glomerular filtration
Glomerular hydrostatic pressure (GHP)
- = BP (+50 mmHg)
Blood colloid osmotic pressure (BCOP):
- = albumin (-25 mmHg)
Capsular hydrostatic pressure (CsHP):
- = pressure of filtrate already present (-15mmHg)
Capsular colloid osmotic pressure (CsCOP)
- = no protein in capsular space (+0 mmHg)
Together the 4 pressures determine the:
Net filtration pressure = 10mmHg
What proportion/percentage of the kidneys plasma flow is filtered?
Plasma makes up 55% of the blood, called renal plasma flow (RPF). How much of this is actually being filtered?
Filtration fraction = 20% of the plasma which enters te kidneys (RPF) is filtered through the glomeruli into the nephrons.
~20% of the RPF is filtered
- glomerular capsule
- nephron
~80% remains in the glomerular capillaries
- the efferent arteriole
- peritubular capillaries
How much plasma is filtered per minute?
Glomerular filtration rate (GFR)
- the amount of plasma filtered per unit time by the kidneys
Renal plasma flow (RPF) x Filtration fraction (FF) = Glomerular filtration rate (GFR)
625mL/min x 20% (FF) = 125 mL/min (GFR)
Normally:
- 180 L/day
- 125 mL/min
- but produces only 1.5L of urine per day, so >99% is reabsorbed
- tightly regulated
- variation for person to person
- declines slowly from age 30
- renal plasma flow and glomerular filtration rate cannot be measured directly, so the renal clearance of substances with particular characteristics is used to estimate them
How much of a substance in the plasma is filtered per minute?
Renal filtered load
Filtered load = the amount of a particular substance (solute) in the plasma is filtered per unit of time (per minute/hour/day)
Filtered load = GFR x solute plasma conc.
How do we measure/quantify how different substances handled by the kidneys?
By working out how much plasma is cleared of a substance per minute: Renal clearance
- clearance is the volume (mL) of plasma that is cleared of a substance by the kidneys per unit time (per minute)
Clearance can be used to:
- quantify how a substance is handled by the kidneys
- estimate glomerular filtration rate (GFR)
describe the renal clearance of creatinine and inulin
- freely filtered
- not secreted
- not reabsorbed
- ALL filtered is excreted in urine
Clearance of creatinine = volume of plasma filtered per minute = GFR
Blood:
55% plasma containing creatinine (and other solutes)
All RBCs and 80% of plasma NOT filtered
- efferent arteriole -> peritubular capillaries
20% of plasma is filtered
- water
- small solutes (including creatinine)
Creatinine is not reabsorbed or secreted so ALL creatinine which is filtered stays in the nephron
This creating will be excreted in the urine
99% of the plasma which was filtered is reabsorbed
‘All’ of the plasma which was filtered has been reabsorbed and ‘cleared’ of creatinine
describe the renal clearance of medications and toxins (eg. PAH)
- freely filtered
- entirely secreted
- not reabsorbed
- ALL in blood is excreted in urine
Clearance of PAH = Volume of plasma flowing through the kidneys per minute = RPF
Blood:
- 55% of plasma containing PAH (and other solutes)
All RBCs and 80% of plasma are NOT filtered
- efferent arteriole -> peritubular capillaries
20% if plasma is filtered
- water
- small solutes (inc. PAH)
PAH is not reabsorbed so ALL PAH which is filtered stays in the nephrons
PAH is entirely secreted so ALL PAH in the blood of the peritubular capillaries is secreted into the nephrons
This PAH will be excreted in the urine
>99% of the plasma which was filtered is reabsorbed
‘all’ of the plasma which has passed through the kidneys has been cleared of PAH
describe the renal clearance of glucose
- freely filtered
- not secreted
- fully reabsorbed in the proximal tubule
- NON excreted in urine
Clearance of glucose = NO plasma is cleared of glucose = 0
Blood:
55% of plasma containing glucose (and other solutes)
All RBCs and 80% of plasma are NOT filtered
- efferent arteriole -> peritubular capillaries
20% of plasma is filtered
- water
- small solutes (inc. glucose)
>99% of the plasma which was filtered is reabsorbed
Glucose is fully reabsorbed
NO glucose will be excreted in the urine
NO plasma has been ‘cleared’ of glucose
describe the renal clearance of sodium
- freely filtered
- not secreted
- almost fully reabsorbed in most parts of the nephron
- small amount excreted in urine
Clearance of sodium = very small volume of plasma per minute
Blood:
- 55% of plasma containing sodium (and other solutes)
All RBCs and 80% of plasma are NOT filtered
- eferent arteriole -> peritubular capillaries
20% of plasma is filtered
- water
- small solutes (inc. sodium)
>99% of the plasma which was filtered is reabsorbed
Almost all of the sodium which is filtered is reabsorbed
A small amount of sodium is not reabsorbed
This sodium will be excreted in the urine
A small amount of the plasma which was filtered has been reabsorbed without sodium and has been ‘cleared’ of sodium
describe renal clearance
clearance is the volume (mL) of plasma that is cleared of a substance by the kidneys per unit time (per minute)
Clearance can be used to:
- quantify how a substance is handled by the kidneys
- estimate glomerular filtration rate (GFR)
- estimate renal plasma flow (RPF)
Clearance (Cx) (in mL/min) = (Ux x V)/Px
- Ux = concentration of X in urine (mmol/L)
- V = volume of urine produced per unit time (mL/min or L/hour)
- Px = concentration of X in plasma
This equation describes the clearance (via the kidneys) for all substances that can be detected in plasma and urine!
How can we measure/estimate glomerular filtration rate (GFR) and renal plasma flow (RPF)?
To be used to measure GFR a substance must:
- be freely filtered
- NOT be reabsorbed from the tubule
- NOT be secreted into the tubule
Two main substances meet the requirements:
Inulin:
- polysaccharide, not metabolised by body
- not founding body, must be injected
Creatinine:
- wast produce produced by muscles
- already in te body, so most commonly used clinically
