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19 Cards in this Set
- Front
- Back
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Clearance (Cx) equation
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Cx = UxV/Px
- Volume of plasma that would be totally cleared of solute in a given time |
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GFR importance
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- Tells how much plasma is being filtered/minute
- Rate will fall for CKD patients - functional nephron # declines - can't filter as much - Overall, gives some indication of overall kidney function |
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End-stage renal disease
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- Patients have ~10% of normal GFR
- Dialysis or transplant are the only options |
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Inulin clearance calculation of GFR
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- Inulin = substance is not metabolized, secreted, or absorbed
- Amount filtered must equal amount excreted! - UxV = GFR * Px from earlier - gets rearranged for GFR! - GFR = UxV/Px - GFR = Cx |
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Inulin alternate methods
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- Creatinine!
- Inulin = gold standard, but very cumbersome... - Must maintain steady [inulin] for sustained periods, etc. - Creatinine gives good estimate without having to put anything into patient - Natural byproduct of mm turnover - Is freely filtred, not reabsorbed...BUT does have some tubular secretion - Thus, GFR tends to be slightly overestimated - Also, have to take muscle mass into account (elderly/kids vs. active adult) - Not good for acute injury either - no previous baseline with which to judge creatinine levels... - Overall good estimation, however |
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Why Plasma creatinine approximates GFR
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- Assume we lose a whole kidney -> only 1/2 nephron units = less filtering = higher Px!
- Excretion rate of creatinine is initially 0.5x the production - Excretion rate slowly climbs until production rate = excretion rate -> Steady State! - However = during this adaptation time the absolute [creatinine] has doubled! *** Thus, the 2x normal [creatinine] tells us we have only 0.5x the nephrons! |
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Glomerulus podocytes
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- Podocytes surround glomerular capillaries in Bowman's capsule
- Allow there to be some space around capillaries for fluid to flow into tubules |
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Ultrafiltration definition
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- Process of separating molecules with semipermeable (selective) membranes and hydrostatic pressure
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Filtration coefficient
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- Measure of how freely filterable something is compared to water
- H20 has coefficient of 1.0 (so does Na+, glucose, inulin) - Albumin = 0.001... bigger, stays in the blood *** Overall if coefficient = 1.0, [x] is same in Bowman's as in bloodstream! |
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Filterability trends
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- Overall the bigger the molecule, the less filterable it is
- For charges (+) > neutral > (-) |
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Size/charge selectivity mechanism
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- Actually thought to be the podocyte! (originally thought space b/w feet too big = 4-14nm)
- Congenital nephrotic syndrome of the Finnish (CNF) - classicly lots of protein in urine - poor filtration! - Podocyte feet were essentially missing! - Nephrin = integral protein that extends into ECM around podocyte extensions - Nephrin interacts with other nephrin at podocyte extension joints = slit processes - Slit process spacing = expected size of filter! - Slit processes also have highly (-) charge - Facilitates filtration of (+) particles much better! |
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3 Starling forces that regulate filtration
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- PG = hydrostatic pressure in glomerular capillaries
- PB = hydrostatic pressure in Bowman's capsules - πG = colloid pressure of glomerular capillaries - Ions are filterable, so not an issue - Proteins are NOT filterable - protein content in plasma that opposes filtration into capsule/tubules - πB = hypothetically would keep fluid in tubules...but protein isn't filterable = NEGLIGIBLE! |
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Filtration (GFR) calculation via forces
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GFR = Kf(ΔP)
GFR = Kf(PG-PB-πG) - PG = 55 mmHg = highest of any capillary bed - for filtration purpose! - PB = 15 mmHg - πG = generated from protein in afferent arterioles - increases along tubule as H2O is continually lost! |
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Skeletal mm. vs. glomerular filtration
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- Skeletal muscle continually decreases hydrostatic pressure in capillary beds
- High filtration at arterial side, almost equal reabsorbtion at venous end of bed - Glomerular capillaries designed to favor filtration for the entire length of the bed! - Via the net result of Starling forces! |
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Overall control of BP and GFR
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PG depends on arterial BP and the resistances of afferent/efferent arterioles
- BP essentially generates the hydrostatic pressure - Afferent/efferent arterioles = major sites of resistance - control points! - Glomerular capillary pressure depends on ΔP between afferent and efferent! |
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Afferent arteriole effects
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- Constriction reduces flow, glomerular hydrostatic pressure -> lower GFR!
- Dilation = opposite! |
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Efferent arteriole effects
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- Constriction = reduces flow, increases hydrostatic pressure (PG)
- However, may increase/decrease GFR depending on degree of vasoconstriction - Goes up initially with increased pressure, decreases as flow falls! - Dilation = decreases PG, increases glomerular flow, decreases GFR! |
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Summary of Starling force influences
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PG - depends on arterial BP and afferent/efferent arteriole resistance
- PB = depends on GFR and downstream resistance - Increases with tubule/ureter obstructions (backing up fluid) - Increased PB = decreased GFR - πG = decrease leads to increased filtration - e.g. = lots of saline injected -> less protein mg/mL -> less πG -> more filtration |
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Albumin concentration calculation at end of glomerular capillary
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If 40mg/mL albumin in plasma
- FF = GFR/RPF = ~21% - Thus, same amount of albumin in 79% of original fluid - 40/0.79 = 50mg/mL |
