PHS 3342 Lecture Notes - Lecture 9: Reabsorption, Chronic Kidney Disease, Blood Pressures
28 Apr 2018
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February 13, 2018
Kidney Physiology (Filtration, Reabsorption, and
Secretion)
Fate of Various Substances Filtered by the Kidneys
Water: mostly reabsorbed (99%)
Sodium: mostly reabsorbed (99.5%)
Glucose: completely reabsorbed
Urea: partially reabsorbed, partially secreted (50/50)
General Definitions
180 L ļ¬uid processed daily, only 1.5 L of urine produced
-Anything ļ¬ltered or secreted but not reabsorbed is excreted in the urine and lost from the body
-Anything ļ¬ltered and subsequently reabsorbed, or not ļ¬ltered at all, enters the venous blood and is saved for the
body
Glomerular ļ¬ltration: produces cell- and protein-free ļ¬ltrate
-Nondiscriminant ļ¬ltration of a protein-free plasma from the glomerulus into Bowmanās capsule
Tubular reabsorption: selectively returns 99% of substances from ļ¬ltrate to blood in renal tubules and collecting ducts
-Selective movement of ļ¬ltered substances from the tubular lumen into the peritubular capillaries
Tubular secretion: selectively moves substances from blood to ļ¬ltrate in renal tubules and collecting ducts
-Selective movement of non ļ¬ltered substances from the peritubular capillaries into the tubular lumen
Plasma clearance for substances handled in different ways:
-Substance ļ¬ltered and not reabsorbed or secreted (ex. insulin): all of the ļ¬ltered plasma is cleared of the substance
- will go into urine
-Substance ļ¬ltered, not secreted, and completely reabsorbed (ex. glucose): none of the ļ¬ltered plasma is cleared of
the substance - none will go into urine
-Substance ļ¬ltered, not secreted, and partially reabsorbed (ex. urea): only a portion of the ļ¬ltered plasma is cleared
of the substance - some will go into urine
-Substance ļ¬ltered and secreted but not reabsorbed (ex. PAH): all of the ļ¬ltered plasma is cleared of the substance,
and the peritubular plasma from which the substance is secreted is also cleared - all goes into urine
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February 13, 2018
Calculation of the Glomerular Filtration Rate (GFR)
If a substance is easily ļ¬ltered and entirely excreted, its clearance is a good estimate of the GFR
-Amount ļ¬ltered = amount excreted ā> plasma concentration x GFR = urine concentration x urine ļ¬ow rate
ā¢GFR = (urine concentration x urine ļ¬ow rate)/plasma concentration
-NB: clearance = volume of plasma from which all the substance has been completed removed and excreted into
the urine
NB: glucose is not a good estimate of GFR - no glucose present in urine, clearance = 0 mL/min
Plasma Inulin and Creatinine as an Estimate of GFR
Plasma inulin: polyfructose that requires a continuous IV infusion to maintain a constant plasma level since it is not
endogenous ā> clearance gives an accurate estimate of GFR
-Rate of appearance in the urine = rate of appearance in the ļ¬ltrate
ā¢Amount excreted in the urine per unit of time = amount ļ¬ltered by the glomerulus per unit of time
ā¢Urine ļ¬ow rate x urine concentration = rate at which plasma is ļ¬ltered x plasma concentration
-GFR = (Uin x V)/Pin
ā¢Uin = urine inulin concentration (mg/mL)
ā¢V = urine ļ¬ow rate (mL/min)
ā¢Pin = plasma inulin concentration (mg/mL)
-*Regular GFR for inulin is 125 mL/min
Creatinine: produced by muscle and is present in plasma; freely ļ¬ltered and is not reabsorbed, secreted, or
metabolized by the nephron ā> clearance gives an accurate estimate of GFR
-Used to estimate or measure GFR in many clinical settings and is routinely performed in suspected cases of renal
disease
ā¢GFR will decrease in chronic kidney disease (manifested by higher plasma creatinine concentrations)
-GFR = (Ucr x V)/Pcr
-*Regular GFR for creatinine is 135 mL/min
Plasma Urea Clearance
Not a good estimate of GFR - blood urea nitrogen is inļ¬uenced by protein intake, GFR, and metabolic rate
-Urea excretion is determined by GFR and urine ļ¬ow rate
-Blood urea nitrogen cannot be used as the only index of GFR
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