REABSORPTION - 2
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Transcript of REABSORPTION - 2
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Lecture 4Dr. Zahoor
REABSORPTION - 2
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We will discuss Reabsorption of - Glucose - Amino acid - Chloride - Urea - Potassium - Phosphate - Calcium - Magnesium
(We have discussed reabsorption of Na+ and water)
REABSORPTION
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Glucose and Amino Acid reabsorption is by secondary active transport (with Na+)
Glucose is filtered by glomeruli but all glucose (100%) is reabsorbed in PCT with Na+ (secondary active transport)
GLUCOSE & AMINO ACID REABSORPTION
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Amino acid is filtered, but reabsorbed in PCT by secondary active transport with Na+
IMPORTANT – In normal person, there is no glucose and amino acids in urine
GLUCOSE & AMINO ACID REABSORPTION
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For both glucose and amino acid specialized Symport carrier, such as Na+ and glucose co-transporter (SGLT) is present in PCT and transfers both Na+ and glucose from lumen to the cell .
NOTE - There is Na+ - K+ pump operating at basolateral membrane, this pump drives the co-transport system at the lumen
GLUCOSE & AMINO ACID REABSORPTION
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Secondary Active Transport for Glucose And Amino Acid
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When glucose and amino acid are in the cell, they passively diffuse down their concentration gradient from basolateral membrane into plasma
From basolateral membrane Glucose is facilitated by carrier such as glucose transporter (GLUT) which is not dependent on energy
GLUCOSE & AMINO ACID REABSORPTION
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What is Tubular maximum ? Tm means maximum capacity of the kidney to reabsorb
a substance . It is due to saturation of carrier system . Tm for glucose is 375 mg/minute Why there is Tm?Because there are carriers specific for a substance in
the cells lining the tubules, when they are saturated, then no more substance can be carried e.g. glucose
Maximum reabsorption rate is reached when all carriers are saturated and they can not carry any more of the substance
TUBULAR MAXIMUM (Tm)
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If substance is filtered beyond its Tm – it will be reabsorbed but will be excreted in the urine also
E.g. normally glucose is filtered below its Tm, therefore all is reabsorbed but in diabetes Mellitus glucose is filtered more than its Tm, therefore excreted in the urine
TUBULAR MAXIMA (Tm)
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Normal Plasma Glucose level is 100mg %
When GFR is 125ml/min, then 125mg of glucose passes in the filtrate in Bowman capsule per minute
GLUCOSE Tm
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Filtered load – quantity of any substance filtered per minute can be calculated
Filtered load of substance = Plasma concentration of substance × GFR Filtered load of Glucose = 100mg /100ml × 125ml/min = 125 mg/min
GLUCOSE Tm
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Tubular maximum (Tm) for glucose is 375mg/min
Normally glucose is filtered 125mg/min, therefore, can be readily reabsorbed because filtered load is much below the Tm of glucose
If filtered load exceeds 375mg/min, which is Tm for glucose, glucose will appear in the urine
GLUCOSE Tm
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Renal threshold is the plasma concentration of glucose at which glucose will appear in the urine, it is
180mg % - 200mg % Why ?Because at this renal threshold (180mg % -
200mg % in plasma) Tm of glucose is reached, therefore, glucose appears in the urine
RENAL THRESHOLD
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Problem to solve We said Tm (glucose filtered load/min) is
375mg/min, at this Tm renal threshold (plasma glucose level) should be 300mg %
But Normal renal threshold for glucose is 180mg
% - 200mg % WHY ?
GLUCOSE Tm & RENAL THRESHOLD
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It is because of two reasons 1. All the nephron doesn’t have same Tm
2. Co-transport carrier may not be working at its maximum capacity when glucose level is high.
Therefore, some of the filtered glucose is not reabsorbed and spill into the urine .
GLUCOSE Tm & RENAL THRESHOLD
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GLUCOSE Tm & RENAL THRESHOLD
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In Diabetes Mellitus, blood glucose is high (more than threshold level) and appears in the urine
WHY Diabetic patient pass more urine?Because, when diabetes is not controlled and
blood glucose level is high, it is filtered and causes osmotic diuresis
APPLIED
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The negatively charged Cl- ion are passively reabsorbed down the electrical gradient created by active reabsorption of Na+
Cl- reabsorption is not directly controlled by kidney
CHLORIDE REABSORPTION
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Urea is waste product obtained from protein metabolism
Urea is passively reabsorbed How?As 65% of water is reabsorbed in PCT,
therefore, filtrate at the end of PCT is decreased from 125ml/min to 44ml/min, therefore, urea is concentrated in the tubular fluid
UREA REABSORPTION
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This high concentration of urea in tubular lumen causes passive diffusion of urea from tubular lumen to peri-tubular capillary plasma
Proximal tubule is partially permeable to urea and about 50% of filtered urea is passively reabsorbed (50% of urea is excreted)
DCT and CT are impermeable to urea, therefore, no urea is absorbed here
ADH increases urea permeability of CT in the medulla of kidney
AppliedIn renal failure, blood urea level increases
UREA REABSORPTION (cont)
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Passive Reabsorption of Urea at the end of proximal tubule
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Most of potassium is located in Intracellular fluid (ICF)
We use words - hyperkalemia – increase K+ level in serum - hypokalemia – decrease K+ level in serumK+ is filtered, reabsorbed and secreted K+ excretion can vary widely from 1% to
110% of filtered load depending on dietary K+ intake, aldosterone level and acid base status
POTASSIUM REGULATION
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K+ is tightly controlled by kidneyK+ is filtered freely in glomerular capillariesK+ is actively reabsorbed in PCT and actively
secreted in principal cell in DCT and CTK+ filtered is almost completely reabsorbed in
PCT and thick ascending limb of loop of henle.
In DCT and CT, K+ is secreted depending on dietary K+ intake
POTASSIUM REGULATION
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Secretion of K+ occurs in principal cells. Aldosterone acts on principal cells in DCT and CT and causes Na+ absorption and K+ secretion
Increased K+ causes increase aldosterone from adrenal cortex directly
At basolateral membrane of principal cell, K+ is actively transported into the cell by Na+-K+ pump
At luminal membrane, K+ is passively secreted into the lumen through K+ channel
POTASSIUM REGULATION
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Potassium Ion Secretion
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APPLIEDIncreased K+ or decreased K+ (hyperkalemia
or hypokalemia) affects the heart and can cause arrhythmias and conduction defect
POTASSIUM REGULATION
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Renal threshold of PO4-3 and Ca2+ is their
normal plasma concentration85% of filtered Phosphate is actively
reabsorbed in PCT by Na+ - PO4 co-transport carrier
15% filtered load is excreted in urine Kidney regulates phosphate and calcium
PHOSPHATE REABSORBTION
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If we take more phosphate in diet, then greater amount of phosphate will be excreted
PO4-3 and Ca2+ are regulated by hormone
parathyroid PTH (parathyroid hormone) – causes Ca2+
reabsorption and inhibits phosphate reabsorption
PTH causes phosphaturia (increase phosphate in urine)
PHOSPHATE REABSORBTION
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60% of plasma Ca2+ is filtered in the glomerular capillaries
PCT and thick ascending limb of Loop of Henle reabsorb more than 90% of filtered Ca2+
DCT and CT reabsorb 8% of filtered Ca2+
Parathyroid hormone increases Ca2+ reabsorption in DCT by activating adrenylate cyclase
CALCIUM REABSORPTION
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Mg2+ is reabsorbed in PCT, thick ascending limb of loop of Henle and DCT
MAGNESIUM REABSORPTION
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Other waste products e.g. uric acid, creatinine, phenol (derived from many foods) are not passively reabsorbed as urea.
Urea is smallest particle of waste products, therefore, it is only waste product i.e. passively reabsorbed (50%) in PCT
OTHER WASTE PRODUCTS
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Urine Composition•Urine is clear and amber in color due to presence of urobilin
•Specific gravity of urine is between 1020 and 1030
•pH – about 6 (normal range 4.5-8)
•Healthy adult passes 1000 to 1500 ml per day
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Urine Dipstick
Test
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