Download 4. REABSORPTION - 2

Lecture 4
Dr. Zahoor
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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)
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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 ?
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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 .
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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
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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
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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
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 This high concentration of urea in tubular lumen causes
passive diffusion of urea from tubular lumen to peritubular 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
Applied
 In renal failure, blood urea level increases
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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 serum
 K+ 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
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 K+ is tightly controlled by kidney
 K+ is filtered freely in glomerular capillaries
 K+ is actively reabsorbed in PCT and actively secreted
in principal cell in DCT and CT
 K+ 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
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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
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Potassium Ion Secretion
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APPLIED
 Increased K+ or decreased K+ (hyperkalemia or
hypokalemia) affects the heart and can cause
arrhythmias and conduction defect
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 Renal threshold of PO4-3 and Ca2+ is their normal
plasma concentration
 85% 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
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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)
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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
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 Mg2+ is reabsorbed in PCT, thick ascending limb of
loop of Henle and DCT
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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
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•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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