1. Proximal Tubular Function.
Prof. K. Sivapalan.

2. Proximal Convoluted Tubules
180 Liters of filtrate goes into the PCT in one day.
Substances needed for the body- glucose, amino acids etc. are reabsorbed.
Some waste products are secreted into the tubule.
Water is freely permeable and follows osmotic gradient. The tubular fluid is isosmotic.
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PCT

3. PCT 15 mm long, 55 µm in diameter.
One layer of cells with tight junctions
The luminal surface has brush border - microvilli.
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PCT

4. Tubular Reabsorption
Endocytosis- some peptide hormones and small proteins.
60 % of the filtered sodium is absorbed in PCT
There are several mechanisms that absorb sodium.
Glucose and amino acids are absorbed along with sodium in the proximal parts of the PCT
Water follows the solutes and the volume of fluid reduces accordingly.
Angeotensin II increases sodium and bicarbonate absorption in proximal tubule.
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PCT

5. Glucose, Amino Acid Absorption
Glucose and sodium bind to SGLT 2, glucose carrier protein in the luminal membrane.
Na+ is pumped out into interstitium by active transport.
This is secondary active transport
Sodium is absorbed indipendantly also.
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6. Transfer Maximum
If blood glucose is 90 mg/dl, amount filtered = 90 x 120 / 100 = 108 mg/min.
Maximum amount that can be absorbed by tubules is 375 mg / min for males and 300 mg in females [TmG].
This works out to blood level of 300 mg/dl.
Glucose appears in urine when arterial glucose level is 200 mg/dl [= 180 mg/dl in venous blood] because all nephrons are not the same.
The renal threshold for glucose is the lowest blood level when glucose appears in urine.
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PCT

7. Tubular Secretion
Substances normally produced - ethereal sulfates, steroid and other glucuronides, 5-hydroxyindolacetic acid
Substances administered – derivatives of hippuric acid [PAH], phenol red, penicillin and a variety of iodinated dyes.
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PCT

8. Absorption of Substances in PCT
Concentration of various substances indicate the rate of absorption.
Regulation of K+- Most of the filtered K+ is absorbed and excreted in distal tubule.
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PCT