3. Physiology
        3.12. Renal
3.12.5. Renal handling of organic substances

Renal handling of organic substances

[Ref: AV6:Chp5]

Organic nutrients

 

Glucose

Tm-limited system

Tm = 375 mg/min

Since GFR = 125mL/min

Tm is reached when plasma glucose
= 300 mg/dL

Renal threshold

Renal threshold is the plasma level of glucose at which glucose first appear in the urine in more than the normal minute amount.

NB:

 

Proteins and peptides

Large proteins

Albumin concentration in filtrate = 10mg/L
--> 1.8g/day
--> Only 100mg/day excreted

Proteins in filtrates are taken up at the luminal membrane
* via endocytosis
* Including hormones

Protein taken up into tubular cells
--> Degraded into amino acid
--> Transport across basolateral membrane into ISF

Endocytic mechanism easily saturated

Small peptides

Completely filterable at Bowman's capsule

--> Catabolized into amino acids
* Within proximal tubular LUMEN
* By peptidases located on the luminal membrane

--> Transported across basolateral membrane into ISF

 

pH dependence of passive reabsorption or secretion

Neutral forms of organic acids and bases are more permeable in lipid membranes than ionized formed.

Neutral forms can diffuse in or out of lumen down the concentration gradient

Ionized forms, once in the lumen, cannot diffuse out.

Thus,

The reverse is also true, ie.

 



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