Management of Fluid and Electrolyte problems in Children

Potassium excretion is controlled primarily by events in the cortical collecting duct.

K⁺ excretion = Urine [K⁺] x Urine volume

This calculation makes a number of assumptions:

1. That the quantity of water in reabsorbed in the medullary collecting duct can be estimated by comparing the rise in the osmolality of the fluid in the terminal cortical collecting duct (which is equal to that of the plasma) with that of the final urine.
2. Potassium is not reabsorbed or secreted in the meduallary collecting duct.
3. The osmolality of the fluid in the terminal cortical collecting duct is known. This is only true if the urine osmolality is greater than the plasma osmolality.

The TTKG will be high (> 7) if mineralocorticoids are acting and will be low (< 2) if they are not.

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