Experimental surgery on the foetal rat in the last 2 days of gestation shows that a hypotonic urine is produced by the kidney (Wells, 1946). A study has been made with radioactive potassium (K42) of the rate of exchange, and also the localization, of this isotope in the kidney of the 20- and 21-day foetal and of the neonatal rat.

The rate of exchange of potassium from the blood-stream to the tissues can be studied by injecting small amounts of the radioactive isotope as bicarbonate into the circulation. The rate at which the adult rat kidney exchanges K42 is higher than that of any other organ (Walker & Wilde, 1952; Ginsburg & Wilde, 1954). In the first 2 minutes following intravenous injection, the radioactivity of the kidney is eight times that of the liver (Ginsburg & Wilde, 1954), and the uptake of the isotope is mainly restricted to the adult cortex (Morel & Guinnebault, 1956).

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