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It is proposed that the nutritional load of phosphate is the primary factor that led to the development of an excretory organ that is based on a high filtration rate of the extracellular fluid. This hypothesis is based on the following arguments: 1) phosphate and potassium impose the highest average nutritional load rates defined as the ratio between the average ion content in foods that satisfies the energy demands of the body per unit time and the respective ionic plasma concentration. Due to the much higher intracellular than extracellular potassium concentration it is argued that a secretory based excretory organ would be more useful for disposal of potassium. 2) Magnesium and calcium nutritional load rates are lower than that of phosphate; in addition the disposal of magnesium should have been dealt with preferably by secretion due to its slightly higher intracellular concentration. The nutritional load rate of calcium is tempered by the inherently low absorption capacity of the intestinal mucosa. 3) In aglomerular teleost fishes the phosphate plasma concentration is significantly elevated vis a vis the mammals and other marine fishes, in comparison to only slight differences in calcium, magnesium and potassium levels. It is suggested that the seemingly peculiar and costly way in which sodium excretion is accomplished in the kidney represents the outcome of the solution to the phosphate nutritional load.

PHOSPHATE LOAD AS THE EVOLUTIONARY DRIVING FORCE FOR THE DEVELOPMENT OF A HIGH FILTRATION RATE BASED EXCRETORY ORGAN: A HYPOTHESIS