Homer Smith working at Mount Desert Island Biological Laboratory (MDIBL) confirmed the findings of previous investigators that the blood of elasmobranchs is slightly hypertonic to seawater leading to passive water absorption through the gills or gastrointestinal tract. He also made the simultaneous measurement of the osmotic pressure of the blood, urine, and seawater in acclimatized fish finding that the blood was hypertonic to the sea and the urine was hypotonic to the blood. Thus, water was absorbed passively and excreted in the urine. He found that the salt concentration of the blood of elasmobranchs was high but lower than that of the ocean and that the major solute component of the elasmobranch blood was urea. Notably, the excretion of urea in the urine was very low so that urea had to be reabsorbed by the kidney. In addition, he observed that the kidney of elasmobranchs was not able to excrete the diffusional load of salt that comes in through the gill and oral surfaces so that salt had to be disposed of extrarenally. The extrarenal excretion of salt was later identified as the function of the rectal gland. Smith’s research highlighted the unique salt challenges faced by elasmobranchs.