G lomerular filtration in the kidney is a continuous process that acts in concert with tubular reabsorption to prevent derangements of body fluid composition. Filtration is regulated by systemic factors, but it is also controlled by an intrinsic mechanism based on the anatomical connection between the distal nephron and the glomerular arterioles. Facing the threat of urinary salt loss, this mechanism causes vasoconstriction and reduces filtration by generating adenosine through the hydrolysis of nucleotide precursors such as 5′-AMP and possibly ATP .