The elimination by the kidneys of excessive or unwanted substances of the body parallels in importance the retention of vital elements in the proper amount for normal chemical processes. The functions are finely balanced and remarkably effective, providing there is no countermanding action at play. The renal tubules are the focal mechanism in the ongoing drama of conservation and elimination; an example is sodium metabolism.
The excretion of sodium depends on the filtration rate of the kidneys, plasma sodium concentration, the effects of antidiuretic hormone (ADH), and possibly, the action of one or more adrenal cortical hormones to favor tubular reabsorption. As pointed out by Smith,1 much less sodium than water is removed by osmotic diuresis, indicating that the reabsorption of sodium in the proximal tubule is essentially independent of the reabsorption of water. It is known also that sodium reabsorption operates as an active process rather than a