In nephrology, the pump leak model is a mechanism for the reabsorption of sodium in the kidney nephrons, in order to achieve and maintain the fluid-balanced euvolemic state within the human body.
The "pump" is the active transport of sodium from the tubular cell into the interstitial space at the basolateral membrane, via carrier proteins, i.e. Na+/K+ ATPase. The "leak" results from the action of active transport (of the Na+/K+ ATPase), which creates a diffusion gradient that opposes its own action by favouring back-diffusion.
Sodium is passively transported from the tubular fluid into the tubular cells at the apical membrane (via ion channels), and then actively pumped out of the tubular cells at the basolateral membrane (via Na+/K+ ATPase), which then enters the blood. The pump-leak model explains the relationship between the transport of ions and cellular metabolism.