The Physiology of the Loop of Henle

This chapter reports the morphology and function of the thick ascending limb (TAL) of the loop of Henle a crucial segment for many tasks of the nephron. Indeed, the TAL is a mainstay not only for the direct reabsorption of sodium, potassium, and divalent cations but also for its indirect role in the urine renal water reabsorption, overall acid-base homeostasis, and ammonia cycle. These functions are described in detail in this chapter, reporting in addition to the well-known mechanisms of action, innovative physiopathologic evidence. In an integrative way the mechanisms of sodium transport along the TAL are illustrated not only as functional to the paracellular uptake of cations but also as key element for the maintenance of the counter-current mechanism. TAL contributes to the acid-base homeostasis by reabsorbing bicarbonate. In addition to this classic view an innovative mechanism of acid secretion recently has been described in association with furosemide administration. Knowing the physiology of the TAL is fundamental for the clinicians who use loop diuretics. These are among the most commonly used drugs, but except the sodium loss, not all the physiologic consequences are known. This chapter attempts to fill this gap.