Water excretion by the kidney is guided by the hypothalamic antidiuretic hormone, arginine vasopressin. The hormone is normally released from the posterior pituitary in response to dehydration (hypernatremia) and pathologically released when the serum Na concentration is normal or low in response to a variety of stimuli, including an inadequate circulation, stress, hypoxia, cortisol deficiency, and neurologic disease. In addition, vasopressin can be produced and secreted ectopically, for example, in patients with small cell carcinoma of the lung. Without vasopressin, the urine is more dilute than plasma, owing to reabsorption of Na in water-impermeable segments of the nephron. When vasopressin is acting, water channels are inserted into the renal collecting duct, allowing equilibration with the surrounding hypertonic renal medulla, concentrating the urine. With normal kidney function and high levels of vasopressin, the urine can be four times as concentrated as normal plasma. Without vasopressin, the urine osmolality can fall as low as 50 mOsm/kg.