What is the role of water balance maintenance in the pathogenesis of diabetes insipidus (DI)?

Updated: Mar 18, 2020
  • Author: Romesh Khardori, MD, PhD, FACP; Chief Editor: George T Griffing, MD  more...
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Answer

The normal range of plasma osmolality is between 275 and 295 mOsm/kg. The ability of the kidneys to modify the concentration of urinary solutes ranges between 50–1200 mOsm/kg. Healthy adults on a normal diet excrete 800–1200 mOsm of solute daily. Thus, to excrete 1000 mOsm of solute, the obligate urinary water excretion would be 1000 mOsm per 1200 mOsm/kg water, which translates into 0.8 kg (0.8 L) of water per day. This urine is maximally concentrated and appears dark yellow or orange in color. If this requirement for obligate water excretion is not met, solutes accumulate, leading to uremia.

Conversely the maximum volume of urine (secondary to limits imposed by renal dilutional capacity) is 20 L of water per day (1000 mOsm per 50 mOsm/kg water). This maximally dilute urine is colorless.

The maintenance of water balance in healthy humans is principally accomplished through 3 robust, interrelated determinants: thirst, AVP, and the kidneys. In addition, recognition of a fourth factor, apelin, has emerged in recent years. Apelin is a bioactive peptide that is widely distributed throughout the body. In the brain, it is expressed in supraoptic and paraventricular nuclei, as well as in other sites, and has specific receptors located on vasopressinergic neurons. Apelin acts as a potent diuretic neuropeptide that inhibits ADH release.


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