Hyponatremia

The Leaky Apartment With A Broken Thermostat

Imagine your blood is an apartment and sodium is the furniture inside. ADH is the thermostat that controls whether windows stay open (low ADH, water leaves) or closed (high ADH, water stays). Normally the thermostat reads the air density (osmolality—how crowded the air feels) and adjusts. In low sodium (hyponatremia), the thermostat breaks and stays set to "close the windows" even when the apartment is already full of humid air, OR the apartment springs a leak and loses furniture (sodium) faster than water evaporates, OR someone left a fire hose running inside. The brain is like a water balloon in the corner—it swells when too much humidity (water) gets trapped, and if you suddenly suck all the humidity out, the balloon shrivels and its skin tears. That torn skin is the myelin around nerves, and once it tears, signals stop flowing.