Complications of dehydration may occur because of the dehydration, and/or because of the underlying disease or situation that causes the fluid loss.

Kidney failure
Kidney failure is a common occurrence, although if it is due to dehydration and is treated early, it is often reversible. As dehydration progresses, the volume of fluid in the intravascular space decreases, and blood pressure may fall. This can decrease blood flow to vital organs like the kidneys, and like any organ with a decreased blood flow; it has the potential to fail to do its job.

Coma
Decreased blood supply to the brain may cause confusion and even coma. If enough organs begin to malfunction, the body itself may fail, and death can occur.

Shock
When the fluid loss overwhelms the body's ability to compensate, blood flow and oxygen delivery to the body's vital organs become inadequate and cell and organ function can begin to fail.

Heat-related illnesses and associated complications
In heat-related illness, the body's attempt to cool itself by sweating may cause dehydration to the point that muscles may go into spasm (heat cramps). It is often the muscles that are being stressed that will spasm (for example, in people who work outside in a hot environment, arm and leg muscles may spasm from lifting and moving heavy objects or equipment; in athletes, leg muscles may fail from running). As fluid loss increases, the patient may be so dehydrated that there is not enough water to sweat and heat exhaustion or heat stroke may occur. Heat stroke is a true medical emergency and 911 or the Emergency Response System should be activated immediately in this situation.

Electrolyte abnormalities
In dehydration, electrolyte abnormalities may occur since important chemicals (such as sodium and potassium) are lost from the body through sweat. For example, patients with profuse diarrhea or vomiting may lose significant amounts of potassium, causing muscle weakness and heart rhythm disturbances. The health care practitioner is often aware of the fluid and electrolyte balance in the dehydrated patient and may decide to monitor electrolyte levels by checking blood tests. Some examples of symptoms caused by abnormal electrolyte levels include muscle weakness due to low potassium, heart rhythm disturbances due to either low or high potassium, and seizures due to low (hyponatremia) or high sodium. In many patients with dehydration, the kidneys are able to compensate and regulate electrolyte levels.

It is reasonable to remember that dehydration does not occur quickly, and sometimes it may take hours to slowly correct the fluid deficit and allow the electrolytes to redistribute themselves appropriately in the different spaces in the body. If rehydration is done too slowly, the patient may remain hypotensive and in shock for too long. If done too quickly, water and electrolyte concentrations within organ cells can be negatively affected, causing cells to swell and eventually die.

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