Dehydration
Over time, a lack of water can lead to dehydration, a condition in which your body doesn’t contain enough fluid to carry out its normal physiological processes. Dehydration can range from mild to severe. Mild to moderate dehydration, which can usually be corrected by drinking adequate amounts of water, is characterized by dry mouth, headache, constipation, fatigue, weakness, thirst, dizziness and decreased urine output. Severe dehydration, which generally requires medical attention, is characterized by extreme thirst, lack of sweating, rapid heartbeat, sunken eyes, extremely dry mouth and little urination.
Heat Exhaustion
Your hypothalamus, which controls thirst and hunger, also controls your body temperature. When you get too hot, you sweat. When the sweat evaporates from your skin, your body temperature drops. If you get hot when you are dehydrated, your body has no fluid to release. As a result, the heat gets trapped and your body temperature continues to rise. Heat exhaustion is characterized by fatigue, headache, clammy skin, increased thirst, rapid heartbeat, dizziness, nausea, vomiting, stomach cramps and increased sweating. Heat exhaustion can progress into heat stroke, a life-threatening condition that can lead to coma, heart attack and stroke.
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Seizures
Dehydration doesn’t just affect your fluid levels, it affects your electrolyte levels as well. Electrolytes, such as sodium and potassium, control the electrical signals in your body, including the ones that control your heart, brain and nervous system. When your electrolyte levels are out of whack, it messes up your body’s electrical signaling. This can lead to involuntary muscle contractions, or seizures, and loss of consciousness. If left untreated, dehydration-induced seizures can lead to death.
Hypovolemic Shock
Hypovolemic shock, or low blood volume shock, is one of the most serious complications of a lack of water, according to MayoClinic.com. Water is a major component of your blood. When you don’t drink enough water for an extended period of time, your blood volume drops and, as a result, your heart cannot pump enough blood -- or oxygen -- to your body. Hypovolemic shock, which can be life-threatening, is characterized by sweating, weakness, pale skin, confusion, decreased urine output, clammy skin and agitation.