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What is dehydration?

Dehydration is an excessive loss of water from the body tissues, often accompanied by an imbalance of sodium, potassium, chloride, and other electrolytes. It can occur whenever fluids are lost and not adequately replaced, especially when an individual does not drink enough fluids. Early dehydration has no symptoms; mild or moderate dehydration can cause symptoms that include: thirst, fatigue, muscle cramps, dizziness, and headache. Severe dehydration can cause more serious symptoms, such as confusion, low blood pressure, unconsciousness, shock, and may even lead to death.

The human body consists of about 60-70% water and must have a continuous supply in order to function. Water enters the body primarily through drinking liquids and secondarily as part of the food that we eat. It is absorbed by the intestines and carried throughout the body. Water comprises the fluids found inside of cells, in the spaces in between cells and tissues, in the lymphatic system, mucous membranes, and in the fluid portion of the blood within our veins and arteries. As needed, fluids can be shifted from one "compartment" or area into another.

Most water is filtered from the blood, reabsorbed, and recirculated several times by the kidneys. Excess water and dissolved wastes are made into urine and eliminated from the body during urination. Additional small amounts of water are continually lost through sweating, breathing, and in stool. The total amount of normal water loss ranges from 1,500 to 2,500 milliliters (mL) per day (about 50-85 ounces per day) based on the following sources:

Stool 50-100 mL/day
Evaporation/Exhaling 500-1000 mL/day
Urine 1000-2000 mL/day on average

Maintaining the balance and conservation of water within the body is a complex process. The kidneys are part of a feedback system that conserves or removes water by concentrating or diluting urine and by controlling the conservation of sodium. Sodium and other electrolytes – potassium, chloride, and bicarbonate – help regulate water balance at the cellular level by maintaining electrical neutrality and the body's acid-base balance.

This feedback system and its components are vital in maintaining a healthy level of water in the body. Sensors in the body perceive and respond to increases and decreases in the amount of water and dissolved substances in the bloodstream. As the number of dissolved particles in blood (osmolality) increases, indicating either a decrease in the amount of water in the blood or an increase in the number of particles, a specialized gland in the brain – the hypothalamus – secretes antidiuretic hormone (ADH). This hormone signals the kidneys to conserve water. Water moves from cells to the blood stream to maintain blood pressure and volume. If not corrected, body tissues dry out, causing cells to shrink and malfunction. As fluid levels decrease, a "thirst" response is triggered by the brain, signaling a person to drink more water. Working together, these feedback systems normally maintain a dynamic fluid balance.

Dehydration occurs when liquid/fluids are lost faster than they can be replaced. This can occur with excessive vomiting, diarrhea, sweating, use of diuretics (medications that increase urine production), and/or not taking in enough fluids through drinking or eating. This can worsen if the person also loses too much sodium (hyponatremia) or too little (hypernatremia) in relationship to the decrease in water. Prolonged dehydration can result in shock and damage to internal organs, particularly the brain, leading to confusion, coma, and possible death.

Anyone can become dehydrated, but the condition tends to be more serious in the young, elderly, and in those with underlying health conditions or weakened immune systems. Infants and children may have difficulty communicating thirst. Because they have faster metabolisms and higher body water content than adults, children also have different fluid requirements than adults. Sweating, vomiting, and diarrhea are common sources of rapid fluid loss in children, and worldwide, diarrheal illnesses are a serious health threat. According to the World Health Organization, diarrheal illness is the second leading cause of death in children under the age of five. The U.S. Centers for Disease Control and Prevention estimates that in developing countries, diarrhea kills 2,200 children every day.

In the elderly, dehydration is a common problem and has been linked to a number of adverse health outcomes. Experts estimate that more than 20% of older people living independently is the U.S. are dehydrated, and similar rates of dehydration have been recorded in older people living in residential treatment facilities in the UK.

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