Dehydration is the loss of water from the body in excess of the amount consumed.
Under typical circumstances the body loses and needs to replace approximately 2 to 3 litres of water daily. Breathing, urinating, defecating, and perspiring all cause water losses that need to be replaced on a daily basis. If water is lost from the bloodstream, the body can compensate somewhat by shifting water from cells into the blood vessels, but this is a very short-term solution. If the lost water is not replenished, the body may suffer serious consequences.
The body is able to monitor the amount of water it needs to function. The thirst mechanism signals the body to drink when the body water content is reduced. Hormones, including anti-diuretic hormone (ADH), work with the kidney to limit the amount of water lost in the urine when the body needs to conserve water. Water intake and output are highly variable but closely matched to less than 0.1% over an extended period through homeostatic control. Electrolyte intake and output are also closely linked, both to each other and to the hydration status.
Failure to match intake and loss of water and minerals, especially sodium and potassium, may lead to dehydration. Depending on the ratio of water to electrolyte loss, dehydration can be classified as isotonic, hypertonic or hypotonic:
Is characterised by isotonic loss of both water and solutes from the extracellular fluid, that is when both water and sodium are lost in equivalent amounts, e.g. through vomiting, diarrhoea or through inadequate intake. There is no osmotic shift of water from the intracellular space to the extracellular space. This type of dehydration accounts for cases of dehydration in young children.
In hypertonic dehydration water loss exceeds salt loss, that is when more water than sodium is lost (e.g. through inadequate water intake, excessive sweating, osmotic diuresis and diuretic drugs). This is characterised by an osmotic shift of water from the intracellular fluid to the extracellular fluid. This type of dehydration is more common in people who have diabetes, and it accounts for approximately 10 to 20 percent of all paediatric cases of dehydration with diarrhoea.
In hypotonic dehydration more sodium than water is lost, e.g. in some instances of high sweat or gastro-intestinal water losses or when water and electrolyte deficits are treated with water replacement only, it is characterised by an osmotic shift of fluid from the extracellular area to the intracellular. It also occurs with excessive intakes of plain water or other liquids with little or no sodium content. This type of dehydration accounts for approximately 10 to 15 percent of all paediatric cases of dehydration with diarrhoea. This complication can be life-threatening if swelling causes pressure on the brain (cerebral oedema). This is called hyponatraemia.
Mild dehydration is common and usually caused by not drinking enough fluids throughout the day. In children, diarrhoea is a common cause.
Fluids are continuously lost through normal body functions such as sweating, breathing and urinating. Common causes of dehydration include gastrointestinal water loss caused by diarrhoea and vomiting, excessive urination, excessive water loss through the skin (sweating) and lungs (breathing) due to heat and/or humidity, physical activity and fever or burns.
One of the most common reasons a person loses excess water is an infection that causes diarrhoea. Diarrhoea may be caused by bacteria, viruses or parasites. A significant amount of water can be lost with each bowel movement – up to 1 litre per hour in extreme cases. Worldwide, more than four million children die each year because of dehydration from diarrhoea.
The rapid loss of water that occurs with severe and sustained vomiting makes dehydration more likely to occur, as it is difficult to restore hydration status by drinking. The risk of dehydration due to vomiting is higher in infants and children, in the elderly, in people with eating disorders (e.g. bulimia) and anyone taking medications that can cause vomiting.
There are certain medications that increase urination beyond normal levels (e.g. diuretics), and there are conditions that affect kidney function, leading to the loss of body water through urine. Conditions that may affect urine production include diabetes and kidney cancer. Diabetes is characterised by elevated blood sugar levels causing sugar to spill into the urine and excessive urination which can lead to significant dehydration.
Sweating or perspiration is the mechanism used by the body to cool itself in conditions of heat, humidity and physical activity. Humidity can play a greater role in dehydration than heat, because the sweat drips from the body rather than evaporating, and thus does not cause a loss of heat from the body. However, high rates of sweating can also occur during sports or other vigorous physical activity in cool and dry conditions, contributing to the risk of dehydration. Heavy clothing limits sweat evaporation meaning that body heat is not dissipated causing the body to lose even more water as it attempts to lose more heat.
These conditions, including sunburn, increase body temperature, requiring more fluid for proper body functioning. Fever is present due to an infection and the body uses a significant amount of water in the form of sweat to cool itself. Burn victims become dehydrated because water seeps into the damaged skin and is lost by evaporation.
Inadequate intake of liquids during hot weather or exercise may also deplete the body’s water stores. Anyone may become dehydrated, but young children, older adults and people with chronic illnesses are most at risk.
A loss of body water equivalent to about 1% of body weight is normally compensated within 24 hours. Thirst stimulates drinking, so intake is increased and there is also a reduction in water loss by the kidneys. If losses are greater than this, reductions in physical and cognitive performance may occur and there may be some impairment of thermoregulation and cardiovascular function
Mild dehydration can cause symptoms such as thirst, headache, weakness, dizziness and fatigue and generally makes people feel tired and lethargic. Symptoms of moderate dehydration may include dry mouth, little or no urine, sluggishness, a rapid heartbeat and lack of skin elasticity. Severe dehydration is a life-threatening medical emergency, and is characterized by extreme thirst, no urine, rapid breathing, altered mental state and cold, clammy skin.
Increasing levels of dehydration with fluid losses of more than 1% of body weight can lead successively to reduction in exercise performance and in the ability to control body temperature. With fluid deficits of 4% and more, severe performance decrements may be observed as well as difficulties in concentration, headaches, irritability and sleepiness, and increases in body temperature and in respiratory rates. Dehydration that causes a loss of 10% or more of body weight can be fatal.
As dehydration progresses, the volume of water in the blood stream decreases, and blood pressure may fall. Cardiovascular function is impaired with increasing levels of dehydration, with a rise in heart rate and difficulties in maintaining the volume of blood that the heart delivers to the tissues. The heart pumps harder to maintain blood flow to the organs, but blood pressure may fall as the blood volume falls. Reduced blood flow to the skin and reduced levels of hydration keep the body from sweating and dissipating heat.
Chronic dehydration can increase the risk of infection, particularly of the urinary tract. The kidneys and other major organs that receive a decreased blood flow may begin to fail. Kidney failure is a common occurrence, although it is reversible if it is due to dehydration and is treated early. Decreased blood supply to the brain may cause confusion, impairing both cognitive function and coordination.