In physiology, dehydration is a lack of total
body water
In physiology, body water is the water content of an animal body that is contained in the tissues, the blood, the bones and elsewhere. The percentages of body water contained in various fluid compartments add up to total body water (TBW). This ...
,
with an accompanying disruption of
metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mild dehydration can also be caused by
immersion diuresis, which may increase risk of
decompression sickness in
divers.
Most people can tolerate a 3-4% decrease in total body water without difficulty or adverse health effects. A 5-8% decrease can cause
fatigue and
dizziness
Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to disequilibrium or a non-specific feeling, such as giddiness or foolishness.
Dizziness is a common medical c ...
. Loss of over ten percent of total body water can cause physical and mental deterioration, accompanied by severe
thirst.
Death occurs at a loss of between fifteen and twenty-five percent of the body water.
[Ashcroft F, Life Without Water in Life at the Extremes. Berkeley and Los Angeles, 2000, 134-138.] Mild dehydration is characterized by thirst and general
discomfort and is usually resolved with
oral rehydration
Oral rehydration therapy (ORT) is a type of fluid replacement used to prevent and treat dehydration, especially due to diarrhea. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium. Oral rehydrati ...
.
Dehydration can cause
hypernatremia (high levels of
sodium ions
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable isot ...
in the blood) and is distinct from
hypovolemia (loss of
blood volume, particularly
blood plasma
Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the int ...
).
Signs and symptoms
The hallmarks of dehydration include
thirst and neurological changes such as
headaches,
general discomfort, loss of
appetite, decreased
urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellul ...
volume (unless
polyuria
Polyuria () is excessive or an abnormally large production or passage of urine (greater than 2.5 L or 3 L over 24 hours in adults). Increased production and passage of urine may also be termed diuresis. Polyuria often appears in conjunction wi ...
is the cause of dehydration),
confusion, unexplained
tiredness
Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
, purple fingernails, and
seizures
An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with lo ...
. The symptoms of dehydration become increasingly severe with greater total body water loss. A body water loss of 1-2%, considered mild dehydration, is shown to impair cognitive performance. While in people over age 50, the body's thirst sensation diminishes with age, a study found that there was no difference in fluid intake between young and old people. Many older people have symptoms of dehydration. Dehydration contributes to
morbidity in the elderly population, especially during conditions that promote
insensible free water losses, such as hot weather. A Cochrane review on this subject defined water-loss dehydration as "people with
serum osmolality
Plasma osmolality measures the body's electrolyte–water balance. There are several methods for arriving at this quantity through measurement or calculation.
Osmolality and osmolarity are measures that are technically different, but functional ...
of 295 mOsm/kg or more" and found that the main symptom in the elderly (people aged over 65) was fatigue.
Cause
Risk factors for dehydration include but are not limited to: exerting oneself in hot and humid weather, habitation at high altitudes, endurance athletics, elderly adults, infants, children and people living with chronic illnesses.
Dehydration can also come as a side effect from many different types of drugs and medications.
In the elderly, blunted response to thirst or inadequate ability to access free water in the face of excess free water losses (especially
hyperglycemia related) seem to be the main causes of dehydration. Excess free water or hypotonic water can leave the body in two ways –
''sensible'' loss such as
osmotic diuresis,
sweating
Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals.
Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. The eccrine sweat glands are distri ...
,
vomiting and
diarrhea, and
''insensible'' water loss, occurring mainly through the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
and
respiratory tract. In humans, dehydration can be caused by a wide range of diseases and states that impair water
homeostasis
In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
in the body. These occur primarily through either impaired thirst/water access or sodium excess.
Diagnosis
Definition
Dehydration occurs when water intake does not replace
free water lost due to normal physiologic processes, including
breathing,
urination,
perspiration
Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammals.
Two types of sweat glands can be found in humans: eccrine glands and apocrine glands. The eccrine sweat glands are distr ...
, or other causes, including
diarrhea, and
vomiting. Dehydration can be life-threatening when severe and lead to
seizure
An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with lo ...
s or respiratory arrest, and also carries the risk of
osmotic cerebral edema if
rehydration is overly rapid.
The term "dehydration" has sometimes been used incorrectly as a proxy for the separate, related condition of
hypovolemia, which specifically refers to a decrease in volume of
blood plasma
Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the int ...
.
The two are regulated through independent mechanisms in humans;
the distinction is important in guiding treatment.
Prevention
For routine activities, thirst is normally an adequate guide to maintain proper hydration. Minimum water intake will vary individually depending on weight, energy expenditure, age, sex, physical activity, environment, diet, and genetics. With exercise, exposure to hot environments, or a
decreased thirst response, additional water may be required. In
athletes in competition, drinking to
thirst optimizes performance and safety, despite weight loss, and as of 2010, there was no scientific study showing that it is beneficial to stay ahead of thirst and maintain weight during exercise.
In warm or humid weather, or during heavy exertion, water loss can increase markedly, because humans have a large and widely variable capacity for sweating. Whole-body sweat losses in men can exceed 2 L/h during
competitive sport, with rates of 3–4 L/h observed during short-duration, high-intensity exercise in the heat.
When such large amounts of water are being lost through perspiration,
electrolytes, especially sodium, are also being lost.
In most athletes exercising and sweating for 4–5 hours with a sweat sodium concentration of less than 50 mmol/L, the total sodium lost is less than 10% of total body stores (total stores are approximately 2,500 mmol, or 58 g for a 70-kg person).
These losses appear to be well tolerated by most people. The inclusion of sodium in fluid replacement drinks has some theoretical benefits
and poses little or no risk, so long as these fluids are hypotonic (since the mainstay of dehydration prevention is the replacement of free water losses).
Treatment
The most effective treatment for minor dehydration is widely considered to be drinking water and reducing fluid loss. Plain water restores only the volume of the blood plasma, inhibiting the thirst mechanism before solute levels can be replenished. Solid foods can contribute to fluid loss from vomiting and diarrhea. Urine concentration and frequency will return to normal as dehydration resolves.
In some cases, correction of a dehydrated state is accomplished by the replenishment of necessary water and
electrolytes (through
oral rehydration therapy, or
fluid replacement by
intravenous therapy). As oral rehydration is less painful, non-invasive, inexpensive, and easier to provide, it is the treatment of choice for mild dehydration. Solutions used for intravenous rehydration must be
isotonic or
hypertonic
In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane- ...
. Pure water injected into the veins will cause the breakdown (
lysis) of red blood cells (
erythrocytes).
When
fresh water is unavailable (e.g. at sea or in a desert),
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
or drinks with significant
alcohol concentration will worsen dehydration.
Urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellul ...
contains a lower solute concentration than seawater; this requires the kidneys to create more urine to remove the excess salt, causing more water to be lost than was consumed from seawater.
If a person is dehydrated and taken to a medical facility,
IVs
Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutrie ...
can also be used.
For severe cases of dehydration where
fainting,
unconsciousness
Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an awareness of self and environment or to respond to any human or environmental stimulus. Unconsciousness may occur as the r ...
, or other severely inhibiting symptoms are present (the patient is incapable of standing or thinking clearly), emergency attention is required. Fluids containing a proper balance of replacement electrolytes are given orally or intravenously with continuing assessment of electrolyte status; complete resolution is normal in all but the most extreme cases.
See also
*
Hydrational fluid
Oral rehydration therapy (ORT) is a type of fluid replacement used to prevent and treat dehydration, especially due to diarrhea. It involves drinking water with modest amounts of sugar and salts, specifically sodium and potassium. Oral rehydr ...
s
*
Terminal dehydration
Terminal dehydration is dehydration to the point of death. Some scholars make a distinction between "terminal dehydration" and "termination by dehydration". Courts in the United StatesNL Cantor (1987). Legal frontiers of death and dying. Indiana Un ...
*
Dryness (medical)
Dryness is a medical condition in which there is local or more generalized decrease in normal lubrication of the skin or mucous membranes.
Examples of local dryness include dry mouth, dry eyes, dry skin ( xeroderma), Diabetic foot and vaginal ...
*
Hypernatremia
References
Further reading
*
*
External links
Definition of dehydration by the U.S. National Institutes of Health's MedlinePlus medical encyclopedia
{{Authority control
Causes of death
Nutrition
Symptoms and signs
Wilderness medical emergencies