Dehydration: what it is, symptoms, causes, consequences and prevention

Dehydration: what it is, symptoms, causes, consequences and prevention

The term dehydration is used to indicate a lack of water in the body. This can be due to a low water intake or a more important loss through, for example, sweating or bleeding.

Symptoms and severity of the condition vary widely between adults and children. In the first case, these can be milder at the beginning, with dizziness and fatigue, but sometimes they can get complicated with real drops in blood pressure and fainting.

In fact, especially during the hot season, elderly subjects may have difficulty in assuming an adequate amount of liquids, because they feel less thirsty. In children, dehydration often appears due to prolonged diarrheal syndromes together with a poor water intake due to feeding difficulties.

Recognizing the early signs of dehydration in adults and children is essential to avoid incurring more serious complications following a lack of water in the body.

What is dehydration: meaning and types?

The term dehydration can indicate a pathological condition caused by a water imbalance in the body with or without other concomitant diseases. In fact, our body contains between 50% and 80% water.

Diarrheic conditions, for example, are the most common reason for dehydration, making it the leading cause of infant mortality globally. Negative fluid balance that can lead to fluid loss can result from:

  • Decreased oral intake of water and beverages.
  • Increased renal, gastrointestinal or haemorrhage fluid excretion.
  • Alteration of the water balance and appearance of other types of fluids: ascites, effusions and capillary extravasations, specific in burns and sepsis.

The decrease in total body water causes reductions in the volumes of both intracellular and extracellular fluids. The clinical manifestations of dehydration are most closely related to the intravascular fluid volume depletion and attempts at physiological compensation that occur.

As dehydration progresses, hypovolemic shock (significant drop in blood pressure) occurs in severe cases, resulting in organ failure and, in severe cases, death.

The volume and distribution of water in the body

In younger subjects, water can take up 55-60% of the body composition (45-50% in young women). The total volume of water is distributed throughout the body as follows:

  • Muscles: 50%.
  • Leather: 20%.
  • Other organs: 20%.
  • Blood: 10%.

About two-thirds of the volume of water in the body is found in the intracellular compartment and only one-third in the extracellular compartment. In this regard, therefore, we will have the plasma volume, which represents the liquid part of the blood, i.e. 5% of the body weight (3.5 liters for a 70 kg man), and the interstitial fluid, 15% of the weight is the fluid between cells and includes some such as:

  • Lymph: 2-3% of body weight. Transcellular fluid: 1.5% of the weight represents the fluid coming from the organs with lumen delimited by the epithelium: digestive secretions, sweat, cerebrospinal fluid, pleural, peritoneal, synovial, intraocular, pericardial, biliary, intraluminal fluid of the thyroid and cochlea ( component of the inner ear). Connective tissue (4.5%).
  • Intracellular volume (VIC) 30-40% of body weight.

Symptoms of dehydration

Recognizing the signs and symptoms of dehydration as early as possible is key to proper treatment and can help prevent more serious and life-threatening cases. There are several common signs to look out for, some more obvious than others.

It’s important to know that if dehydration becomes more severe, it can lead to signs of mental and physical decline that will require immediate emergency action. If the signs of severe dehydration are worrying enough, they may even warrant medical attention.

Among the most frequent we find:


  • Feeling thirsty.
  • Dry mouth.
  • Urinate and sweat less than usual.
  • Dark colored urine.
  • Dry skin.
  • Feeling tired.
  • Dizziness.

Dehydration in infants and young children: symptoms

  • Dry mouth and tongue.
  • I cry without tears.
  • No wet diaper for 3 hours or more.
  • High fever.
  • Sleepy behavior.
  • Irritability.
  • Eyes that look sunken.

Dehydration can be mild or it can be severe enough to be life-threatening. It may also require medical attention if symptoms include:

  • Confusion.
  • Fainting.
  • Poor urination.
  • Rapid heartbeat.
  • Rapid breathing.
  • Shock.

When does it become a medical emergency?

When dehydration becomes severe, more important symptoms may appear, such as those listed below, which should trigger emergency medical attention. Among them we mention:

  • Severe diarrhea (especially in the elderly or children).
  • Presence of faecal material with blood.
  • Diarrheic episodes occurring for more than 3 days.
  • Disorientation.
  • Persistent vomiting sensation.

Intense dehydration can also increase the formation of thrombi, responsible for vascular events such as heart attacks and strokes. Therefore people subject to coagulation phenomena (fibrillation, arrhythmias, venous insufficiency) must ensure that during hot days they have a constant and sufficient intake of water or other liquids.

Similarly, people suffering from kidney disease or gallstones also need to actively avoid dehydration by consuming water while avoiding fizzy drinks.

Causes of dehydration and related diseases

Water plays a key role in maintaining multiple physiological functions in our body. The human body is made up of water in a variable percentage between 55% and 65%.

Two-thirds of this amount is intracellular and one-third is extracellular (inside and outside the cell, respectively). One fifth of extracellular water is intravascular (that is, it is found inside veins and arteries).

The body has a system that works, in a complex way, to keep these volumes constant (homeostasis). The primary control of water homeostasis occurs through the so-called osmoreceptors present in the brain.

Being sensed by these osmoreceptors, dehydration stimulates the thirst center in the hypothalamus, which leads to the need for water intake. These osmoreceptors can also control water retention by the kidneys.

When the hypothalamus detects lower water concentrations in the blood, it will cause the posterior pituitary to release antidiuretic hormone (ADH), which stimulates the kidneys to reabsorb more water.

The drop in blood pressure, which often accompanies dehydration, triggers the secretion of the hormone renin. Renin converts angiotensin I to angiotensin II, which increases the release of aldosterone from the adrenal glands.

Aldosterone increases the absorption of sodium and water by the kidney. Using these mechanisms, the body regulates fluid volume and the concentration of sodium and water.

Dehydration can be classified based on osmolarity and severity. Serum sodium (Na) is a biomarker of osmolarity, assuming the patient has normal blood glucose. Dehydration can be isonatremic (130 to 150 mEq/L), hyponatremic (< 130 mEq/L), or hypernatremic (>150 mEq/L).

Isonatremic dehydration is the most common (80%). The two types of dehydration (hypernatremic and hyponatremic) each account for about 5-10% of cases. Changes in serum sodium reflect the composition of fluid losses and therefore various pathophysiological effects.

Isoosmotic dehydration

It occurs in haemorrhages, burns from plasmatic transfer, loss of gastrointestinal fluids (diarrhea, vomiting), peritonitis, ascites, polyuric phase of acute renal failure, sweating, diuretics, diabetes, alcoholism. Initially, there is a loss of plasma fluid replaced by interstitial fluid without changing the osmolarity of the EVC (extracellular volume).

No change occurs at the level of VIC (intracellular volume).

Hyperosmotic dehydration

It occurs through reduction of water intake, diabetes insipidus (neurogenic or nephrogenic), diabetes mellitus, alcoholism, administration of lithium salts, fever, excessive evaporation through sweat or lungs (e.g. artificial respiration), diarrhea, burns, treatment with mannitol.

Initially, the liquid lost is plasma which becomes hyperosmotic and attracts the interstitial fluid as its osmolarity increases, there is consecutive escape of VIC to the interstitium, eventually leading to a reduction in the volume of both VIC and VEC .

Hypoosmotic dehydration

It may be due to profuse sweating, excessive intake of health diuretics, renal salt loss due to adrenal insufficiency, fluid accumulation in the cavities – ileus, peritonitis.

Initially, loss of salt leads to loss of water, but excess salt will lead to a decrease in VEC osmolarity and a shift of water from VEC to VIC resulting in a decrease in extracellular fluid and an increase in intracellular fluid.

Categories at risk of dehydration

The categories of people most subject to dehydration, especially in the summer, are represented by:

Elderly people

The elderly may suffer from dehydration when exposed to high temperatures due to the decreased perception of thirst, which occurs physiologically in some aged people.

This alteration of the senses causes a decrease in the oral intake of fluids necessary for the optimal functioning of the body. Many seniors also require the…