Dehydratation of the organism

Overview

In clinical practice, dehydration refers to a decrease in total body water that results in an imbalance between extracellular fluid (ECF) and intracellular fluid (ICF). The clinical presentation of the patient depends on the amount of water lost and the relative loss of electrolytes, which determines the tonicity of the extracellular fluid and the direction in which water shifts between compartments.

Water makes up 50 to 60 percent of body weight, and maintaining it at that percentage is essential for circulatory stability, cellular function, and electrolyte homeostasis.

Classification

Dehydration is classified according to the osmolarity of the extracellular fluid:

• Isotonic (normotonic)
• Hypotonic
• Hypertonic

These categories describe whether the loss of water and electrolytes is proportional or disproportionate.

Isotonic Dehydration

Definition: The loss of water and electrolytes in equal proportions leading to:

• Reduced extracellular fluid volume
• Normal extracellular fluid osmolarity
• No water shift between compartments

Causes:

• Bleeding
• Burns
• Gastrointestinal fluid losses such as diarrhea and vomiting
• Loss of fluid to third spaces such as ileus, pancreatitis, peritonitis

Consequences:

• Reduced venous return
• Reduced cardiac output → hypotension and tachycardia
• Risk of hypovolemic shock

Hypotonic Dehydration

Definition: A greater loss of salts than water leading to:

• Reduced extracellular fluid osmolarity
• Water moving into cells → cellular edema

Causes:

• Replacement of sweat losses with water only
• Addison’s disease (aldosterone deficiency → sodium loss)
• Diuretics
• Renal salt wasting

Consequences:

• Symptoms of hypovolemia
• Brain edema → headache, confusion, seizures
• Nausea, vomiting, muscle cramps

Hypertonic Dehydration

Definition: Greater loss of water than salts leading to:

• Increased extracellular fluid osmolarity
• Water moving out of cells → cellular dehydration

Causes:

• Insufficient water intake
• Fever, heavy sweating, hyperventilation
• Osmotic diuresis (diabetes mellitus with glucosuria)
• Diabetes insipidus (central ADH deficiency or renal ADH resistance)

Consequences:

• Symptoms of hypovolemia (e.g., oliguria, orthostatic dizziness)
• Intense thirst, dry mucous membranes, dry skin
• Neurological dysfunction: irritability → confusion → coma
• Brain cell shrinkage

Clinical Features

Symptoms:

• Thirst
• Dizziness
• Lethargy
• Altered consciousness

Signs:

• Dry skin and mucous membranes
• Decreased skin turgor
• Tongue furrows
• Sunken eyes
• Sunken fontanelle in infants

Pathophysiological Consequences

Circulatory System:

• Hypovolemia → reduced preload → circulatory failure
• Severe cases → hypovolemic shock

Central Nervous System:

• Hypotonic dehydration → cerebral edema
• Hypertonic dehydration → brain shrinkage with risk of intracranial hemorrhage

Kidneys:

• Reduced renal perfusion → prerenal azotemia
• Prolonged hypoperfusion → acute tubular necrosis

Electrolyte Disturbances:

• Hypernatremia or hyponatremia
• Hyperkalemia or hypokalemia

Hemostasis:

• Increased blood viscosity → thromboembolic risk

Diagnosis

• Physical examination including skin, mucosa, eyes, blood pressure, and heart rate
• Laboratory tests including serum sodium, serum potassium, serum osmolarity, urine osmolarity, urine sodium, creatinine, and urea
• Assessment of body weight and fluid intake and output

Therapy

• Treatment of the underlying cause
• Fluid replacement depending on the type of dehydration:
  • Isotonic dehydration → isotonic saline
  • Hypotonic dehydration → isotonic or hypertonic saline with slow correction
  • Hypertonic dehydration → careful administration of hypotonic fluids to avoid cerebral edema
• Continuous monitoring of consciousness, electrolytes, and urine output

Sources

• Guyton and Hall. Textbook of Medical Physiology. 13th ed. Elsevier; 2015. Chapter 25: The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema
• Stefan Silbernagl, Florian Lang. Color Atlas of Pathophysiology. Thieme; 2020. Chapter 5: Kidney, Salt and Water Balance. Page 122
• AMBOSS. Dehydration and hypovolemia. https://next.amboss.com/us/article/Ig0Y92?q=dehydration+and+hypovolemia. Accessed December 2025.
• Pathophysiology lecture materials, Second Faculty of Medicine, Charles University