Normal Fluid Distribution
All water in the human body is strictly divided into two major spaces. Its total amount reaches 30–50 liters, which is equivalent to 60% of an adult's body weight.
- Intracellular Fluid (ICF)
This is the largest fluid reservoir, accounting for 40% of body weight. Fluid here is located directly inside cell membranes (including blood cells). Physically, it exists in two states: some molecules move freely, while others are firmly bound to cytoplasmic colloidal structures.
- Extracellular Fluid (ECF)
It makes up the remaining 20% of body weight and serves as a buffer and transport medium. The extracellular compartment is heterogeneous and includes:
- Blood plasma — the intravascular portion (5%).
- Interstitial fluid — bathes tissues and accounts for 15%.
The interstitial sector also includes all transcellular (cavity) fluids. These include lymph, cerebrospinal fluid (CSF), aqueous humor, perilymph and endolymph of the inner ear, as well as the fluid within the pleural and peritoneal cavities.
Mechanisms of Water Movement
The key factor driving water movement between cells and the interstitial space is osmotic pressure.
The process follows a simple physical principle: water always moves toward where the concentration of dissolved substances (solutes and proteins) is higher. In other words, fluid flows down its gradient toward higher osmotic pressure.
This process is triggered immediately upon any imbalance and continues until osmotic equilibrium is restored on both sides of the membrane. The visual result of these shifts is a change in compartment volumes: cells either swell by taking in water or shrink by releasing it.
Fluid Excess: Types of Hyperhydration
Hyperhydration occurs with the pathological accumulation of fluid. Depending on the ratio of incoming water and electrolytes, three forms are distinguished:
- Isotonic hyperhydration. Water and ions enter the body in physiological proportions (similar to plasma). The osmolarity of the extracellular environment remains normal, so no pressure gradient arises. Fluid does not enter cells but accumulates outside. Result: extracellular volume increases, intracellular volume remains unchanged.
- Hypotonic hyperhydration. Known as "water intoxication." It occurs with a massive intake of pure water without salts. The extracellular environment becomes diluted, and its osmolarity drops. Trying to equalize pressure, water rushes into cells along the concentration gradient. Result: both sectors expand, and dangerous cellular edema develops.
- Hypertonic hyperhydration. Occurs with an excess of ions against a backdrop of water deficiency. Extracellular osmolarity rises sharply. To dilute the salty environment, the extracellular space "draws" water out of cells. Result: extracellular volume increases, while cells lose water and shrink (intracellular volume decreases).
Dehydration: Types of Fluid Loss
Dehydration is fluid loss, which is also classified by osmotic principles:
- Isotonic dehydration. Proportional loss of water and salts. Extracellular osmolarity does not change, so cellular fluid stays in place. Result: circulating blood volume and interstitial volume decrease, intracellular volume is normal.
- Hypertonic dehydration. A person loses predominantly pure water while retaining salts. Extracellular fluid concentrates (osmolarity rises). As compensation, water moves out of cells. Result: both compartments suffer, extracellular fluid volume decreases, and marked cellular dehydration occurs.
- Hypotonic dehydration. Occurs if the body loses more ions than water. Extracellular osmolarity drops sharply. Fluid from vessels and the interstitium moves into cells, where the solute concentration is now relatively higher. Result: a critical drop in extracellular fluid volume coupled with cellular hyperhydration (edema).