Intracellular fluid space
Intracellular fluid represents the largest fluid reservoir in our body. It accounts for an average of 31% of total body weight, which in absolute figures is about 24 liters. An important clinical feature of this compartment is its inertness: when pathological processes develop, the volume of fluid inside cells begins to change significantly later and more slowly than in the surrounding extracellular environment.
Water inside the cell is not homogeneous and exists in three different physicochemical states:
- Bound water — firmly bound to various hydrophilic organic as well as inorganic molecules.
- Adhered ("attracted") water — accumulates and is held on the surface of large colloidal molecules.
- Free (mobile) water — this particular fraction is the most dynamic. Its volume changes most significantly and rapidly with any fluctuations in cell vitality, both under absolute norm and during the development of pathology.
Extracellular fluid space
The extracellular compartment accounts for an average of 22% of body weight (about 15 liters). It is not a single entity, but is structurally subdivided into three functionally different fluids: blood, interstitial, and transcellular.
Blood plasma (intravascular fluid) Located inside the vascular bed and accounts for approximately 4% of body weight (from 2 to 2.5 liters). In its composition, plasma is a complex solution:
- About 90% is accounted for by the fluid itself (water).
- 9% consists of various organic substances, with the lion's share (about 6%) being proteins.
- 1% is represented by inorganic substances.
Intercellular (interstitial) fluid This is the largest pool of the extracellular sector, accounting for 18% of body weight (about 12 liters). Interstitial fluid directly bathes tissue cells. Its chemical composition is extremely close to that of blood plasma, and a constant, free exchange of components occurs between these two media.
Transcellular fluid Occupies the smallest volume — only about 1.5% of body weight. This fluid is strictly localized in specialized spaces and cavities of the body. The transcellular pool traditionally includes:
- Cerebrospinal fluid (CSF).
- Synovial fluid, which lubricates joints and tendons.
- Digestive secretions (gastric and intestinal juice).
- Primary urine (fluid located in the cavity of the glomerular capsule and renal tubules).
- Contents of serous cavities (pleural, abdominal, and pericardial cavities).
- Aqueous humor filling the chambers of the eye.
Water metabolism disorders (dyshydrations)
Normally, the body maintains a strict balance of water metabolism. If an excess of fluid accumulates, it is referred to as a positive balance. If a fluid deficit occurs, the balance becomes negative.
Absolutely all water metabolism disorders in pathologically physiology are united by a common term — dyshydration. Globally, they are divided into two large groups:
- Hypohydration — a pathological decrease in fluid volume below the physiological norm, the extreme manifestation of which is severe dehydration.
- Hyperhydration (hyperhydria) — excessive accumulation of fluid in the body. In clinical practice, the most frequent and noticeable form of hyperhydration is edema.
For accurate diagnosis, dyshydrations are classified according to two main criteria.
1. By extracellular fluid osmolality:
- Hypoosmolar form: plasma osmolality drops below 280 mosm/kg H₂O.
- Isoosmolar form: plasma osmolality remains within normal limits.
- Hyperosmolar form: plasma osmolality increases and exceeds 300 mosm/kg H₂O.
2. By predominant localization (affected sector):
- Cellular dyshydration.
- Extracellular dyshydration.
- Mixed form (when both compartments are affected).
Briefly: answers to topical questions
How does the chemical composition of interstitial fluid differ from blood plasma?
The chemical composition of interstitial fluid and blood plasma is similar, but the key difference lies in the protein concentration. In blood plasma, it is significantly higher and is about 70 g/l.
What are the main pathogenetic factors in the development of edema?
The main pathogenetic factors in the development of edema indicated in the sources:
- Hydrodynamic/hemodynamic (hydrostatic): disruption of Starling forces balance; an increase in hydrostatic intracapillary or venous pressure enhances fluid filtration into the interstitium and inhibits resorption.
- Oncotic (hypoproteinemic): a decrease in blood oncotic pressure and/or an increase of it in the intercellular fluid; the main link is a decrease in the effective oncotic suction force of blood plasma.
- Osmotic: in hypoosmolar hyperhydration, the osmotic pressure in the interstitium is lower than in the cell, water moves from the interstitium into the cytoplasm, which leads to cell swelling.
- Membranogenic: an increase in vascular-tissue permeability facilitates the exit of water and plasma proteins into the interstitium.
- Lymphatic: a decrease in lymphatic drainage makes it difficult to remove excess filtrate and protein from the interstitium.
What organic substances are part of blood plasma, besides proteins?
The cited sources do not specify a concrete list of organic substances in blood plasma other than proteins. Only the following is confirmed: blood plasma contains about 90% water and 10% dissolved substances (dry residue); organic substances make up 9%, inorganic ones — 1%; proteins make up about 2/3 of the solid residue, while low-molecular-weight substances and electrolytes make up about 1/3.
To quickly memorize the volumes of extracellular fluid in descending order, use the abbreviation IPT: Interstitial (18%) → Plasma (4%) → Transcellular (1.5%). Together they make up those very 22% of the extracellular sector.
Class and exam questions
Which of the fluid compartments reacts first to the development of a disease?
Extracellular fluid is always the first to react. The intracellular compartment is much more stable, so the volume of fluid inside the cells themselves begins to change much later and at a significantly slower rate.
Which fraction of intracellular water is depleted during changes in cell activity?
Free (or mobile) water is the most dynamic. It is its volume that undergoes the most significant changes with any shifts in cellular activity, whereas bound and adhered water remain relatively stable.
What is classified as transcellular fluid and what is its volume?
These are highly specialized fluids, the total volume of which is about 1.5% of body weight. This group includes cerebrospinal fluid, synovial fluid of joints, gastric and intestinal juices, primary urine, aqueous humor of the eye chambers, and fluids of serous cavities (pleura, pericardium, peritoneum).
How is dyshydration diagnosed based on plasma osmolality?
Doctors assess the concentration of osmotically active substances. If the indicator drops below 280 mosm/kg H₂O, hypoosmolar dyshydration is diagnosed; if it exceeds 300 mosm/kg H₂O, hyperosmolar dyshydration. At normal values, an isoosmolar disorder is discussed.
What else to study on this topic
- Mechanisms of fluid exchange between blood plasma and the interstitial space.
- Pathogenesis of edema formation in various types of hyperhydration.
- Clinical consequences of cellular and extracellular hypohydration (dehydration).
- The role of hydrophilic molecules and colloids in binding intracellular water.
- Causes of plasma osmolality shift beyond 280–300 mOsm/kg H₂O.
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