Fluid Compartments and Osmolarity

Intracellular fluid (ICF)

Fluid located inside cells.

Extracellular fluid (ECF)

Fluid located outside cells.

Interstitial fluid (ISF)

Extracellular fluid surrounding tissue cells.

Plasma

The fluid portion of blood within blood vessels.

Transcellular fluid

Fluid in epithelial-lined cavities, such as cerebrospinal and pleural fluid.

Two barriers separating body-fluid compartments

Cell membranes and blood-vessel walls.

Cell membrane function in fluid balance

Separates ICF from ECF and selectively controls movement of substances.

Why do ICF and ECF have different compositions

Membrane selectivity, charged proteins, and unequal ion distribution.

Main cation in ECF

Sodium (Na⁺).

Main cation in ICF

Potassium (K⁺).

Na⁺/K⁺ ATPase pump

A membrane pump that helps maintain high Na⁺ outside cells and high K⁺ inside cells.

Osmosis

Net movement of water across a selectively permeable membrane toward the higher effective solute concentration.

Osmolarity

The concentration of osmotically active particles per litre of solution.

Normal body-fluid osmolarity

Approximately 300 mOsm/L.

Isotonic solution

A solution with similar effective osmolarity to the fluid it is compared with.

Hypertonic ECF

ECF with a higher effective solute concentration than the ICF.

Hypotonic ECF

ECF with a lower effective solute concentration than the ICF.

Effect of hypertonic ECF on cells

Water moves out of cells, causing them to shrink.

Effect of hypotonic ECF on cells

Water moves into cells, causing them to swell.

Why is brain-cell swelling dangerous

The skull limits expansion, so swelling can disrupt brain function.

Oedema (edema)

Swelling caused by expansion of interstitial fluid volume.Hypertonicity of ECF

Possible causes of ECF hypertonicity

Water loss, inadequate water intake, or excess solute relative to water.

Effect of ECF hypertonicity on cells

Cells lose water and shrink.

Hypotonicity of ECF

A condition in which ECF osmolarity is reduced, causing water to enter cells.

Possible causes of ECF hypotonicity

Renal failure, rapid water ingestion, excessive vasopressin, or hypotonic saline administration.

Effect of ECF hypotonicity on cells

Cells gain water and swell.

Major concern with hypotonicity

Brain-cell swelling can impair neurological function.

Possible symptoms of severe hypotonicity

Headache, lethargy, vomiting, convulsions, and coma.

Water balance

Water input must equal water output over time.

Why is kidney regulation important for water balance

The kidneys adjust water reabsorption and urine output to control body water.

What happens when vasopressin increases

More water is reabsorbed, urine volume decreases, and body water is conserved.