Fluid Compartments & Total Body Water

Vocabulary flashcards covering key terms and concepts from the lecture on fluid compartments, total body water, and fluid balance.

Fluid Compartments

The anatomical spaces in which body fluid is contained, primarily the intracellular fluid (ICF) and extracellular fluid (ECF) compartments.

Total Body Water (TBW)

The sum of all water within the body’s fluid compartments; averages ~50 % of body weight in females and ~60 % in males.

Intracellular Fluid (ICF)

Fluid located inside the body’s cells; makes up roughly two-thirds of TBW (≈33 % body weight in females, 40 % in males).

Extracellular Fluid (ECF)

Fluid outside the cells, divided into plasma and interstitial fluid; about one-third of TBW (≈17 % body weight in females, 20 % in males).

Interstitial Fluid

The portion of ECF found between tissue cells; ≈12.75 % of body weight in females and 15 % in males.

Intravascular Fluid (Plasma)

The liquid component of blood within vessels; ≈4.25 % of body weight in females and 5 % in males.

TBW Calculation Formula

TBW = (⅓ ECF) + (⅔ ICF); ECF = (¼ plasma) + (¾ interstitial fluid).

Osmolality

The concentration of solute particles per kilogram of solvent; normal blood osmolality is ~275–299 mOsm/kg and is largely determined by ECF sodium.

Tonicity

The effect an extracellular solution’s osmolality has on cell volume (isotonic, hypotonic, hypertonic).

Isotonic Solution

An ECF with ~0.9 % NaCl; causes no net volume change in cells.

Hypotonic Solution

An ECF with NaCl <0.9 % (low osmolality); water moves into cells causing them to swell, leading to hypovolemia.

Hypertonic Solution

An ECF with NaCl >0.9 % (high osmolality); water leaves cells causing them to shrink, producing hypervolemia.

Aquaporin Channels

Protein pores in cell membranes that permit rapid, bidirectional water movement between ICF and ECF.

Sodium (Na⁺)

The principal ECF cation that drives osmotic balance and largely sets plasma osmolality.

Albumin

A major plasma protein that maintains the effective osmolality (oncotic pressure) of plasma and prevents excess fluid loss to interstitial spaces.

Fluid Balance

Equilibrium achieved when daily fluid intake (~2.4–3.2 L) equals daily output (urine, stool, skin, lungs) in a healthy adult.

Daily Water Gains

Sources include drinking (1.4–1.8 L), food (0.7–1 L), and metabolic oxidation (0.3–0.4 L).

Daily Water Losses

Routes include urine (1.4–1.8 L), stool (~0.1 L), skin (0.3–0.5 L), and lungs (0.6–0.8 L).

Age-Related TBW Changes

TBW percentage decreases with age and increased body fat; infants have high TBW while older adults have reduced TBW and impaired fluid conservation.

Dehydration Risk in Infants

Heightened by low body fat, high metabolic rate, large surface area, low body weight, and immature renal fluid conservation.

Dehydration Risk in Older Adults

Results from declining renal function, increased cutaneous water loss, reduced thirst perception, and decreased skin turgor.

Body Fat and TBW

Higher adipose tissue (which contains little water) lowers total body water percentage, contributing to sex differences in TBW.