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Fluid Compartments
& Osmolarity

Understand how your body distributes water across compartments and uses osmolarity to maintain cellular balance.

Anatomy & Physiology

Why Fluid Balance Matters

About 60% of your total body weight is water. This water isn't sitting in one big pool: it is carefully divided into distinct compartments, each with a specific composition. Even a small shift in water or solute concentration between compartments can disrupt cell function, blood pressure, and organ performance.

Total Body Water Distribution

Total Body Water (TBW): ~60% body weight Intracellular Fluid (ICF) ~2/3 of TBW (~28 L in 70 kg adult) Inside all cells
High \(\text{K}^+\), \(\text{Mg}^{2+}\), \(\text{PO}_4^{3-}\)
 High protein Extracellular Fluid (ECF) ~1/3 of TBW (~14 L in 70 kg adult) Plasma ~3 L In blood vessels
High \(\text{Na}^+\), \(\text{Cl}^-\)
 Interstitial ~11 L Between cells
High \(\text{Na}^+\), \(\text{Cl}^-\)

Key Definitions

Solute
A substance dissolved in a solvent (e.g., \(\text{Na}^+\), glucose, urea dissolved in water).
Osmolarity
Total solute concentration per liter of solution, measured in \( \text{mOsm/L} \).
Tonicity
The ability of a solution to cause water movement across a cell membrane relative to the cell interior.

The Osmolarity Equation

Example: NaCl dissociates into 2 particles (\(\text{Na}^+ + \text{Cl}^-\)). A \( 0.15 \; \text{M} \) NaCl solution has an osmolarity of \( 2 \times 0.15 = 0.30 \; \text{Osm/L} = 300 \; \text{mOsm/L} \).

Glucose does not dissociate, so \( n = 1 \). A \( 0.30 \; \text{M} \) glucose solution also has an osmolarity of \( 300 \; \text{mOsm/L} \).

Osmosis: Water Follows Solute

Low solute Hypotonic Semi-permeable membrane High solute Hypertonic
\(\text{H}_2\text{O}\)
 Water Solute

Tonicity vs. Osmolarity

Osmolarity Tonicity
Measures Total solute per liter Effect on cell volume
Includes All solutes (penetrating + non-penetrating) Only non-penetrating solutes
Quantitative? Yes: \( \text{mOsm/L} \) No: relative (hypo/iso/hyper)
Example Urea raises osmolarity Urea does not change tonicity (it crosses membranes)
Clinical use Lab values, serum measurement Predicting cell shrink/swell

Cell Response to Tonicity

Hypotonic Cell swells (lysis risk) Isotonic Normal No net change (equilibrium) Hypertonic Cell shrinks (crenation)

Starling Forces at the Capillary

Fluid movement between plasma and interstitial fluid is governed by four pressures collectively called Starling forces:

Hydrostatic Pressure
Blood pressure pushing fluid out of the capillary into the interstitial space.
Oncotic Pressure
Colloid osmotic pressure from plasma proteins (albumin) pulling fluid into the capillary.

How to Solve Osmolarity Problems

1
Identify each solute in the solution and its molar concentration \( C_i \).
2
Determine the dissociation factor \( n_i \): how many particles each molecule produces. NaCl → 2, \(\text{CaCl}_2\) → 3, glucose → 1.
3
Multiply and sum: \( \text{Osmolarity} = \sum n_i \times C_i \).
4
Compare to plasma (\( \approx 300 \; \text{mOsm/L} \)) to determine if the solution is hypotonic, isotonic, or hypertonic.
5
Assess tonicity separately: exclude penetrating solutes (urea, ethanol) to predict cell volume change.

Example 1: Normal Saline

Problem: Calculate the osmolarity of 0.9% NaCl (normal saline). Is it isotonic to plasma?

1
0.9% NaCl = 0.9 g per 100 mL = 9 g/L. Molar mass of NaCl = \( 58.44 \; \text{g/mol} \).
2
Molarity: \( C = \frac{9}{58.44} \approx 0.154 \; \text{M} \).
3
NaCl dissociates into 2 particles: \(\text{Na}^+ + \text{Cl}^-\), so \( n = 2 \).
4
Osmolarity = \( 2 \times 0.154 = 0.308 \; \text{Osm/L} \approx \) 308 mOsm/L.

Example 2: Mixed Solution

Problem: A solution contains \( 0.1 \; \text{M} \) NaCl and \( 0.1 \; \text{M} \) glucose. What is its osmolarity? Is it hypo-, iso-, or hypertonic?

1
NaCl: \( n = 2 \), so contribution = \( 2 \times 0.1 = 0.2 \; \text{Osm/L} \).
2
Glucose: \( n = 1 \), so contribution = \( 1 \times 0.1 = 0.1 \; \text{Osm/L} \).
3
Total osmolarity = \( 0.2 + 0.1 = 0.3 \; \text{Osm/L} = \) 300 mOsm/L.

Example 3: The Urea Trap

Problem: A solution contains \( 0.3 \; \text{M} \) urea. Is it isotonic, hypotonic, or hypertonic?

1
Urea does not dissociate: \( n = 1 \). Osmolarity = \( 1 \times 0.3 = 300 \; \text{mOsm/L} \).
2
Osmolarity = 300 mOsm/L → isosmotic to plasma.
3
But urea freely crosses membranes → it is a penetrating solute → effective osmoles = 0.
4
Tonicity: the solution is effectively hypotonic → cells will swell.

Fluid Shifts Between Compartments

ICF ~28 L
High \(\text{K}^+\)
 High protein ISF ~11 L
High \(\text{Na}^+\)
 Low protein Plasma ~3 L
High \(\text{Na}^+\)
 High protein
\(\text{H}_2\text{O}\)
\(\text{H}_2\text{O}\)
\(\text{H}_2\text{O}\)
\(\text{H}_2\text{O}\)
 Cell membrane (osmosis) Capillary wall (Starling forces) All compartments are in osmotic equilibrium at ~300 mOsm/L

Common Mistakes

✕ Confusing Osmolarity with Tonicity
Osmolarity counts all solutes; tonicity only counts non-penetrating ones. Urea raises osmolarity but not tonicity.
✕ Forgetting Dissociation
NaCl produces 2 particles, \(\text{CaCl}_2\) produces 3. A 0.15 M NaCl solution is 300 mOsm/L, not 150.
✕ Mixing Up ICF and ECF Ions
\(\text{K}^+\) is the major ICF cation; \(\text{Na}^+\) is the major ECF cation. Swapping these on exams is a common error.

Summary Table

ICF Interstitial Plasma
Volume ~28 L ~11 L ~3 L
% of TBW ~67% ~26% ~7%
Major cation \(\text{K}^+\) \(\text{Na}^+\) \(\text{Na}^+\)
Major anion \(\text{PO}_4^{3-}\), protein \(\text{Cl}^-\) \(\text{Cl}^-\), protein
Protein level High Low High
Osmolarity ~300 ~300 ~300

Question 1 of 5

What percentage of total body water is found in the intracellular fluid compartment?

Question 2 of 5

What is the osmolarity of a \( 0.1 \; \text{M} \) \(\text{CaCl}_2\) solution?

Question 3 of 5

A 300 mOsm/L urea solution is placed around red blood cells. What will happen to the cells?

Question 4 of 5

Which ion is the major cation of the intracellular fluid?

Question 5 of 5

In Starling's equation, which force primarily pulls fluid back into the capillary?