Body Compartments

1. What is a mole (mol)?

👉 A measure of the amount of a substance.

2. What is molarity (M)?

👉 Moles of solute per liter of solution (mol/L).

3. What does 0.9% NaCl mean?

👉 0.9 g NaCl per 100 mL, or 9 g/L. This equals approximately 154 mM NaCl.

4. What is an osmole (osmol)?

👉 A measure of the number of dissolved particles in a solution, not the number of molecules originally added.

5. What is the difference between molarity and osmolarity?

👉 Molarity measures the concentration of molecules; osmolarity measures the concentration of dissolved particles after dissociation.

6. How do you calculate osmolarity?

👉 Osmolarity = Molarity × Number of particles produced.

Examples:

• NaCl → 2 particles → 0.15 M × 2 = 300 mOsm/L.

• CaCl₂ → 3 particles → 0.15 M × 3 = 450 mOsm/L.

• Glucose → 1 particle → 0.15 M × 1 = 150 mOsm/L.

Memory trick: Count how many pieces the compound breaks into.

7. What happens to osmolarity when NaCl, CaCl₂, and glucose dissolve?

👉 NaCl separates into Na⁺ and Cl⁻ (2 particles); CaCl₂ separates into Ca²⁺ and 2 Cl⁻ (3 particles); glucose stays intact (1 particle).

8. What is the difference between osmolarity and osmolality?

👉 Osmolarity = osmoles per liter of solution; osmolality = osmoles per kilogram of solvent. They are approximately equal in most body fluids.

9. What are the two major body fluid compartments?

👉 Intracellular fluid (ICF) = fluid inside cells; extracellular fluid (ECF) = fluid outside cells.

10. What are the two subdivisions of extracellular fluid?

👉 Plasma = fluid inside blood vessels; interstitial fluid = fluid surrounding cells outside blood vessels.

11. What is the 60-40-20 rule?

👉 60% of body weight is total body water, 40% is ICF, and 20% is ECF.

12. For a 70-kg individual, how much fluid is in each compartment?

• Total body water = 42 L.

• ICF = 28 L.

• ECF = 14 L.

• Interstitial fluid = 10.5 L.

• Plasma = 3.5 L.

13. What is the primary difference between pitting edema and dehydration?

👉 Pitting edema = excessive fluid accumulation in the interstitial space; dehydration = loss of body water.

14. Why does pitting edema occur in hypertension?

👉 Increased vascular hydrostatic pressure pushes fluid out of capillaries into the interstitial space, causing fluid accumulation, especially in the feet and ankles.

15. How can decreased kidney function cause increased blood volume?

👉 The kidneys retain excess sodium and water, increasing ECF volume and blood volume, which can increase vascular pressure and contribute to edema.

16. What is osmosis?

👉 Movement of water across a semipermeable membrane toward the side with higher effective solute concentration.

17. What is osmotic pressure?

👉 The pressure generated by solutes that cannot cross a membrane, causing water to move toward the side with higher effective solute concentration.

18. What is the van't Hoff equation for osmotic pressure?

• Π = osmotic pressure.

• n = number of particles produced per molecule.

• C = solute concentration.

• R = gas constant.

• T = absolute temperature (Kelvin).

19. What is the most important rule to remember about osmosis?

👉 Water moves toward the side with higher effective osmolarity, provided the solute cannot freely cross the membrane.

20. What is diffusion?

👉 Movement of solutes from an area of higher concentration to an area of lower concentration to equalize concentrations.

21. What is the difference between osmosis and diffusion?

👉 Osmosis = water moves toward higher effective solute concentration; diffusion = solutes move down their concentration gradient.

Memory trick:

• Osmosis = water moves.

• Diffusion = solute moves.

22. What is Fick's First Law of Diffusion?

• J = rate of solute movement (flux).

• D = diffusion coefficient.

• A = surface area.

• ΔC = concentration difference.

• Δx = distance across which diffusion occurs.

High yield: Greater surface area and concentration gradient increase diffusion; greater distance decreases diffusion.

23. What is the reflection coefficient (σ)?

👉 Measures how impermeable a membrane is to a particular solute and therefore how effectively that solute can generate osmotic pressure.

Reflection coefficient	Meaning
σ = 1	Solute cannot cross; maximal osmotic effect
σ = 0	Solute freely crosses; no sustained osmotic effect
0–1	Partial permeability; partial osmotic effect

24. What happens to osmotic pressure when a solute can freely cross a membrane?

👉 Its reflection coefficient approaches 0, so it cannot generate sustained osmotic pressure or drive net water movement.

25. Can the same solute generate osmotic pressure across one membrane but not another?

👉 Yes! It depends on the permeability of the specific membrane.

Example: NaCl can generate osmotic pressure across a cell membrane when it cannot cross, but does not generate sustained osmotic pressure across a barrier that is freely permeable to it.

26. What is oncotic pressure?

👉 Osmotic pressure generated by large proteins, especially albumin, that helps pull water into the vascular compartment.

27. What happens when serum albumin increases?

👉 Plasma oncotic pressure increases, pulling more water from the interstitial space into the vascular compartment.

Memory trick: Albumin attracts water into blood vessels.

28. How does the proximal tubule use osmosis to reabsorb water?

👉 Na⁺ and glucose are transported from the tubular lumen into the interstitial space, increasing interstitial osmolarity. Water follows by osmosis.

29. Why does L-glucose cause osmotic diuresis in the professor's example?

👉 L-glucose is not transported by the Na⁺-glucose cotransporter, so it remains in the tubular lumen. This prevents normal solute and water reabsorption, leaving more water in the urine.

Important distinction:

• D-glucose = transported and reabsorbed.

• L-glucose = not transported by this transporter and remains in the lumen.

30. What happens to ICF and ECF during the six major volume changes?

👉 Remember: All volume changes start in the ECF. Water then shifts between ECF and ICF if their effective osmolarities differ.

Condition	What happens?	ECF volume	ICF volume	ECF osmolarity	ICF osmolarity	Clinical example
Isotonic expansion	Gain isotonic fluid	↑	No change	↔	↔	Isotonic saline IV
Hypertonic expansion	Gain hypertonic fluid	↑	↓	↑	↑	Hypertonic saline IV
Hypotonic expansion	Gain hypotonic fluid	↑	↑	↓	↓	SIADH, psychogenic polydipsia
Isotonic contraction	Lose isotonic fluid	↓	No change	↔	↔	Hemorrhage
Hypertonic contraction	Lose hypotonic fluid	↓	↓	↑	↑	Diabetes insipidus, alcohol-related water loss
Hypotonic contraction	Lose hypertonic fluid	↓	↑	↓	↓	Addison disease

Clinical Correlation

Condition	Type
Isotonic saline IV	Isotonic expansion
Hypertonic saline IV	Hypertonic expansion
SIADH	Hypotonic expansion
Psychogenic polydipsia	Hypotonic expansion
Hemorrhage	Isotonic contraction
Diabetes insipidus	Hypertonic contraction
Alcohol-related water loss	Hypertonic contraction
Addison disease	Hypotonic contraction

Must-know Darrow-Yannet reasoning

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