Overview
This lecture covers the organization of body fluid compartments, their composition, normal distributions, key principles of osmolarity, and how different infusions or diseases cause volume and osmolarity shifts in these compartments.
Body Fluid Compartments
- The body consists of water organized into compartments: intracellular fluid (ICF) and extracellular fluid (ECF).
- ICF is all water within cells; ECF is outside cells and includes interstitial fluid (ISF) and plasma.
- ISF surrounds cells and is non-circulating; plasma is the circulating part of ECF.
- About 60% of body weight is water: 2/3 intracellular, 1/3 extracellular.
- Female and obese individuals have less total body water due to higher fat content.
Distribution and Composition
- ICF contains mainly potassium (K⁺), magnesium (Mg²⁺), proteins, and organic phosphates (e.g., ATP).
- ECF contains mainly sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻); plasma also has proteins.
- Capillary walls are permeable to Na⁺ and Cl⁻, so their concentrations are similar in plasma and ISF.
- Cell membranes restrict Na⁺ and Cl⁻ from entering cells, keeping ICF and ECF compositions distinct.
Osmolarity and Water Shifts
- Osmolarity is the concentration of osmotically active particles; normal value is ~300 mOsm/L.
- Water moves between compartments to equalize osmolarity; solutes like Na⁺/Cl⁻ in ECF cause water to shift out of cells if ECF becomes hyperosmolar.
- Adding hypertonic saline or losing water (e.g., fever) raises ECF osmolarity, causing water to leave cells (cell shrinkage).
- Adding pure water or hypotonic fluid lowers ECF osmolarity, causing water to enter cells (cell swelling).
Effects of IV Fluids
- Isotonic saline (0.9% NaCl, 150 mmol/L, 300 mOsm/L): Expands ECF only, no fluid shift between ECF/ICF.
- Hypotonic saline: Lowers ECF osmolarity, causes water to move into cells, expanding ICF and ECF.
- Hypertonic saline: Raises ECF osmolarity, causes water to leave cells, ECF expands, ICF contracts.
Key Terms & Definitions
- Intracellular Fluid (ICF) — water inside cells, mostly K⁺ and proteins.
- Extracellular Fluid (ECF) — water outside cells, includes ISF and plasma.
- Interstitial Fluid (ISF) — ECF surrounding cells, non-circulating.
- Plasma — circulating ECF within blood vessels, rich in proteins.
- Osmolarity — concentration of osmotically active particles per liter of solution.
- Osmolality — concentration of osmotically active particles per kilogram of water.
- Isotonic — solution with osmolarity equal to body fluids (~300 mOsm/L).
- Hypertonic — solution with higher osmolarity than body fluids.
- Hypotonic — solution with lower osmolarity than body fluids.
- Volume Expansion/Contraction — increase/decrease in ECF volume, often refers to ECF unless specified.
Action Items / Next Steps
- Review diagrams of fluid compartments and practice labeling ICF, ECF, ISF, and plasma.
- Memorize normal values for body water distribution and plasma osmolarity.
- Read textbook sections on fluid therapy and effects of IV fluids on compartments.