Overview
This lecture covers the distribution of body fluids into compartments, the movement of water between them, and the key electrolytes involved in body fluid regulation.
Body Fluid Compartments
- The human body consists of solids (40–50% of body weight) and water (50–60% of body weight, varying by sex).
- Total body water in an adult male is about 40 liters, divided into intracellular fluid (ICF) and extracellular fluid (ECF).
- ICF is two-thirds of total body water (≈25 liters), located inside cells.
- ECF is one-third of total body water (≈15 liters), and splits into plasma (20% of ECF) and interstitial fluid (80%).
- Plasma is fluid within blood vessels; interstitial fluid is found between cells.
Water Movement and Osmosis
- Water intake occurs mainly via the gastrointestinal (GI) tract (drinks and food), roughly 2,500 mL/day.
- Water first enters ECF, then redistributes between ICF and ECF passively by osmosis.
- Water movement is passive and depends on osmotic gradients; no energy (ATP) is required.
- Osmosis causes water to move to areas of higher solute concentration across semi-permeable membranes.
- Fluid loss occurs through the kidneys, GI tract, lungs, and skin (≈2,500 mL/day).
- The lymphatic system returns excess interstitial fluid to plasma.
Electrolytes and Their Distribution
- Major ECF electrolytes: sodium (Na⁺), potassium (K⁺), calcium (Ca²⁺), and chloride (Cl⁻).
- Typical plasma concentrations: Na⁺ 145 mmol/L, K⁺ 4 mmol/L, Ca²⁺ 2.5 mmol/L, Cl⁻ 115 mmol/L.
- Electrolyte concentrations are similar in plasma and interstitial fluid.
- Electrolyte exchange between ECF and ICF requires transport proteins (e.g., sodium-potassium pump using ATP).
- Major ICF concentrations: Na⁺ 12 mmol/L, K⁺ 155 mmol/L, Ca²⁺ <0.5 mmol/L, Cl⁻ 4 mmol/L.
- Sodium is the main ECF cation; potassium is the main ICF cation.
- Chloride is the main ECF anion; phosphate is the main ICF anion.
- Proteins (esp. albumin) are present in plasma but not in interstitial fluid and help maintain osmotic gradients.
- Bicarbonate (HCO₃⁻) is important for acid-base balance.
Clinical Importance and Regulation
- Severe disturbances in electrolyte levels (especially K⁺ and Ca²⁺) can cause cardiac arrhythmias and be fatal.
- Homeostatic mechanisms regulate changes in ECF, not ICF.
Key Terms & Definitions
- Intracellular fluid (ICF) — Fluid inside cells, holding two-thirds of body water.
- Extracellular fluid (ECF) — Fluid outside cells, including plasma and interstitial fluid.
- Osmosis — Passive movement of water across a semi-permeable membrane toward higher solute concentration.
- Cation — Positively charged ion (e.g., Na⁺, K⁺, Ca²⁺).
- Anion — Negatively charged ion (e.g., Cl⁻, phosphate, bicarbonate).
- Sodium-potassium pump — Cellular transporter that exchanges Na⁺ out of cells for K⁺ into cells using ATP.
- Albumin — The main plasma protein, maintains osmotic pressure in blood vessels.
- Lymphatic system — Returns excess interstitial fluid to the blood.
Action Items / Next Steps
- Review normal electrolyte values and their distribution in body compartments.
- Understand mechanisms of fluid movement and the role of osmosis.
- Prepare for questions on compartmentalization and electrolyte imbalances.