Body Fluid Compartments & Ion Balance Quiz base video -3

The body contains several major fluid compartments that together maintain internal balance. These compartments differ in size, contents, and how they exchange substances. Understanding them is essential for interpreting clinical conditions such as dehydration, edema, electrolyte imbalance, and blood loss. 1. Plasma and Interstitial Fluid Plasma and interstitial fluid make up the extracellular fluid (ECF). They have almost identical ionic composition because capillary membranes are highly permeable to most ions. The key difference is: Plasma has more proteins, mainly albumin. Capillaries restrict protein movement, so proteins remain mostly in plasma. Donnan Effect Plasma proteins carry negative charges, which affects the distribution of ions: They attract cations (like sodium and potassium), so cation levels in plasma are slightly higher than in interstitial fluid. They repel anions (like chloride), giving interstitial fluid slightly higher anion levels. These differences are small, so clinically the ionic composition of plasma and interstitial fluid is considered nearly the same. 2. Composition of Extracellular Fluid ECF contains: High: Sodium, chloride, bicarbonate Low: Potassium, magnesium, phosphate, organic ions The kidneys regulate these ions to keep the environment stable for cells. 3. Intracellular Fluid (ICF) The fluid inside cells differs significantly from ECF because the cell membrane: Is highly permeable to water Is not permeable to many ions ICF composition: High: Potassium, phosphate, magnesium, sulfate, proteins Low: Sodium, chloride, calcium Cells contain about four times more protein than plasma. 4. Measuring Fluid Compartments — Indicator Dilution Principle The volume of a body fluid compartment can be measured by injecting a substance (indicator) that: Mixes evenly throughout the compartment Stays only in that compartment Is not metabolized or excreted After mixing, a sample is taken, and the final concentration is used to calculate the volume. Basic formula: Volume = (Amount injected) / (Final concentration) Example: If 10 mg dye is injected and final concentration is 0.01 mg/mL → volume = 1000 mL 5. Measuring Specific Fluid Volumes Different substances measure different compartments: Total Body Water: tritium-labeled water, deuterium-labeled water, antipyrine Extracellular Fluid: radioactive sodium, chloride, iothalamate, thiosulfate, inulin Plasma Volume: 125I-albumin or Evans blue dye (binds plasma proteins) 6. Calculated Volumes Some compartments cannot be measured directly and must be computed: Intracellular Fluid = Total Body Water – Extracellular Fluid Interstitial Fluid = Extracellular Fluid – Plasma Blood Volume = Plasma Volume ÷ (1 – Hematocrit) For example: Plasma = 3 L, Hematocrit = 0.40 → Blood volume = 5 L 7. Blood Volume Measurement Blood can also be measured by labeling a patient’s red blood cells with radioactive chromium and tracking dilution after reinjection.