Electrolytes and Body Fluids — MCQs

Q: At what age is the extracellular fluid equal to the intracellular fluid?

1 year. ***1 year*** - At **birth**, the **extracellular fluid (ECF)** is significantly larger than the **intracellular fluid (ICF)**, approximately **45-50% versus 30-35%** of total body weight. - During the **first year of life**, ECF volume **decreases** while ICF volume **increases** until they reach **equilibrium at approximately 1 year of age**, with each constituting about **30-35%** of total body weight. - This represents a critical developmental transition in body fluid compartment maturation. *2 months* - At two months of age, the **ECF** volume is still considerably **larger** than the ICF volume. - The transition towards equal fluid compartments is incomplete at this early stage. *3 months* - At three months, the **ECF** volume remains **higher** than the ICF volume. - The process of fluid compartment redistribution is ongoing but has not yet reached equilibrium. *14 days* - In the **neonatal period** (14 days), the **ECF** volume is at its **highest** relative to the ICF volume. - The infant's body contains a disproportionately large amount of ECF compared to ICF at this stage.

Q: Which extracellular fluid is characterized by high potassium and low sodium levels?

Endolymph (inner ear fluid). ***Endolymph (inner ear fluid)*** - **Endolymph** is unique among extracellular fluids, featuring a very **high potassium concentration** and a **low sodium concentration**, resembling intracellular fluid. - This ionic composition is crucial for the function of hair cells in the **cochlea** and **vestibular system**, enabling sound transduction and balance. *Synovial fluid* - **Synovial fluid** generally has electrolyte concentrations similar to **plasma**, with higher sodium and lower potassium. - Its primary role is **lubrication** and nutrition of articular cartilage, not electrochemical signaling based on potassium gradients. *Perilymph* - **Perilymph** is found in the **scala vestibuli** and **scala tympani** of the cochlea, surrounding the endolymphatic duct. - Its ionic composition is similar to **extracellular fluid** (plasma), characterized by **high sodium** and **low potassium**. *CSF* - **Cerebrospinal fluid (CSF)** has electrolyte concentrations that are generally similar to **plasma**, though slightly different due to the **blood-brain barrier**. - It maintains **homeostasis** of the brain and spinal cord, with **high sodium** and **low potassium** relative to the intracellular environment.

Q: Prolonged immobilization leads to?

Hypercalcemia due to increased bone resorption. ***Hypercalcemia due to increased bone resorption*** - Prolonged **immobilization** reduces mechanical stress on bones, leading to decreased osteoblast activity and increased **osteoclast activity**. - This imbalance results in a net increase in **bone resorption**, releasing calcium into the bloodstream and causing **hypercalcemia**. - This is a well-established complication of prolonged bed rest and is clinically significant in bedridden patients. *Hypocalcemia due to altered calcium metabolism* - **Hypocalcemia** is typically associated with conditions like **hypoparathyroidism**, vitamin D deficiency, or malabsorption. - Immobilization primarily affects **bone turnover** in a way that *increases* serum calcium, rather than decreasing it. - The reduced mechanical loading leads to bone loss, not calcium retention in bones. *Hypokalemia* - **Hypokalemia** (low potassium) is usually caused by conditions like **diuretic use**, gastrointestinal losses, or certain endocrine disorders. - While immobilization can have various systemic effects, it does not directly lead to **hypokalemia**. - Potassium metabolism is distinct from the bone resorption process triggered by immobilization. *Hyperkalemia* - **Hyperkalemia** (high potassium) is often linked to **renal failure**, certain medications (e.g., ACE inhibitors, potassium-sparing diuretics), or massive tissue breakdown. - Immobilization does not typically cause **hyperkalemia** through any direct mechanism. - The primary electrolyte disturbance in immobilization is related to calcium, not potassium.

Practice by Chapter

Body Fluid Compartments and Composition

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Osmolality and Tonicity

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Sodium and Water Balance

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Potassium Homeostasis

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Calcium and Phosphate Regulation

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Magnesium Metabolism

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Fluid Shifts Between Compartments

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Edema Formation Mechanisms

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Dehydration Physiology

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Disorders of Electrolyte Balance

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