Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 141: Major cation in ECF

A. Na+ (Correct Answer)
B. Ca2+
C. K+
D. Mg2+

Explanation: ***Na+*** - **Sodium (Na+)** is the primary cation found in the **extracellular fluid (ECF)**, playing a crucial role in maintaining **osmotic pressure**, fluid balance, and **nerve and muscle function**. - Its concentration in the ECF is significantly higher than in the intracellular fluid (ICF), a gradient maintained by the **Na+/K+ ATPase pump**. *Ca2+* - **Calcium (Ca2+)** is an important cation, but its concentration in the ECF is considerably lower than sodium's. - While essential for **bone health**, muscle contraction, and **neurotransmitter release**, it does not represent the major cation of the ECF. *K+* - **Potassium (K+)** is the major cation of the **intracellular fluid (ICF)**, not the ECF. - Its primary role is in maintaining **resting membrane potential** and cellular excitability. *Mg2+* - **Magnesium (Mg2+)** is an important cation involved in many enzymatic reactions and **neuromuscular function**, but its concentration in the ECF is much lower than that of sodium. - It is predominantly found within cells and bone.

Question 142: In intracellular fluid, which of the following has least concentration?

A. Potassium
B. Magnesium
C. Protein
D. Calcium (Correct Answer)

Explanation: ***Calcium*** - The concentration of **free ionized calcium** in the intracellular fluid is kept extremely low (around 0.1 µM) compared to extracellular fluid. - This low intracellular concentration is crucial for its role as a **second messenger** in many cellular processes and is maintained by active transport mechanisms. *Potassium* - **Potassium** is the most abundant intracellular cation, with high concentrations actively maintained inside cells. - It plays a vital role in maintaining **cell volume**, **nerve impulse transmission**, and **muscle contraction**. *Magnesium* - **Magnesium** is also found in relatively high concentrations within the intracellular fluid, second only to potassium among cations. - It is crucial for **enzyme activity**, **ATP metabolism**, and **DNA/RNA synthesis**. *Protein* - **Proteins** are highly concentrated within the intracellular fluid, constituting a large portion of the cell's mass and volume. - They serve diverse functions, including **enzymatic catalysis**, **structural support**, and **transport**, contributing significantly to intracellular osmotic pressure.

Question 143: Which of the following is the most abundant extracellular ion?

A. Potassium
B. Sodium (Correct Answer)
C. Calcium
D. Chloride

Explanation: ***Sodium*** - **Sodium** is the most abundant extracellular ion with a concentration of approximately **140 mEq/L** in the extracellular fluid - It is the primary **extracellular cation** and plays a crucial role in regulating **extracellular fluid volume**, **osmotic pressure**, and **blood pressure** - Essential for **nerve impulse transmission** and **muscle contraction** *Chloride* - **Chloride** is the most abundant extracellular **anion** with a concentration of approximately **103 mEq/L** - While it is the predominant anion, its absolute concentration is lower than sodium - Important for maintaining **acid-base balance** and **osmotic pressure** *Potassium* - **Potassium** is primarily an **intracellular cation** with extracellular concentration of only **4-5 mEq/L** - Although critical for **nerve and muscle function**, it is not abundant in the extracellular space *Calcium* - **Calcium** has a much lower extracellular concentration of approximately **2.5 mEq/L** (or 5 mg/dL) - Important for **bone formation**, **muscle contraction**, and **blood clotting**, but not the most abundant extracellular ion

Question 144: 5 g mannitol was injected intravenously. 40% of mannitol is excreted. After equilibrium, plasma concentration of mannitol is 30 mg%. Calculate extracellular fluid volume.

A. 18 L
B. 14 L
C. 10 L (Correct Answer)
D. 24 L

Explanation: ***10 L*** - The amount of mannitol retained in the body is 5 g - (40% of 5 g) = 5 g - 2 g = **3 g**. - Extracellular fluid volume (ECFV) is calculated by dividing the retained amount of substance by its plasma concentration: ECFV = 3000 mg / 30 mg/dL = **100 dL = 10 L**. *18 L* - This value would result if a different amount of retained mannitol or plasma concentration were used, not aligning with the given problem's parameters. - It implies either a miscalculation of the retained substance or an incorrect conversion during the volume calculation. *14 L* - This answer would imply a different calculation of the retained mannitol, potentially not accounting for the exact percentage excreted. - It is not consistent with the given dose, excretion percentage, and final plasma concentration. *24 L* - This volume is significantly larger than what would be expected, suggesting a substantial overestimation of the retained substance or an underestimation of the plasma concentration. - Such a large volume for extracellular fluid is physiologically improbable given the parameters.

Question 145: The concentration of sodium ion in extracellular fluid is ___.

A. 10 mmol/L
B. 140 mmol/L (Correct Answer)
C. 25 mmol/L
D. 100 mmol/L

Explanation: ***140 mmol/L*** - This value represents the typical and **normal concentration of sodium ions** ([Na+]) in the **extracellular fluid** (ECF). - Sodium is the **primary cation** determining ECF osmolality and volume. *10 mmol/L* - This concentration is significantly **too low** for extracellular fluid sodium and would indicate severe **hyponatremia**, incompatible with normal physiological function. - Such low levels are more characteristic of **intracellular fluid sodium** concentrations, which are actively maintained at low levels by the Na+/K+-ATPase pump. *25 mmol/L* - This value is also considerably **lower than the normal range** for extracellular sodium, suggesting severe hyponatremia. - It does not reflect the physiological concentration required for maintaining crucial bodily functions like nerve impulse transmission and fluid balance. *100 mmol/L* - While closer to the normal range than 10 or 25 mmol/L, this value is still below the typical physiological concentration of sodium in the ECF. - It would indicate **moderate hyponatremia**, which can have significant clinical consequences.

Question 146: Organs involved in calcium homeostasis are all except

A. Intestines
B. Skin
C. Lungs (Correct Answer)
D. Kidneys

Explanation: ***Lungs*** - The **lungs** are primarily involved in gas exchange (oxygen and carbon dioxide) and do not play a direct role in the regulation of **calcium homeostasis**. - While other organs contribute to calcium balance through absorption, excretion, or hormone production, the lungs' physiological functions are unrelated to calcium metabolism. *Intestines* - The **intestines**, particularly the small intestine, are crucial for the **absorption of dietary calcium** under the influence of **active vitamin D**. - Without proper intestinal absorption, calcium levels in the body cannot be maintained. *Skin* - The **skin** is essential for the endogenous synthesis of **vitamin D3 (cholecalciferol)** when exposed to ultraviolet B (UVB) radiation. - This **vitamin D3** is then metabolized into active forms that regulate calcium and phosphate levels. *Kidneys* - The **kidneys** play a vital role in calcium homeostasis by **reabsorbing calcium** from the filtrate and excreting excess calcium. - They also hydroxylate calcidiol to form the active hormone **calcitriol** (1,25-dihydroxyvitamin D), which significantly influences calcium levels.

Question 147: Sodium content of one liter of isotonic saline is:-

A. 140 mEq
B. 154 mEq (Correct Answer)
C. 70 mEq
D. 40 mEq

Explanation: ***154 mEq*** - Isotonic saline, also known as **0.9% sodium chloride**, contains 0.9 grams of NaCl per 100 mL, or 9 grams per liter. - To convert grams to mEq, we use the formula: mEq = (weight in mg / molecular weight) * valence. Given that **molecular weight of NaCl is approximately 58.5 g/mol** and its valence is 1, a liter contains (9000 mg / 58.5) * 1 = **153.8 mEq**, which is rounded to 154 mEq. *140 mEq* - This value is close to the normal **physiological range of serum sodium** but does not represent the precise sodium content of isotonic saline. - Using 140 mEq/L would indicate a slightly **hypotonic solution** compared to standard 0.9% saline. *70 mEq* - This value signifies a significantly **hypotonic solution**, which would not be considered isotonic in a clinical context. - Infusing a solution with 70 mEq sodium per liter would lead to **dilutional hyponatremia** and fluid shifts into the intracellular space. *40 mEq* - This is an extremely **hypotonic solution**, far from the sodium concentration of isotonic saline. - A solution with only 40 mEq of sodium per liter would cause severe **fluid and electrolyte disturbances**, including rapid intracellular fluid shifts.

Question 148: Interstitial fluid volume can be measured by:

A. Tritium oxide - Sodium thiosulfate
B. Inulin - Serum albumin labelled with radioactive Iodine (Correct Answer)
C. Inulin - Radioactive sodium
D. Aminopyrine - Sucrose

Explanation: ***Inulin - Serum albumin labelled with radioactive Iodine*** - The **interstitial fluid volume** is calculated by subtracting the plasma volume from the extracellular fluid volume. - **Inulin** is used to measure **extracellular fluid volume** because it freely distributes throughout the extracellular space but does not enter cells. - **Serum albumin labeled with radioactive iodine** measures **plasma volume** as it stays primarily within the bloodstream due to its large size. *Tritium oxide - Sodium thiosulfate* - **Tritium oxide** (or D2O) is used to measure **total body water (TBW)**, as it distributes throughout all fluid compartments. - **Sodium thiosulfate** is used to measure **extracellular fluid volume**, similar to inulin. *Inulin - Radioactive sodium* - While **inulin** measures **extracellular fluid volume**, **radioactive sodium** (typically 24Na) also measures extracellular fluid volume but can slightly overestimate it due to slow intracellular penetration. - This combination doesn't directly provide a method for exclusively calculating interstitial fluid by subtraction from plasma volume. *Aminopyrine - Sucrose* - **Aminopyrine** is primarily used to measure the **volume of distribution of specific drugs** or gastric acid secretion, not fluid compartments. - **Sucrose** can be used to measure **extracellular fluid volume** as it does not readily cross cell membranes, similar to inulin, but it's not the primary combination for measuring interstitial fluid from the given options.

Question 149: The effect seen due to decreased serum calcium concentration is

A. Depression of nervous system
B. Excitability of muscle (Correct Answer)
C. Increased renal absorption
D. Relaxation of muscle

Explanation: ***Excitability of the muscle*** - A decrease in serum calcium concentration (**hypocalcemia**) reduces the threshold potential for sodium channels, making nerve and muscle cells **more excitable**. - This increased excitability can lead to symptoms like **tetany**, muscle spasms, and even convulsions. *Depression of Nervous system* - This is typically seen with **hypercalcemia** (increased serum calcium), where elevated calcium levels stabilize nerve membranes, making them less excitable. - **Hypocalcemia**, conversely, leads to neuronal hyperexcitability, not depression. *Increase the renal absorption* - Renal calcium reabsorption is primarily regulated by **parathyroid hormone (PTH)**. Low serum calcium stimulates PTH release, which *increases* renal calcium reabsorption to restore calcium levels. - This is a *physiological response* to hypocalcemia, not an *effect* of hypocalcemia on neural or muscular function. *Relaxation of muscle* - Muscle relaxation requires ATP and the re-sequestration of calcium into the sarcoplasmic reticulum, and is not a direct consequence of low extracellular calcium. - Instead, **hypocalcemia** causes increased muscle **contraction** and spasms due to enhanced neuromuscular excitability.

Question 150: 5 percent dextrose is

A. Hypertonic
B. Hypotonic (Correct Answer)
C. Isotonic
D. Normotonic

Explanation: **Hypotonic** - 5% dextrose in water (D5W) is initially **isotonic in the bag**, but once administered intravenously, the **dextrose is rapidly metabolized** by the body's cells. - This leaves behind free water, which acts as a hypotonic solution, causing water to shift from the extracellular space into the cells. *Hypertonic* - A hypertonic solution has a **higher concentration of solutes** than the body's fluids, causing water to move out of the cells. - D5W's effect after metabolism is the opposite, leading to a hypotonic state. *Isotonic* - An isotonic solution has a solute concentration similar to that of the body's fluids, causing no net water movement into or out of cells. - While D5W is isotonic in the bag, its physiological effect after glucose metabolism is **not isotonic**. *Normotonic* - Normotonic is another term for isotonic, meaning it has a normal or equivalent tonicity compared to body fluids. - As explained, D5W acts as a **hypotonic solution** in the body once the dextrose is utilized.

Practice by Chapter

Body Fluid Compartments and Composition

Practice Questions

Osmolality and Tonicity

Practice Questions

Sodium and Water Balance

Practice Questions

Potassium Homeostasis

Practice Questions

Calcium and Phosphate Regulation

Practice Questions

Magnesium Metabolism

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Fluid Shifts Between Compartments

Practice Questions

Edema Formation Mechanisms

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Dehydration Physiology

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Disorders of Electrolyte Balance

Practice Questions

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