Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids Indian Medical PG Practice Questions and MCQs

Question 111: What is the approximate percentage of free or ionized calcium in the body?

A. 10%
B. 25%
C. 50% (Correct Answer)
D. 75%

Explanation: ### Explanation **Correct Answer: C. 50%** In the human body, plasma calcium exists in three distinct fractions. Understanding the distribution of these fractions is crucial for clinical practice: 1. **Ionized (Free) Calcium (~50%):** This is the physiologically active form. It is responsible for vital functions such as neuromuscular excitability, cardiac contractility, and blood coagulation. It is the only fraction regulated by parathyroid hormone (PTH). 2. **Protein-bound Calcium (~40%):** Most of this is bound to albumin. This fraction acts as a reservoir but is physiologically inactive and cannot cross capillary membranes. 3. **Complexed Calcium (~10%):** This is calcium bound to small diffusible anions like citrate, phosphate, and bicarbonate. **Analysis of Incorrect Options:** * **A (10%):** This represents the **complexed fraction** of calcium, not the free ionized form. * **B (25%):** This value does not correspond to any major physiological calcium fraction in plasma. * **D (75%):** This is an overestimation; nearly half of plasma calcium is always "sequestered" by proteins or anions. **NEET-PG High-Yield Pearls:** * **pH and Calcium:** Alkalosis (e.g., hyperventilation) increases calcium binding to albumin, **decreasing** ionized calcium levels. This can trigger tetany even if total serum calcium is normal. * **Corrected Calcium Formula:** Since 40% is protein-bound, if a patient has hypoalbuminemia, the "total calcium" will appear low. Use the formula: *Corrected Ca = Measured Ca + [0.8 × (4.0 - Albumin)]*. * **Storage:** Remember that **99%** of total body calcium is stored in the bone (hydroxyapatite); the percentages discussed above refer only to the **1%** found in extracellular fluid.

Question 112: Which of the following is a true statement regarding extracellular fluid (ECF) and intracellular fluid (ICF)?

A. ECF is rich in potassium (K+).
B. ECF volume is twice that of the ICF.
C. ECF is rich in organic anions.
D. ECF has high sodium and low potassium concentration. (Correct Answer)

Explanation: **Explanation:** The distribution of electrolytes between fluid compartments is maintained by the **Na+-K+ ATPase pump**, which actively pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell. This creates a distinct chemical gradient: the **Extracellular Fluid (ECF)** is characterized by high concentrations of **Sodium (Na+)**, Chloride (Cl-), and Bicarbonate (HCO3-), while the **Intracellular Fluid (ICF)** is rich in **Potassium (K+)**, Magnesium (Mg2+), and organic phosphates. **Analysis of Options:** * **Option D (Correct):** As per the physiological gradient, ECF sodium is high (~142 mEq/L) and potassium is low (~4 mEq/L). * **Option A:** Incorrect. Potassium is the primary **intracellular** cation (~140 mEq/L). * **Option B:** Incorrect. In a standard 70kg adult, Total Body Water (TBW) is 60% of body weight. Of this, **ICF makes up 2/3 (40%)** and **ECF makes up 1/3 (20%)**. Thus, ICF volume is actually twice that of ECF. * **Option C:** Incorrect. Organic anions (like proteins and organic phosphates) are found in much higher concentrations inside the cell (ICF) compared to the ECF. **High-Yield NEET-PG Pearls:** 1. **Indicator Dilution Method:** Used to measure compartments. * **TBW:** Tritiated water (D2O). * **ECF:** Inulin, Mannitol, or Sucrose. * **Plasma:** Evans Blue dye or Radio-iodinated albumin. 2. **ICF Volume** cannot be measured directly; it is calculated as **TBW minus ECF**. 3. **Interstitial Fluid (ISF)** is calculated as **ECF minus Plasma volume**.

Question 113: Intracellular and interstitial body fluids have similar:

A. Total osmotic pressure (Correct Answer)
B. Colloid osmotic pressure
C. Chloride ion concentrations
D. Potassium ion concentrations

Explanation: **Explanation:** **1. Why Total Osmotic Pressure is Correct:** The cell membrane is highly permeable to water but selectively permeable to solutes. According to the principle of **osmotic equilibrium**, water moves freely between the intracellular fluid (ICF) and the interstitial fluid (ISF) until the concentration of osmotically active particles is equal on both sides. Therefore, the **total osmotic pressure** (approximately 280–300 mOsm/L) is essentially the same in all fluid compartments. If a difference in osmolarity occurs, water shifts rapidly to restore equality. **2. Why Other Options are Incorrect:** * **Colloid Osmotic Pressure (COP):** This is exerted by proteins (mainly albumin). The ICF has a high protein concentration, whereas the ISF has very low protein levels. Thus, COP is significantly higher in the ICF. * **Chloride Ion Concentrations:** Chloride is the primary extracellular anion. Its concentration is high in the ISF (~108 mEq/L) but very low in the ICF (~4–10 mEq/L). * **Potassium Ion Concentrations:** Potassium is the major intracellular cation. Its concentration is high in the ICF (~140 mEq/L) and low in the ISF (~4 mEq/L), maintained by the Na⁺-K⁺ ATPase pump. **High-Yield NEET-PG Pearls:** * **Major Cations:** ICF = Potassium ($K^+$); ECF = Sodium ($Na^+$). * **Major Anions:** ICF = Phosphates and Proteins; ECF = Chloride ($Cl^-$) and Bicarbonate ($HCO_3^-$). * **Gibbs-Donnan Effect:** Explains why plasma has a slightly higher osmotic pressure than interstitial fluid due to non-diffusible plasma proteins. * **Osmolarity vs. Osmolality:** In clinical practice, these are used interchangeably, but osmolality (mOsm/kg) is more accurate as it is independent of temperature.

Question 114: Which of the following statements about body water is true?

A. Water constitutes 60% of body weight. (Correct Answer)
B. Plasma volume constitutes 10% of the body water.
C. Extracellular fluid volume can be determined by dilution methods.
D. 10% of body water is intracellular water.

Explanation: ### Explanation **1. Why Option A is Correct:** In an average healthy adult male, total body water (TBW) constitutes approximately **60% of the total body weight**. This value is slightly lower in females (approx. 50%) due to a higher proportion of subcutaneous fat, as adipose tissue contains very little water compared to muscle. **2. Analysis of Incorrect Options:** * **Option B:** Plasma volume is a sub-component of the Extracellular Fluid (ECF). It constitutes only about **5% of the total body weight** or roughly **8% of the total body water**. * **Option C:** This statement is technically a general principle, but in the context of this specific question, Option A is the fundamental physiological constant. However, to be precise, ECF is measured using substances that distribute only in the extracellular space (e.g., Inulin, Mannitol, or Sodium thiosulfate). * **Option D:** Intracellular Fluid (ICF) is the largest fluid compartment, accounting for **40% of body weight** or **two-thirds (approx. 67%)** of total body water, not 10%. **3. NEET-PG High-Yield Pearls:** * **The 60-40-20 Rule:** 60% of body weight is Total Body Water; 40% is Intracellular Fluid (ICF); 20% is Extracellular Fluid (ECF). * **ECF Breakdown:** Of the 20% ECF, 15% is Interstitial fluid and 5% is Plasma. * **Measurement Markers (Indicator Dilution Method):** * **Total Body Water:** Deuterium oxide ($D_2O$), Tritiated water, or Antipyrine. * **ECF Volume:** Inulin (Gold Standard), Mannitol, Sucrose. * **Plasma Volume:** Evans Blue dye (T-1824) or Radio-iodinated Albumin ($I^{125}$). * **Blood Volume:** $Cr^{51}$ labeled RBCs. * **Calculated Values:** Interstitial fluid volume = ECF volume – Plasma volume. ICF volume = TBW – ECF volume.

Question 115: What is metabolic water?

A. Water ingested orally
B. Water infused intravenously
C. Water produced during cellular metabolism (Correct Answer)
D. None of the above

Explanation: **Explanation:** **Metabolic water** (also known as oxidation water) refers to the water molecules generated within a living organism through the oxidation of energy-containing nutrients (carbohydrates, fats, and proteins) during cellular respiration. In the final stage of the electron transport chain, oxygen acts as the final electron acceptor and combines with hydrogen ions to form water ($C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6\mathbf{H_2O} + \text{Energy}$). On average, a sedentary adult produces approximately **250–350 ml** of metabolic water per day, contributing to about 10% of the total daily water intake. **Analysis of Options:** * **Option A (Water ingested orally):** This is referred to as "preformed water" or exogenous intake. It accounts for the majority of daily water gain (~2100 ml/day). * **Option B (Water infused intravenously):** This is an artificial exogenous source used in clinical settings for fluid resuscitation or maintenance; it is not biological metabolic water. * **Option D:** Incorrect, as Option C is the standard physiological definition. **High-Yield Facts for NEET-PG:** * **Yield per Nutrient:** Fat produces the most metabolic water per gram (**1.07 ml/g**), followed by carbohydrates (0.60 ml/g) and proteins (0.41 ml/g). * **Clinical Significance:** In patients with **oliguric renal failure**, metabolic water must be accounted for when calculating "Insensible Water Loss" to prevent fluid overload. * **Survival Physiology:** Desert animals (like the Kangaroo rat) rely almost exclusively on metabolic water to survive without drinking.

Question 116: Where is the majority of the body's sodium present?

A. Extracellular fluid
B. Intracellular fluid
C. Plasma
D. Bone (Correct Answer)

Explanation: **Explanation:** The distribution of sodium in the body is a high-yield concept often misunderstood due to the focus on fluid compartments. While sodium is the primary cation of the extracellular fluid, the **majority of total body sodium (~40-45%) is stored in the skeleton (bone).** 1. **Why Bone is Correct:** Approximately 40-45% of total body sodium is located in the bone matrix. It exists in two forms: a **non-exchangeable pool** (embedded in hydroxyapatite crystals) and an **exchangeable pool** on the crystal surface. This exchangeable pool acts as a reservoir that can be mobilized during periods of severe hyponatremia or metabolic acidosis. 2. **Analysis of Incorrect Options:** * **Extracellular Fluid (ECF):** While sodium is the *dominant* cation of the ECF (concentration ~140 mEq/L), the ECF as a whole contains only about 50% of total body sodium. Bone contains a higher total percentage. * **Intracellular Fluid (ICF):** Sodium concentration is very low inside cells (~10-14 mEq/L) due to the action of the Na⁺-K⁺ ATPase pump. Only about 5-10% of body sodium is intracellular. * **Plasma:** Plasma is a sub-compartment of the ECF. It contains only about 11-12% of the total body sodium. **Clinical Pearls for NEET-PG:** * **Total Body Sodium:** Roughly 50% is in ECF, 40-45% in Bone, and 5-10% in ICF. * **Exchangeable Sodium:** Only about 70% of total body sodium is "exchangeable." Most of the sodium in the bone matrix is non-exchangeable. * **Osmolality:** Sodium and its associated anions (Cl⁻ and HCO₃⁻) are the primary determinants of ECF osmolality and volume.

Question 117: Calculate the osmolarity of normal saline.

A. 333 mOsmol / L
B. 300 mOsmol / L (Correct Answer)
C. 280 mOsmol / L
D. 320 mOsmol / L

Explanation: **Explanation:** **1. Why Option B is Correct:** Normal Saline (NS) is a **0.9% w/v solution of Sodium Chloride (NaCl)**. This means there are 0.9 grams of NaCl in 100 mL, or **9 grams in 1 Liter**. To calculate osmolarity: * **Molar Mass of NaCl:** ~58.5 g/mol. * **Molarity:** 9 g / 58.5 ≈ 0.154 mol/L (or 154 mmol/L). * **Dissociation:** NaCl dissociates into two particles (Na⁺ and Cl⁻). * **Calculation:** 154 mmol/L × 2 = **308 mOsmol/L**. In clinical practice and standard examinations like NEET-PG, this is rounded to **300 mOsmol/L**, making it nearly isotonic with human plasma (approx. 285–295 mOsmol/L). **2. Why Other Options are Incorrect:** * **Option A (333 mOsmol/L):** This value does not correspond to standard crystalloids; it is too high for isotonic saline. * **Option C (280 mOsmol/L):** While this is close to the actual osmolarity of plasma, NS is slightly hyperosmolar compared to blood. * **Option D (320 mOsmol/L):** This is an overestimation. While 308 is the theoretical value, 300 is the standard accepted answer in most physiological contexts. **Clinical Pearls for NEET-PG:** * **Composition:** NS contains 154 mEq/L of Na⁺ and 154 mEq/L of Cl⁻. * **High-Yield Risk:** Large volumes of NS can lead to **Hyperchloremic Metabolic Acidosis** due to the high chloride content. * **Comparison:** Ringer’s Lactate (RL) is more "physiological" with an osmolarity of ~273 mOsmol/L. * **Free Water:** NS provides 0 mL of free water; it stays entirely in the extracellular compartment, making it the fluid of choice for **hypovolemic shock**.

Question 118: Total body water constitutes what percentage of body weight?

A. 80%
B. 60% (Correct Answer)
C. 33%
D. 25%

Explanation: **Explanation:** The correct answer is **60% (Option B)**. In a standard 70 kg adult male, Total Body Water (TBW) constitutes approximately 60% of the total body weight (roughly 42 liters). This percentage is a fundamental physiological constant used to calculate fluid distribution across compartments. **Breakdown of Options:** * **Option A (80%):** This is incorrect for adults but relevant for **neonates**, whose TBW is significantly higher (approx. 75–80%) due to lower fat content. * **Option C (33%):** This represents the fraction of TBW that resides in the **Extracellular Fluid (ECF)** compartment (1/3 of TBW), not the total body weight. * **Option D (25%):** This does not correspond to a primary body fluid compartment percentage relative to total weight. **High-Yield NEET-PG Pearls:** 1. **The 60-40-20 Rule:** TBW is 60% of body weight; Intracellular Fluid (ICF) is 40%; Extracellular Fluid (ECF) is 20%. 2. **Gender Variation:** Women have a lower TBW (approx. 50%) because they generally have a higher proportion of adipose tissue. 3. **Adipose Tissue Effect:** Fat is hydrophobic and contains very little water. Therefore, obese individuals have a lower percentage of TBW compared to lean individuals. 4. **Measurement:** TBW is measured using the **indicator dilution method** with substances like **Deuterium oxide ($D_2O$)**, Tritiated water, or Antipyrine.

Question 119: What is the normal range for serum osmolality?

A. 255-270 mosm/L
B. 270-285 mosm/L (Correct Answer)
C. 285-300 mosm/L
D. 300-325 mosm/L

Explanation: **Explanation:** Serum osmolality is a measure of the concentration of solutes (particles) per liter of solution in the blood. It is primarily determined by sodium, glucose, and urea. **1. Why Option B is Correct:** The physiological normal range for serum osmolality is typically **275–295 mOsm/kg H₂O** (often rounded to **270–285 mOsm/L** in clinical textbooks). This range is tightly regulated by the hypothalamus-pituitary axis via Antidiuretic Hormone (ADH) and the thirst mechanism to maintain cellular integrity and fluid balance. **2. Why Other Options are Incorrect:** * **Option A (255-270 mOsm/L):** This represents a **hypo-osmolar** state. It is seen in conditions like SIADH (Syndrome of Inappropriate ADH) or water intoxication, leading to cellular edema (brain swelling). * **Option C (285-300 mOsm/L):** While the upper limit (295-300) is borderline, this range generally leans toward mild dehydration. * **Option D (300-325 mOsm/L):** This represents a **hyper-osmolar** state. It is characteristic of severe dehydration, Diabetes Insipidus, or Hyperglycemic Hyperosmolar State (HHS). **Clinical Pearls for NEET-PG:** * **Calculated Osmolality Formula:** $2[Na^+] + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}$. Sodium is the most significant contributor (the "major extracellular cation"). * **Osmolar Gap:** The difference between measured and calculated osmolality. A gap $>10$ suggests the presence of unmeasured toxins like ethanol, methanol, or ethylene glycol. * **ADH Trigger:** A mere **1-2% change** in osmolality is sufficient to trigger ADH release from the posterior pituitary.

Question 120: Which of the following statements concerning intracellular fluid is true?

A. Statement 2 only
B. Statements 1 and 2
C. Statements 1 and 3 (Correct Answer)
D. Statements 1, 2, and 3

Explanation: To understand the distribution of body fluids, one must recall the composition of the **Intracellular Fluid (ICF)** versus the **Extracellular Fluid (ECF)**. ### **Explanation of Statements** * **Statement 1 (True):** The ICF constitutes approximately **40% of the total body weight** (or 2/3rd of the Total Body Water). In a standard 70 kg adult, this equals roughly 28 liters. * **Statement 2 (False):** Sodium ($Na^+$) is the primary cation of the **Extracellular Fluid**. In contrast, the primary cation of the **Intracellular Fluid** is **Potassium ($K^+$)**. The concentration of $Na^+$ inside the cell is low (approx. 10-14 mEq/L) due to the continuous activity of the $Na^+$-$K^+$ ATPase pump. * **Statement 3 (True):** The ICF contains significantly higher concentrations of **proteins** and **phosphates** compared to the ECF. Proteins are essential for cellular structure and enzymatic functions, while phosphates serve as vital components of ATP and buffer systems. ### **Why Option C is Correct** Since Statements 1 and 3 are physiologically accurate and Statement 2 is incorrect, **Option C** is the only valid choice. ### **High-Yield Facts for NEET-PG** * **Total Body Water (TBW):** 60% of body weight (0.6 × weight). * **60-40-20 Rule:** 60% TBW, 40% ICF, 20% ECF. * **Marker for ICF Volume:** There is no direct marker for ICF. It is calculated as **TBW minus ECF volume**. * **Markers for ECF Volume:** Inulin (Gold Standard), Mannitol, or Thiosulfate. * **Osmolality:** Despite different ionic compositions, the osmolality of ICF and ECF is always in **equilibrium** (approx. 285–295 mOsm/kg).

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

Practice Questions

Fluid Shifts Between Compartments

Practice Questions

Edema Formation Mechanisms

Practice Questions

Dehydration Physiology

Practice Questions

Disorders of Electrolyte Balance

Practice Questions

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