Electrolytes and Body Fluids Practice Questions

Q: What is the most osmotically active intracellular cation?

K+. **Explanation:** The correct answer is **K+ (Potassium)**. Osmotic activity is determined by the concentration of particles in a specific compartment. In the intracellular fluid (ICF), Potassium is the predominant cation, with a concentration of approximately 140–150 mEq/L. Because it is the most abundant solute within the cell, it exerts the greatest osmotic pressure, effectively determining the intracellular volume. **Analysis of Options:** * **Na+ (Sodium):** This is the most osmotically active **extracellular** cation. While it is the primary determinant of ECF volume and plasma osmolarity, its intracellular concentration is very low (approx. 10–14 mEq/L). * **Mg+2 (Magnesium):** Magnesium is the second most abundant intracellular cation. While physiologically vital as a cofactor for enzymatic reactions, its concentration (approx. 20–30 mEq/L) is significantly lower than that of Potassium. * **Protein:** Proteins are the most abundant intracellular **anions** (along with organic phosphates). While they contribute to the oncotic pressure, they are not cations. **High-Yield Clinical Pearls for NEET-PG:** * **Na+-K+ ATPase Pump:** This active transporter maintains the high intracellular K+ and high extracellular Na+ gradient, consuming roughly 30% of a cell's ATP. * **Gibbs-Donnan Effect:** This describes the behavior of charged particles near a semi-permeable membrane when one ion (like protein) is non-diffusible; it influences the distribution of other mobile ions. * **Osmolarity vs. Osmolality:** In clinical practice, these are often used interchangeably, but remember that normal plasma osmolality is **280–295 mOsm/kg H2O**.

Q: Hypertonic dehydration would result from which of the following conditions?

Diabetes mellitus. **Explanation:** **Hypertonic dehydration** occurs when water loss exceeds solute loss, leading to an increase in extracellular fluid (ECF) osmolarity. This creates an osmotic gradient that draws water out of the cells, causing intracellular dehydration. **Why Diabetes Mellitus is correct:** In uncontrolled Diabetes Mellitus, hyperglycemia leads to **osmotic diuresis**. Glucose acts as an osmotically active particle in the renal tubules, dragging out a disproportionately large volume of free water compared to electrolytes. This results in a net loss of hypotonic fluid, leaving the remaining body fluids hypertonic. **Analysis of Incorrect Options:** * **Gastrointestinal fluid loss (Option A):** Most GI secretions (vomiting/diarrhea) are roughly **isotonic**. Loss of these fluids typically leads to isotonic dehydration (decreased ECF volume with normal osmolarity). * **Primary hypoadrenocorticism (Addison’s Disease) (Option C):** Deficiency of aldosterone leads to excessive sodium loss in urine. This results in **hypotonic dehydration**, where salt loss exceeds water loss, leading to decreased ECF osmolarity. * **SIADH (Option D):** This condition involves excessive water retention due to high ADH levels, leading to **hypotonic overhydration** (hyponatremia), not dehydration. **High-Yield NEET-PG Pearls:** * **Diabetes Insipidus:** Another classic cause of hypertonic dehydration (pure water loss). * **Clinical Sign:** In hypertonic dehydration, the **thirst mechanism** is intensely activated due to cellular dehydration in the hypothalamus. * **Fluid of Choice:** Treatment usually involves hypotonic fluids (e.g., 0.45% Saline or 5% Dextrose) to restore osmolarity gradually. * **Formula:** Plasma Osmolarity $\approx 2[Na^+] + \frac{Glucose}{18} + \frac{BUN}{2.8}$. In DM, the high glucose significantly raises osmolarity.

Q: What is the approximate quantity of water lost in expired air each 24 hours?

350 ml. **Explanation:** The correct answer is **350 ml**. **1. Why 350 ml is correct:** Water loss through the respiratory tract is a component of **insensible water loss** (loss that occurs without being consciously perceived). As atmospheric air is inhaled, it is warmed to body temperature (37°C) and completely saturated with water vapor (100% relative humidity) by the mucous membranes of the respiratory passages. This humidification process requires water to evaporate from the respiratory tract lining. Under normal resting conditions in a temperate climate, an average adult loses approximately **300 to 400 ml** of water per day through expired air. **2. Why other options are incorrect:** * **200 ml (Option A):** This value is too low for standard respiratory loss. However, it is approximately the amount of water lost daily in **feces**. * **600 ml (Option C):** This value is more representative of the water lost through **insensible perspiration via the skin** (diffusion through the stratum corneum, distinct from active sweating). * **800 ml (Option D):** This is too high for daily respiratory loss under normal conditions. Total insensible loss (Skin + Lungs) is roughly 700–900 ml/day. **High-Yield Clinical Pearls for NEET-PG:** * **Effect of Climate:** In cold, dry weather, the water vapor pressure of the atmosphere decreases to nearly zero. This increases the gradient for evaporation, leading to higher water loss from the lungs (often causing the "dry throat" sensation in winter). * **Tachypnea:** Patients with increased respiratory rates (e.g., fever, respiratory distress) will have significantly higher water loss through expired air. * **Total Daily Water Output:** Usually totals ~2500 ml (Urine: 1500 ml; Skin: 600 ml; Lungs: 350 ml; Feces: 100 ml; Sweat: 100 ml).

Q: A 32-year-old man visits for a periodic health examination. He has no complaints. He is 170 cm tall and weighs 75 kg. What are the approximate volumes of total body water, intracellular fluid, and extracellular fluid in this patient, respectively?

45L, 30L, 15L. To solve this question, we apply the **60-40-20 Rule**, which is the gold standard for calculating body fluid compartments in a healthy adult male. ### 1. Calculation Logic (The "Why") Total Body Water (TBW) is approximately **60% of total body weight**. * **TBW:** 60% of 75 kg = $0.6 \times 75 = \mathbf{45L}$. TBW is further divided into Intracellular Fluid (ICF) and Extracellular Fluid (ECF): * **ICF:** 2/3 of TBW (or 40% of body weight) = $2/3 \times 45 = \mathbf{30L}$. * **ECF:** 1/3 of TBW (or 20% of body weight) = $1/3 \times 45 = \mathbf{15L}$. This matches **Option B** (45L, 30L, 15L). ### 2. Analysis of Incorrect Options * **Option A (40L, 30L, 10L):** Incorrectly assumes TBW is ~53% and uses an incorrect 3:1 ratio for ICF:ECF. * **Option C (45L, 35L, 10L):** While TBW is correct, the distribution is wrong. ICF and ECF must follow the 2:1 ratio. * **Option D (50L, 25L, 25L):** Overestimates TBW (66%) and incorrectly assumes ICF and ECF are equal in volume. ### 3. NEET-PG High-Yield Pearls * **Gender Variations:** In adult females, TBW is lower (**~50%**) due to a higher proportion of subcutaneous fat (fat is hydrophobic and contains little water). * **Age Variations:** Newborns have the highest TBW (**~75-80%**), which decreases with age. * **ECF Sub-compartments:** ECF is composed of Interstitial Fluid (3/4 of ECF) and Plasma (1/4 of ECF). * **Indicator Dilution Method:** Remember the markers used to measure these volumes: * **TBW:** Deuterium oxide ($D_2O$), Tritiated water, or Antipyrine. * **ECF:** Inulin (Gold Standard), Mannitol, or Thiosulfate. * **Plasma:** Evans Blue dye or Radio-iodinated albumin. * **ICF:** Calculated indirectly (TBW minus ECF).

Q: Potassium in which compartment is responsible for cardiac and neural function?

Intracellular. **Explanation:** The correct answer is **A. Intracellular**. **Why Intracellular?** Potassium ($K^+$) is the primary intracellular cation, with approximately 98% of the body's total potassium stored inside cells (at a concentration of ~140-150 mEq/L). The **Resting Membrane Potential (RMP)** of excitable tissues, such as cardiac myocytes and neurons, is primarily determined by the ratio of intracellular to extracellular potassium. Because the cell membrane is highly permeable to $K^+$ at rest, the high intracellular concentration allows $K^+$ to leak out, creating the negative electrical charge necessary for cellular excitability. Without this massive intracellular reservoir, the electrochemical gradient required for action potentials in the heart and nerves would collapse. **Why the other options are incorrect:** * **B. Extracellular:** While extracellular $K^+$ levels (3.5–5.0 mEq/L) are clinically monitored and can trigger arrhythmias if abnormal, it is the **intracellular** pool that establishes the gradient and provides the bulk of the ion responsible for maintaining the RMP. * **C & D. Intravascular and Extravascular:** These are sub-compartments of the Extracellular Fluid (ECF). While they are important for transport and homeostasis, they do not represent the primary site where potassium exerts its fundamental physiological role in membrane excitability. **High-Yield Clinical Pearls for NEET-PG:** * **Nernst Equation:** Used to calculate the equilibrium potential of $K^+$, which is roughly -90mV (close to the RMP of -70 to -90mV). * **Insulin & Beta-2 Agonists:** These shift $K^+$ from the ECF into the **Intracellular** compartment by stimulating the $Na^+/K^+$ ATPase pump (used in the emergency management of hyperkalemia). * **Hypokalemia on ECG:** Look for U-waves, T-wave flattening, and ST depression. * **Hyperkalemia on ECG:** Look for Tall tented T-waves, widened QRS, and loss of P-waves.

Q: 0.9% sodium chloride solution contains how many mmol/L of sodium?

154. **Explanation:** The concentration of sodium in 0.9% Sodium Chloride (Normal Saline) is derived from basic chemical calculations. A 0.9% solution means there are **0.9 grams of NaCl in 100 mL** of water, which equates to **9 grams per Liter (1000 mL)**. To find the millimoles (mmol): 1. **Molecular Weight of NaCl:** Sodium (23) + Chloride (35.5) = 58.5 g/mol. 2. **Calculation:** 9 grams / 58.5 (molecular weight) = 0.1538 mol/L. 3. **Conversion:** 0.1538 mol/L × 1000 = **154 mmol/L**. Since NaCl dissociates completely into Na⁺ and Cl⁻, the solution contains 154 mmol/L of Sodium and 154 mmol/L of Chloride, resulting in an osmolarity of 308 mOsm/L. **Analysis of Incorrect Options:** * **B (145):** This represents the upper limit of the normal physiological range for serum sodium (135–145 mEq/L). Normal saline is actually slightly hypertonic compared to plasma. * **C (131):** This is the sodium concentration found in **Ringer’s Lactate (RL)**, which is more physiological than Normal Saline. * **D (130):** This is the sodium concentration found in **Hartmann's Solution** (a variation of RL). **Clinical Pearls for NEET-PG:** * **Isotonicity:** While 0.9% NaCl is called "Normal" saline, its chloride content (154 mmol/L) is significantly higher than plasma chloride (96–106 mmol/L). * **Complication:** Large volume resuscitation with 0.9% NaCl can lead to **Hyperchloremic Metabolic Acidosis** due to the high chloride load. * **Fluid of Choice:** It is the preferred fluid for patients with **hyponatremia, hypovolemic shock, and metabolic alkalosis** (due to its chloride-rich nature).

Q: Which of the following would NOT be likely to produce hypernatremia?

Hyperglycemia. ### Explanation **Hypernatremia** is defined as a serum sodium concentration >145 mEq/L. It occurs when there is either a net gain of sodium or, more commonly, a net loss of free water. **Why Hyperglycemia is the Correct Answer:** Hyperglycemia typically causes **hyponatremia** (specifically, hypertonic hyponatremia). Glucose is osmotically active; as blood glucose levels rise, water is drawn out of the intracellular compartment into the extracellular fluid (ECF) to maintain osmotic balance. This shift of water dilutes the existing sodium in the ECF, lowering its concentration. * **High-Yield Rule:** For every 100 mg/dL increase in blood glucose above normal, the serum sodium concentration decreases by approximately **1.6 mEq/L**. **Analysis of Incorrect Options:** * **Near drowning in salt water:** This involves the aspiration and ingestion of highly concentrated saline, leading to a direct gain of sodium in the ECF, causing hypernatremia. * **Diabetes Insipidus (DI):** Whether central or nephrogenic, DI is characterized by a deficiency or resistance to ADH. This leads to the excretion of large volumes of dilute urine (free water loss), which concentrates the serum sodium. * **Watery diarrhea:** Gastrointestinal fluids are usually hypotonic (containing more water than electrolytes). Loss of hypotonic fluid results in a disproportionate loss of water compared to sodium, leading to hypernatremia. **Clinical Pearls for NEET-PG:** * **Pseudohyponatremia:** Seen in severe hyperlipidemia or hyperproteinemia (the sodium concentration is actually normal, but the lab measurement is skewed). * **Correction Rate:** In chronic hypernatremia, avoid lowering sodium faster than **0.5 mEq/L per hour** (or 10-12 mEq/L per day) to prevent **Cerebral Edema**. * **Drug of Choice:** Desmopressin (dDAVP) for Central DI; Thiazides for Nephrogenic DI.

Q: What is the normal plasma osmolality in an adult?

280-290 mOsm/L. **Explanation:** The normal plasma osmolality in a healthy adult is tightly regulated between **280–295 mOsm/kg H₂O** (often simplified to **280–290 mOsm/L** in clinical exams). Osmolality represents the concentration of particles dissolved in a fluid. In plasma, this is primarily determined by sodium ($Na^+$), chloride ($Cl^-$), bicarbonate ($HCO_3^-$), glucose, and urea. **Why Option C is correct:** The body maintains this narrow range to ensure cellular integrity. The hypothalamus senses even a 1% change in osmolality via osmoreceptors, triggering thirst or the release of Antidiuretic Hormone (ADH) from the posterior pituitary to restore balance. **Analysis of Incorrect Options:** * **Option A (320–330 mOsm/L):** This represents severe hyperosmolality. It is seen in conditions like severe dehydration, Diabetes Insipidus, or Hyperosmolar Hyperglycemic State (HHS). * **Option B (300–310 mOsm/L):** This is considered high-normal to mildly elevated. While not always symptomatic, it is outside the physiological baseline. * **Option D (260–270 mOsm/L):** This represents hypoosmolality, typically caused by hyponatremia or water intoxication (SIADH), which can lead to cerebral edema. **High-Yield Clinical Pearls for NEET-PG:** 1. **Calculated Osmolality Formula:** $2[Na^+] + \frac{\text{Glucose}}{18} + \frac{BUN}{2.8}$. Sodium is the most significant contributor (the "determinant") of plasma osmolality. 2. **Osmolar Gap:** The difference between measured and calculated osmolality. A gap **>10 mOsm/L** suggests the presence of unmeasured toxins like ethanol, methanol, or ethylene glycol. 3. **Effective Osmolality (Tonicity):** Urea is an "ineffective osmole" because it crosses cell membranes freely; therefore, tonicity is primarily $2[Na^+] + \frac{\text{Glucose}}{18}$.

Q: 5 percent dextrose solution is considered:

Isotonic. **Explanation:** The tonicity of a solution is determined by its effective osmolality relative to plasma (normal range: 275–295 mOsm/L). **Why Isotonic is Correct:** A 5% Dextrose (D5W) solution contains 50 grams of glucose per liter. Its calculated osmolarity is approximately **252–278 mOsm/L**, which is nearly identical to the osmolarity of human plasma. Therefore, **in the IV bag (in vitro)**, D5W is considered an **isotonic** solution. **Analysis of Incorrect Options:** * **A. Hypotonic:** While D5W is isotonic in the bag, it behaves as a hypotonic solution **in vivo** (inside the body). Once infused, the dextrose is rapidly metabolized by insulin, leaving behind "free water." However, by standard classification based on initial concentration, it is categorized as isotonic. * **C. Normotonic:** This is not a standard physiological term used to describe IV fluids; "isotonic" is the correct terminology. * **D. Hypertonic:** A solution is hypertonic if its osmolarity is significantly higher than 300 mOsm/L (e.g., 10% Dextrose or 3% Normal Saline). D5W does not meet this threshold. **High-Yield Clinical Pearls for NEET-PG:** 1. **The "Physiological Paradox":** Remember that D5W is **Isotonic in the bottle but Hypotonic in the body.** 2. **Contraindication:** Because it becomes hypotonic in vivo, D5W should **never** be used for fluid resuscitation in patients with head injuries, as the free water can cross the blood-brain barrier and worsen **cerebral edema**. 3. **Caloric Value:** 1 liter of D5W provides 170 kcal (50g x 3.4 kcal/g). 4. **Drug of Choice:** It is the preferred fluid for treating **Hypernatremia** (to replace a pure water deficit).

Question 1

What is the most osmotically active intracellular cation?

Accepted Answer

K+

Answer

Na+

Answer

Mg+2

Answer

Protein

Question 2

Hypertonic dehydration would result from which of the following conditions?

Accepted Answer

Diabetes mellitus

Answer

Gastrointestinal fluid loss

Answer

Primary hypoadrenocorticism

Answer

Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH)

Question 3

What is the approximate quantity of water lost in expired air each 24 hours?

Accepted Answer

350 ml

Answer

200 ml

Answer

600 ml

Answer

800 ml

Question 4

A 32-year-old man visits for a periodic health examination. He has no complaints. He is 170 cm tall and weighs 75 kg. What are the approximate volumes of total body water, intracellular fluid, and extracellular fluid in this patient, respectively?

Accepted Answer

45L, 30L, 15L

Answer

40L, 30L, 10L

Answer

45L, 35L, 10L

Answer

50L, 25L, 25L

Question 5

Potassium in which compartment is responsible for cardiac and neural function?

Accepted Answer

Intracellular

Answer

Extracellular

Answer

Intravascular

Answer

Extravascular

Question 6

0.9% sodium chloride solution contains how many mmol/L of sodium?

Accepted Answer

154

Answer

145

Answer

131

Answer

130

Question 7

Which of the following would NOT be likely to produce hypernatremia?

Accepted Answer

Hyperglycemia

Answer

Near drowning in salt water

Answer

Diabetes Insipidus

Answer

Watery diarrhea

Question 8

What is the difference between plasma and interstitial fluid regarding their protein and ion content?

Accepted Answer

Interstitial fluid contains less protein and the same ions as plasma.

Answer

Interstitial fluid contains more protein and the same ions as plasma.

Answer

Interstitial fluid contains the same protein and fewer ions than plasma.

Answer

Interstitial fluid contains less protein and fewer ions than plasma.

Question 9

What is the normal plasma osmolality in an adult?

Accepted Answer

280-290 mOsm/L

Answer

320-330 mOsm/L

Answer

300-310 mOsm/L

Answer

260-270 mOsm/L

Question 10

5 percent dextrose solution is considered:

Accepted Answer

Isotonic

Answer

Hypotonic

Answer

Normotonic

Answer

Hypertonic

Electrolytes and Body Fluids — MCQs

Electrolytes and Body Fluids — MCQs

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