Anatomy Quiz: Fluid Compartments And Osmolarity
20 questions · exam conditions
0:00
Fluid Compartments And OsmolarityQuestion 1 of 20

A person drinks 1 liter of distilled water. After absorption, what is the most likely sequence of events regarding osmolarity and fluid distribution?

Plasma osmolarity increases, water moves from ICF to ECF, cell volume decreases
Plasma osmolarity decreases, water moves from ECF to ICF, cell volume increases
Plasma osmolarity remains constant, no net water movement occurs between compartments
Plasma osmolarity decreases, water moves from ICF to ECF, cell volume decreases
Plasma osmolarity increases, water moves from ECF to ICF, cell volume increases
← Back to quizzes

Anatomy Quiz

Anatomy Quiz: Fluid Compartments And Osmolarity

Practice Fluid Compartments And Osmolarity in Anatomy with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Fluid Compartments And Osmolarity, giving you a quick way to practice the rules, question types, and explanations that matter most for Anatomy.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A person drinks 1 liter of distilled water. After absorption, what is the most likely sequence of events regarding osmolarity and fluid distribution?

  1. Plasma osmolarity increases, water moves from ICF to ECF, cell volume decreases
  2. Plasma osmolarity decreases, water moves from ECF to ICF, cell volume increases (correct answer)
  3. Plasma osmolarity remains constant, no net water movement occurs between compartments
  4. Plasma osmolarity decreases, water moves from ICF to ECF, cell volume decreases
  5. Plasma osmolarity increases, water moves from ECF to ICF, cell volume increases
Explanation: When you encounter questions about fluid and electrolyte balance, focus on how water moves to equalize osmolarity between body compartments. Distilled water is hypotonic (contains no solutes), so it will dilute whatever it mixes with. When you drink 1 liter of distilled water, it's absorbed into the bloodstream, immediately diluting the plasma and decreasing its osmolarity. This creates an osmotic gradient between the extracellular fluid (ECF) and intracellular fluid (ICF). Since water always moves from areas of lower osmolarity to higher osmolarity to achieve equilibrium, water will flow from the now-diluted ECF into the cells. As water enters the cells, they swell and increase in volume. Answer B correctly describes this sequence: plasma osmolarity decreases, water moves from ECF to ICF, and cell volume increases. Answer A incorrectly suggests plasma osmolarity increases - but adding pure water can only dilute, never concentrate the plasma. Answer C wrongly assumes no change occurs, ignoring the fundamental principle that hypotonic solutions always create osmotic gradients. Answer D makes the critical error of suggesting water moves from ICF to ECF despite acknowledging that plasma osmolarity decreases - this contradicts basic osmotic principles since water should move toward the higher concentration. Remember this key pattern: hypotonic solutions (like distilled water) cause cells to swell, while hypertonic solutions cause cells to shrink. The direction of water movement always follows the osmotic gradient, moving toward the compartment with higher solute concentration.

Question 2

A laboratory analysis shows that a patient's plasma osmolarity is 310 mOsm/L (normal: 280-295 mOsm/L). Which compensatory mechanism would be most appropriate to restore normal fluid balance?

  1. Increased aldosterone secretion to retain more sodium in the kidneys
  2. Increased ADH secretion to retain more water in the kidneys (correct answer)
  3. Decreased aldosterone secretion to excrete more sodium in the kidneys
  4. Decreased ADH secretion to excrete more water in the kidneys
  5. Increased insulin secretion to promote cellular glucose uptake
Explanation: When you encounter questions about fluid balance and osmolarity, focus on the relationship between plasma concentration and the body's regulatory responses. Osmolarity measures the concentration of dissolved particles in plasma - when it's elevated, the blood is too concentrated and needs dilution. With a plasma osmolarity of 310 mOsm/L (above the normal range of 280-295), the patient's blood is hyperconcentrated. The body needs to reduce this concentration by retaining water, not by manipulating sodium levels. ADH (antidiuretic hormone) is the primary hormone that regulates water balance by making the kidney's collecting ducts more permeable to water, allowing more water reabsorption and less water loss in urine. Increased ADH secretion would retain water, diluting the plasma and lowering osmolarity back to normal levels, making option B correct. Option A is wrong because increasing aldosterone would retain more sodium, which would actually worsen the high osmolarity by adding more particles to the blood. Option C incorrectly suggests decreasing aldosterone - while this might reduce sodium retention slightly, it doesn't address the core problem of needing more water retention. Option D represents the opposite of what's needed; decreasing ADH would cause more water loss through urine, further concentrating the plasma and worsening the elevated osmolarity. Remember this key principle: high osmolarity = concentrated blood = need more water retention via ADH. When plasma osmolarity rises, think ADH increase; when it falls, think ADH decrease. This hormone specifically targets water balance, while aldosterone primarily affects sodium and potassium balance.

Question 3

During severe dehydration, a patient loses primarily water with minimal electrolyte loss. What changes would be expected in the major fluid compartments?

  1. ICF volume decreases more than ECF volume, cells shrink significantly (correct answer)
  2. ECF volume decreases more than ICF volume, cells swell slightly
  3. Both ICF and ECF volumes decrease equally, cell size unchanged
  4. ECF volume increases while ICF volume decreases, cells shrink
  5. ICF volume increases while ECF volume decreases, cells swell
Explanation: When you encounter questions about fluid balance, focus on osmotic gradients and where water moves to maintain equilibrium between compartments. During severe dehydration with primarily water loss, the body loses hypotonic fluid (water with minimal electrolytes). This creates a relative increase in solute concentration in the remaining extracellular fluid (ECF), making it hypertonic compared to the intracellular fluid (ICF). Water always moves across cell membranes to equalize osmotic pressure. Since the ECF becomes more concentrated, water moves out of cells into the ECF to restore balance. This movement causes cells to shrink significantly as they lose water. While both fluid compartments lose volume overall due to dehydration, the ICF loses proportionally more volume because water shifts from the ICF to the ECF. Option B incorrectly suggests ECF volume decreases more and cells swell - this would happen if electrolytes were lost instead of water, making ECF hypotonic. Option C assumes equal volume loss with unchanged cell size, which ignores the osmotic gradient created by losing hypotonic fluid. Option D incorrectly states ECF volume increases - while ECF gains water from ICF, the overall ECF volume still decreases due to the initial fluid loss. Remember this pattern: losing water creates hypertonic ECF → water leaves cells → cells shrink. Losing electrolytes creates hypotonic ECF → water enters cells → cells swell. Always consider what type of fluid is lost and the resulting osmotic changes between compartments.

Question 4

An athlete loses 3 kg of body weight during exercise, primarily through sweating. Sweat contains approximately 50 mEq/L sodium compared to 140 mEq/L in plasma. What effect would this have on plasma osmolarity?

  1. Plasma osmolarity would decrease because more water than sodium is lost
  2. Plasma osmolarity would increase because sweat is hypotonic relative to plasma (correct answer)
  3. Plasma osmolarity would remain unchanged because equal proportions of water and sodium are lost
  4. Plasma osmolarity would decrease because sodium loss exceeds water loss
  5. Plasma osmolarity would increase because sodium is concentrated in the remaining plasma
Explanation: When you encounter questions about fluid loss and osmolarity, focus on comparing the concentration of solutes in what's being lost versus what remains in the body. Let's analyze this step by step. The athlete loses 3 kg (essentially 3 liters) of sweat containing 50 mEq/L sodium, while plasma contains 140 mEq/L sodium. Since sweat has a lower sodium concentration than plasma, it's hypotonic relative to plasma. When you lose hypotonic fluid, you're losing proportionally more water than sodium compared to what's in your blood. Here's what happens: as the hypotonic sweat is lost, the remaining plasma becomes more concentrated because relatively more water than sodium has been removed. This increases plasma osmolarity. Answer B correctly identifies that plasma osmolarity increases because sweat is hypotonic relative to plasma. Answer A incorrectly suggests osmolarity decreases. While more water than sodium is indeed lost, this actually concentrates the remaining plasma, increasing osmolarity. Answer C is wrong because the proportions aren't equal - sweat is hypotonic, so water and sodium aren't lost in the same ratio as found in plasma. Answer D makes a factual error by claiming sodium loss exceeds water loss, which contradicts the hypotonic nature of sweat. Remember this pattern: when the body loses hypotonic fluid (lower solute concentration than plasma), the remaining plasma becomes more concentrated and osmolarity increases. When hypertonic fluid is lost, osmolarity decreases. Always compare the lost fluid's concentration to plasma concentration.

Question 5

A patient receives an IV infusion of 5% dextrose in water (D5W). Once the glucose is metabolized, what happens to the remaining water?

  1. Remains in the vascular space, increasing blood volume permanently
  2. Distributes throughout total body water, with most entering the ICF (correct answer)
  3. Distributes only within the ECF, maintaining osmotic balance
  4. Is immediately excreted by the kidneys to prevent fluid overload
  5. Remains in the interstitial space, causing localized tissue swelling
Explanation: When you encounter questions about IV fluid distribution, think about what happens after the solute is metabolized and how the remaining water behaves based on osmotic principles. D5W initially acts as an isotonic solution because of the glucose content. However, once cells metabolize the glucose, you're left with pure water that becomes hypotonic relative to body fluids. This free water doesn't stay in the vascular space—it distributes according to the normal water distribution in the body. Since about 60% of total body water is intracellular fluid (ICF) and 40% is extracellular fluid (ECF), most of this water will move into cells down the osmotic gradient. This makes option B correct. Option A is wrong because water doesn't permanently increase blood volume—it redistributes throughout all body compartments, not just the vascular space. Option C incorrectly suggests the water stays only in the ECF; this would only happen if the solution remained isotonic, but the metabolized glucose leaves behind hypotonic water that crosses cell membranes. Option D assumes immediate renal excretion, but healthy kidneys don't instantly excrete free water—the water first distributes throughout body compartments according to osmotic gradients. For anatomy and physiology exams, remember that free water (hypotonic solutions) always distributes proportionally throughout total body water, with the majority ending up intracellular. This principle applies whether the free water comes from metabolized D5W, excessive water intake, or other hypotonic sources.

Question 6

A patient with chronic kidney disease has difficulty concentrating urine. If this patient becomes moderately dehydrated, what would be the expected relationship between urine osmolarity and plasma osmolarity?

  1. Urine osmolarity would be much higher than plasma osmolarity (>1000 mOsm/L)
  2. Urine osmolarity would be slightly higher than plasma osmolarity (350-400 mOsm/L) (correct answer)
  3. Urine osmolarity would equal plasma osmolarity (isosthenuric)
  4. Urine osmolarity would be lower than plasma osmolarity (hypotonic urine)
  5. Urine osmolarity would fluctuate unpredictably relative to plasma osmolarity
Explanation: When you encounter questions about kidney disease and urine concentration, focus on understanding how chronic kidney disease progressively impairs the kidney's ability to concentrate urine, but doesn't eliminate it entirely in moderate cases. In chronic kidney disease, the kidneys lose their ability to maximally concentrate urine due to damage to the nephrons, particularly the loop of Henle and collecting duct system. However, some concentrating ability typically remains until very advanced stages. When a patient with moderate chronic kidney disease becomes dehydrated, their kidneys will still respond to antidiuretic hormone (ADH) and attempt to conserve water, but they cannot achieve the normal maximum urine concentration of 1200-1400 mOsm/L that healthy kidneys produce. Answer B is correct because the damaged kidneys can still concentrate urine somewhat above plasma osmolarity (normally ~290 mOsm/L), reaching concentrations of 350-400 mOsm/L, but cannot achieve maximal concentration. Answer A represents normal kidney function during dehydration - patients with chronic kidney disease cannot concentrate urine this effectively. Answer C describes isosthenuria, where urine osmolarity equals plasma osmolarity, which occurs in severe, end-stage kidney disease when concentrating ability is completely lost. Answer D describes hypotonic urine, which would indicate the kidneys are actually diluting urine despite dehydration - this doesn't occur in chronic kidney disease. Remember: chronic kidney disease causes a progressive loss of concentrating ability. In moderate disease, some concentration occurs but is significantly impaired compared to normal function.

Question 7

A patient receives an intravenous infusion of 0.9% NaCl (isotonic saline). Assuming the solution distributes only within the extracellular fluid compartment, what is the primary effect on fluid compartment volumes?

  1. Intracellular fluid volume increases, extracellular fluid volume decreases
  2. Both intracellular and extracellular fluid volumes increase proportionally
  3. Extracellular fluid volume increases, intracellular fluid volume remains unchanged (correct answer)
  4. Intracellular fluid volume decreases, extracellular fluid volume remains unchanged
  5. Both intracellular and extracellular fluid volumes decrease proportionally
Explanation: When you encounter questions about IV fluid administration, focus on the tonicity of the solution and how it affects osmotic balance between fluid compartments. Normal saline (0.9% NaCl) is isotonic, meaning it has the same osmolarity as body fluids. This is crucial because it prevents water movement across cell membranes. When isotonic saline enters the bloodstream, it stays within the extracellular compartment (blood plasma and interstitial fluid) because there's no osmotic gradient driving water into or out of cells. Since the question states the solution distributes only within the extracellular space, the added volume directly increases extracellular fluid volume. Because the solution is isotonic, cells neither gain nor lose water, so intracellular fluid volume remains unchanged. This makes option C correct. Option A incorrectly suggests intracellular volume increases while extracellular decreases - this would require a hypotonic solution that draws water into cells. Option B assumes water enters both compartments proportionally, but isotonic solutions don't cross cell membranes to enter the intracellular space. Option D suggests intracellular volume decreases, which would only occur with a hypertonic solution that pulls water out of cells. Remember this key pattern: isotonic solutions expand only the extracellular compartment, hypotonic solutions cause cellular swelling, and hypertonic solutions cause cellular shrinkage. On anatomy and physiology exams, always identify the solution's tonicity first - it determines the direction of water movement and which compartments are affected.

Question 8

A laboratory reports the following values for a dehydrated patient: plasma sodium 150 mEq/L (normal 135-145), plasma osmolarity 320 mOsm/L (normal 280-295). What type of dehydration is indicated?

  1. Isotonic dehydration with proportional water and sodium loss
  2. Hypotonic dehydration with greater sodium loss than water loss
  3. Hypertonic dehydration with greater water loss than sodium loss (correct answer)
  4. Hypervolemic dehydration with excess sodium retention
  5. Isosmotic dehydration with normal electrolyte concentrations
Explanation: When analyzing fluid and electrolyte imbalances, you need to examine both sodium levels and plasma osmolarity together to understand what type of dehydration has occurred. The relationship between water and sodium loss determines the classification. Looking at these lab values, the plasma sodium is elevated at 150 mEq/L (above the normal 135-145 range) and osmolarity is high at 320 mOsm/L (above normal 280-295). This pattern indicates that more water has been lost relative to sodium, concentrating the remaining sodium in the blood. This defines hypertonic dehydration, making C correct. Option A is wrong because isotonic dehydration would show normal sodium levels (135-145 mEq/L) and normal or slightly elevated osmolarity, since water and sodium are lost proportionally. Option B describes hypotonic dehydration, which would present with low sodium levels (below 135 mEq/L) and low osmolarity because more sodium than water is lost. Option D is incorrect because hypervolemic dehydration isn't a recognized classification of dehydration - dehydration by definition involves fluid volume loss, not excess. Remember this pattern: in dehydration, look at sodium levels to determine the type. High sodium = hypertonic (more water lost), normal sodium = isotonic (proportional loss), low sodium = hypotonic (more sodium lost). The osmolarity will follow the same pattern as sodium concentration, providing confirmation of your assessment.

Question 9

A patient loses 2 liters of isotonic fluid through hemorrhage. What is the most accurate description of the resulting changes in fluid compartments?

  1. Only ECF volume decreases; ICF volume and osmolarity remain unchanged (correct answer)
  2. Both ECF and ICF volumes decrease proportionally; osmolarity increases in both
  3. ECF volume decreases initially, then ICF volume decreases as water redistributes
  4. ICF volume decreases more than ECF volume; cells shrink significantly
  5. Both ECF and ICF volumes decrease equally; osmolarity remains constant
Explanation: When you encounter questions about fluid loss, focus on the type of fluid lost and how it affects osmotic balance between compartments. The key distinction is whether the lost fluid is isotonic, hypotonic, or hypertonic. Hemorrhage involves losing whole blood, which is isotonic to body fluids. Since isotonic fluid has the same osmolarity as both extracellular fluid (ECF) and intracellular fluid (ICF), its loss doesn't create an osmotic gradient between compartments. Without an osmotic imbalance, there's no driving force for water to move across cell membranes. Therefore, only the ECF volume decreases directly from the blood loss, while ICF volume remains stable. The osmolarity in both compartments stays unchanged because you're losing solutes and water in the same proportion they existed. Option A correctly describes this scenario. Option B is wrong because it suggests both compartments lose volume and osmolarity increases - this would occur with hypotonic fluid loss, not isotonic. Option C incorrectly implies a two-phase redistribution process that doesn't happen with isotonic losses. Option D describes what would happen with hypertonic fluid loss, where the remaining ECF becomes hypotonic relative to ICF, causing cells to swell rather than shrink. Study tip: Remember the isotonic rule - when isotonic fluid is lost or gained, only the ECF changes volume while ICF remains stable. This applies to hemorrhage, isotonic saline administration, and similar clinical scenarios. Focus on whether osmotic gradients are created to predict water movement between compartments.

Question 10

A solution contains 150 mM NaCl and 100 mM glucose. What is the total osmolarity of this solution, and how would it affect red blood cells?

  1. 250 mOsm/L; cells would swell because the solution is hypotonic
  2. 400 mOsm/L; cells would shrink because the solution is hypertonic (correct answer)
  3. 300 mOsm/L; cells would remain unchanged because the solution is isotonic
  4. 350 mOsm/L; cells would shrink moderately because the solution is hypertonic
  5. 250 mOsm/L; cells would shrink because the solution is hypertonic
Explanation: When calculating osmolarity, you need to consider how many particles each solute produces when dissolved. This determines the solution's effect on cell membranes through osmosis. NaCl dissociates completely into Na⁺ and Cl⁻ ions, so 150 mM NaCl produces 300 mOsm/L (150 × 2). Glucose doesn't dissociate, so 100 mM glucose contributes 100 mOsm/L directly. The total osmolarity is 300 + 100 = 400 mOsm/L. Since normal blood plasma is approximately 300 mOsm/L, this 400 mOsm/L solution is hypertonic (higher solute concentration than inside red blood cells). Water will move out of the cells toward the higher concentration, causing the cells to shrink through crenation. Choice A incorrectly calculates osmolarity as 250 mOsm/L, failing to account for NaCl's dissociation, and wrongly predicts cell swelling. Choice C gives 300 mOsm/L by ignoring glucose entirely—a common mistake of only considering the electrolyte. This leads to the incorrect conclusion that cells remain unchanged. Choice D calculates 350 mOsm/L, which suggests partial dissociation of NaCl (perhaps counting it as 2.33 particles instead of 2), leading to an intermediate but incorrect value. Study tip: Always remember that ionic compounds like NaCl multiply by their number of dissociated particles when calculating osmolarity, while non-electrolytes like glucose contribute 1:1. Normal plasma osmolarity (~300 mOsm/L) is your reference point for determining if solutions are hypo-, iso-, or hypertonic.

Question 11

A patient with diabetes insipidus produces large volumes of dilute urine. Based on the table shown, what would be the expected change in plasma osmolarity over 6 hours without treatment?

  1. Decrease from 290 to approximately 270 mOsm/L due to excessive water retention
  2. Increase from 290 to approximately 315 mOsm/L due to excessive water loss (correct answer)
  3. Remain constant at 290 mOsm/L due to compensatory mechanisms
  4. Decrease from 290 to approximately 250 mOsm/L due to dilute urine production
  5. Increase from 290 to approximately 330 mOsm/L due to sodium retention
Explanation: Diabetes insipidus involves ADH deficiency or resistance, leading to inability to concentrate urine. Large volumes of dilute urine cause significant water loss while retaining solutes, concentrating the plasma and increasing osmolarity to around 315 mOsm/L. A is incorrect because water is lost, not retained. C is incorrect because compensation is impaired without functional ADH. D shows too extreme a decrease and wrong direction. E overestimates the increase and incorrectly attributes it to sodium retention.

Question 12

A patient presents with severe dehydration after prolonged vomiting and diarrhea. Laboratory analysis shows plasma osmolarity of 320 mOsm/L (normal: 280-295 mOsm/L) and elevated hematocrit. Which sequence of fluid shifts would most likely occur as the body attempts to maintain cellular function?

  1. Water moves from intracellular to extracellular compartment, then ADH release increases water reabsorption in kidneys (correct answer)
  2. Water moves from extracellular to intracellular compartment, then aldosterone release increases sodium retention
  3. Sodium moves from intracellular to extracellular compartment, then renin-angiotensin system activates
  4. Potassium moves from extracellular to intracellular compartment, then atrial natriuretic peptide is released
Explanation: With elevated plasma osmolarity (320 mOsm/L), the extracellular fluid becomes hypertonic relative to intracellular fluid. Water will move from the intracellular compartment to the extracellular compartment to equilibrate osmotic gradients, causing cellular dehydration. The hypothalamus detects this increased osmolarity and releases ADH, which increases water reabsorption in the collecting duct to help restore normal osmolarity. Choice B is incorrect because water would move out of, not into, cells when ECF osmolarity is high. Choice C focuses on sodium movement rather than the primary water shifts, and choice D describes inappropriate ion movement and hormone response for this scenario.

Question 13

A laboratory experiment measures osmolarity in different fluid compartments of a healthy subject before and after intravenous administration of 0.9% NaCl (isotonic saline). Which outcome would be expected 30 minutes post-infusion?

  1. Intracellular fluid volume increases with no change in intracellular osmolarity
  2. Extracellular fluid volume increases with no change in extracellular osmolarity (correct answer)
  3. Both intracellular and extracellular fluid volumes increase proportionally with decreased total body osmolarity
  4. Extracellular fluid volume decreases due to rapid kidney filtration with increased extracellular osmolarity
Explanation: Isotonic saline (0.9% NaCl) has the same osmolarity as normal body fluids (~280-295 mOsm/L). When administered intravenously, it expands the extracellular fluid compartment without creating an osmotic gradient across cell membranes. Therefore, no water movement occurs between intracellular and extracellular compartments, intracellular volume remains unchanged, and extracellular osmolarity remains the same while extracellular volume increases. Choice A is incorrect because intracellular volume doesn't change with isotonic solutions. Choice C is wrong because only ECF volume increases, not both compartments. Choice D is incorrect because isotonic saline doesn't increase osmolarity and the kidneys don't immediately filter out isotonic fluid.

Question 14

During surgery, a patient receives an accidental infusion of 3% NaCl (hypertonic saline). Blood samples taken 15 minutes later would most likely show which pattern of changes compared to pre-infusion values?

  1. Decreased plasma sodium concentration, increased red blood cell volume, decreased plasma osmolarity
  2. Increased plasma sodium concentration, decreased red blood cell volume, increased plasma osmolarity (correct answer)
  3. Unchanged plasma sodium concentration, increased red blood cell volume, unchanged plasma osmolarity
  4. Increased plasma sodium concentration, unchanged red blood cell volume, decreased plasma osmolarity
Explanation: Hypertonic saline (3% NaCl) has much higher osmolarity than normal plasma (~900 mOsm/L vs ~290 mOsm/L). This increases plasma sodium concentration and plasma osmolarity directly. The hypertonic plasma creates an osmotic gradient that pulls water out of red blood cells, causing them to shrink (decreased RBC volume). The high osmolarity also triggers osmoreceptors to release ADH, but the immediate effect is cellular dehydration. Choice A describes the opposite effects. Choice C incorrectly suggests no osmolarity change with hypertonic solution. Choice D is wrong because RBC volume would change due to osmotic water movement, and osmolarity would increase, not decrease.

Question 15

A patient with diabetes insipidus has deficient ADH production. Laboratory monitoring shows plasma osmolarity of 305 mOsm/L and urine osmolarity of 150 mOsm/L. Which statement best describes the fluid compartment status in this condition?

  1. Intracellular compartment is expanded due to excessive water retention, while extracellular osmolarity remains normal
  2. Interstitial fluid increases while plasma volume decreases, creating compartmental osmolarity differences
  3. Extracellular compartment maintains normal volume through aldosterone compensation, while intracellular compartment shrinks
  4. Both fluid compartments are volume-depleted and hypertonic due to excessive free water loss in urine (correct answer)
Explanation: When you encounter questions about diabetes insipidus and fluid compartments, focus on how ADH deficiency disrupts the body's ability to concentrate urine and retain water. In diabetes insipidus, deficient ADH means the kidneys cannot reabsorb water effectively in the collecting ducts. This leads to massive free water loss through dilute urine (note the low urine osmolarity of 150 mOsm/L compared to normal ~300-1200 mOsm/L). The body loses water faster than solutes, causing both extracellular and intracellular compartments to become volume-depleted and hypertonic. The elevated plasma osmolarity (305 mOsm/L vs normal ~285-295 mOsm/L) confirms this hypertonicity, and since water moves freely between compartments, both become concentrated and shrunken. Choice A is wrong because there's no water retention—quite the opposite occurs with excessive water loss. Choice B incorrectly suggests compartmental osmolarity differences, but osmolarity equilibrates across compartments since water moves freely through cell membranes. Choice C mentions aldosterone compensation maintaining normal extracellular volume, but aldosterone primarily affects sodium retention, not free water loss, and cannot compensate for the massive water losses in diabetes insipidus. The correct answer is D because both compartments lose volume due to excessive urinary water loss, and both become hypertonic as water is lost while solutes remain concentrated. Remember: In diabetes insipidus, think "water escapes"—the body loses free water, making everything more concentrated (hypertonic) and volume-depleted. The laboratory values of high plasma osmolarity and low urine osmolarity are classic hallmarks of this condition.

Question 16

During a physiology lab, students measure osmolarity in different solutions and predict cellular responses. A red blood cell is placed in a solution with osmolarity of 250 mOsm/L. After equilibration, which cellular and solution characteristics would be observed?

  1. Cell volume increases, final cell osmolarity equals 290 mOsm/L, solution osmolarity increases toward 290 mOsm/L
  2. Cell volume decreases, final cell osmolarity decreases to 250 mOsm/L, solution osmolarity remains 250 mOsm/L
  3. Cell volume increases, both cell and solution reach equilibrium osmolarity between 250-290 mOsm/L (correct answer)
  4. Cell volume remains unchanged, cell osmolarity stays at 290 mOsm/L, solution osmolarity stays at 250 mOsm/L
Explanation: The red blood cell initially has normal osmolarity (~290 mOsm/L) and is placed in a hypotonic solution (250 mOsm/L). Water will move into the cell down the osmotic gradient, causing cell swelling. As water enters the cell, it dilutes the cell's contents while the solution becomes slightly more concentrated due to water loss. Both compartments reach osmotic equilibrium at an osmolarity between the initial values, closer to 250 mOsm/L since the solution volume is much larger than the cell volume. Choice A incorrectly suggests the final osmolarity approaches the cell's initial value. Choice B is wrong because the cell would swell, not shrink, in hypotonic solution. Choice D incorrectly suggests no equilibration occurs.

Question 17

A patient accidentally drinks 2 liters of distilled water within 30 minutes. Assuming normal kidney function but insufficient time for complete renal compensation, which combination of changes in fluid compartments would most likely occur?

  1. ECF volume increases significantly, ICF volume increases moderately, both compartments become hypotonic
  2. ECF volume decreases, ICF volume increases significantly, ECF becomes hypertonic while ICF becomes hypotonic
  3. ECF volume increases moderately, ICF volume increases significantly, both compartments become hypotonic (correct answer)
  4. ECF volume remains unchanged, ICF volume decreases, ECF becomes hypotonic while ICF becomes hypertonic
Explanation: Drinking 2 liters of distilled water (hypotonic solution) initially enters the extracellular compartment, decreasing ECF osmolarity. This creates an osmotic gradient that drives water into cells, causing significant intracellular swelling. The water distributes between compartments based on osmotic equilibration, with more water ultimately entering the larger intracellular compartment (~2/3 of total body water). Both compartments become hypotonic as the excess water dilutes existing solutes. Choice A incorrectly suggests ECF volume increases more than ICF. Choice B is wrong about the direction of ECF volume change and osmolarity effects. Choice D incorrectly states ECF volume is unchanged and ICF volume decreases, which contradicts basic osmotic principles.

Question 18

A patient with heart failure develops peripheral edema. Laboratory results show normal plasma osmolarity but decreased plasma protein levels. Which mechanism best explains the fluid shift pattern in this scenario?

  1. Decreased plasma colloid osmotic pressure allows excessive filtration from capillaries to interstitial space, expanding interstitial fluid volume (correct answer)
  2. Increased plasma hydrostatic pressure prevents normal reabsorption from interstitial space, causing intracellular fluid expansion
  3. Normal plasma osmolarity maintains equilibrium between all fluid compartments, preventing any significant volume changes
  4. Decreased plasma proteins increase plasma osmolarity locally, drawing water from cells into the vascular compartment
Explanation: In heart failure with decreased plasma proteins, colloid osmotic pressure (oncotic pressure) is reduced. Normally, plasma proteins create an osmotic force that helps retain fluid in capillaries and promotes reabsorption from interstitial space. With decreased proteins, this reabsorptive force is diminished while capillary hydrostatic pressure may be elevated due to heart failure. This shifts the Starling forces toward increased filtration and decreased reabsorption, causing fluid accumulation in interstitial space (edema). Choice B incorrectly focuses on intracellular expansion rather than interstitial. Choice C ignores the protein effects on fluid movement. Choice D wrongly states that decreased proteins increase osmolarity.

Question 19

An athlete loses 3 liters of sweat during intense exercise. The sweat contains primarily water and sodium chloride at concentrations lower than plasma. Immediately after exercise, before any fluid replacement, which fluid compartment changes would be expected?

  1. Both ECF and ICF volumes decrease proportionally, with no change in osmolarity of either compartment
  2. ECF volume increases while ICF volume decreases, with ECF becoming hypotonic and ICF becoming hypertonic
  3. ICF volume decreases more than ECF volume, and both compartments become hypotonic
  4. ECF volume decreases more than ICF volume, and both compartments become slightly hypertonic (correct answer)
Explanation: When you encounter fluid balance questions, focus on two key principles: where the fluid loss originates and how osmolarity changes drive water movement between compartments. Sweat loss primarily comes from the extracellular fluid (ECF), specifically from plasma that's filtered through sweat glands. Since sweat is hypotonic compared to plasma (lower sodium concentration), you're losing more water relative to solutes. This creates two effects: direct volume loss from ECF and increased osmolarity in remaining body fluids. The correct answer is D because ECF loses volume directly through sweating, while the resulting hypertonicity causes some water to shift from ICF to ECF, but this doesn't fully compensate for the ECF loss. Both compartments become slightly hypertonic due to the net loss of hypotonic fluid. Option A is wrong because sweat loss isn't proportional between compartments—ECF is directly affected first, and osmolarity must change since hypotonic fluid is lost. Option B incorrectly suggests ECF volume increases and describes opposite osmolarity changes; ECF can't gain volume when you're losing fluid externally. Option C correctly identifies that both become hypotonic, but this contradicts the loss of hypotonic sweat, which should concentrate remaining fluids, and incorrectly suggests ICF volume drops more than ECF. Remember this pattern: when the body loses hypotonic fluid (sweat, respiratory losses), think "concentration effect"—remaining fluids become more concentrated (hypertonic), and the compartment losing fluid directly (usually ECF) is affected most severely.

Question 20

Examine the diagram showing fluid shifts between compartments. If a patient develops heart failure with fluid retention, which pattern of compartment changes would be most likely?

  1. Increased plasma volume, decreased interstitial volume, normal ICF volume
  2. Decreased plasma volume, increased interstitial volume, decreased ICF volume
  3. Increased plasma volume, increased interstitial volume, normal ICF volume (correct answer)
  4. Normal plasma volume, decreased interstitial volume, increased ICF volume
  5. Decreased plasma volume, normal interstitial volume, increased ICF volume
Explanation: Heart failure causes sodium and water retention, expanding the ECF compartment. This affects both plasma (increased blood volume) and interstitial spaces (edema formation). The retained fluid is isotonic, so ICF volume remains normal as there's no osmotic gradient. A is incorrect because interstitial volume increases (edema). B incorrectly shows decreased plasma volume. D and E don't reflect the ECF expansion characteristic of heart failure.