Body fluid compartments Practice Questions
40 free Body fluid compartments practice questions for the Physiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Total body water in an average adult male constitutes approximately what percentage of body weight?
- A 40%
- B 50%
- C 60%
- D 70%
Correct answer: 60%
Total body water in adult males is about 60% of body weight. This proportion varies with age, sex, and body composition.
The largest body fluid compartment is the:
- A Plasma
- B Interstitial fluid
- C Intracellular fluid
- D Transcellular fluid
Correct answer: Intracellular fluid
Intracellular fluid accounts for about two-thirds of total body water. It is the fluid contained within cells.
Extracellular fluid constitutes approximately what fraction of total body water?
- A One-third
- B One-half
- C Two-thirds
- D Three-fourths
Correct answer: One-third
Extracellular fluid makes up about one-third of total body water. It includes plasma and interstitial fluid.
Plasma volume represents approximately what percentage of total body water?
- A 5%
- B 10%
- C 15%
- D 25%
Correct answer: 5%
Plasma volume accounts for about 5% of total body water. It is the intravascular component of extracellular fluid.
Interstitial fluid volume is best described as:
- A Fluid confined within the blood vessels
- B Fluid contained within the cells
- C Fluid between cells
- D Fluid pooled within enclosed body cavities
Correct answer: Fluid between cells
Interstitial fluid surrounds and bathes cells. It forms the largest component of extracellular fluid.
Which fluid compartment contains the highest concentration of potassium?
- A Plasma
- B Interstitial fluid
- C Intracellular fluid
- D Transcellular fluid
Correct answer: Intracellular fluid
Potassium is the major intracellular cation. Its high intracellular concentration is maintained by the Na⁺/K⁺ ATPase.
The primary cation in extracellular fluid is:
- A Potassium
- B Calcium
- C Sodium
- D Magnesium
Correct answer: Sodium
Sodium is the predominant extracellular cation. It plays a key role in fluid balance and osmotic pressure.
Which substance distributes freely across cell membranes and equilibrates between fluid compartments?
- A Sodium
- B Chloride
- C Potassium
- D Water
Correct answer: Water
Water moves freely across cell membranes through aquaporins. This allows rapid equilibration between compartments.
Osmolality of body fluids is primarily determined by the concentration of:
- A Circulating plasma proteins such as albumin
- B Triglycerides and other circulating lipids
- C Electrolytes
- D Glucose and other small sugars
Correct answer: Electrolytes
Electrolytes, especially sodium and its associated anions, are the main determinants of body-fluid osmolality and drive water distribution between compartments.
Which method is commonly used to measure extracellular fluid volume?
- A Inulin dilution
- B Deuterium oxide dilution
- C Total body potassium measurement
- D Hemoglobin concentration
Correct answer: Inulin dilution
Inulin distributes in extracellular fluid but does not enter cells. It is used to measure extracellular fluid volume.
Intracellular fluid volume can be estimated by:
- A Subtracting extracellular fluid from total body water
- B Directly measuring the plasma volume with a dye
- C Using a sodium dilution technique
- D Using an albumin dilution technique
Correct answer: Subtracting extracellular fluid from total body water
Intracellular fluid volume is calculated indirectly. It equals total body water minus extracellular fluid volume.
Which fluid compartment is most affected during acute hemorrhage?
- A Intracellular fluid
- B Interstitial fluid
- C Plasma
- D Transcellular fluid
Correct answer: Plasma
Hemorrhage causes direct loss of plasma volume. Compensatory shifts from interstitial fluid occur later.
Transcellular fluid includes all of the following EXCEPT:
- A Cerebrospinal fluid
- B Synovial fluid
- C Interstitial fluid
- D Pleural fluid
Correct answer: Interstitial fluid
Transcellular fluid includes specialized fluids like CSF and pleural fluid. Interstitial fluid is part of extracellular fluid, not transcellular.
Which factor most strongly influences the distribution of water between intracellular and extracellular compartments?
- A Hydrostatic pressure
- B Osmotic gradients
- C Protein concentration
- D Capillary permeability
Correct answer: Osmotic gradients
Osmotic gradients across cell membranes drive water movement. These gradients are determined by effective osmoles.
Edema formation primarily reflects excess fluid accumulation in the:
- A Plasma
- B Intracellular fluid
- C Interstitial fluid
- D Transcellular fluid
Correct answer: Interstitial fluid
Edema is due to increased interstitial fluid volume. It results from imbalances in Starling forces or lymphatic drainage.
Which protein contributes most to plasma oncotic pressure?
- A Globulin
- B Fibrinogen
- C Albumin
- D Hemoglobin
Correct answer: Albumin
Albumin is the most abundant plasma protein. It plays a major role in maintaining oncotic pressure.
A hypotonic intravenous fluid will initially cause water to move:
- A Out of cells
- B Into cells
- C Only into plasma
- D Only into interstitial fluid
Correct answer: Into cells
Hypotonic fluids lower extracellular osmolality. Water then moves into cells, causing cellular swelling.
Which compartment experiences the greatest relative change during dehydration?
- A Intracellular fluid
- B Plasma
- C Transcellular fluid
- D Cerebrospinal fluid
Correct answer: Plasma
Plasma volume decreases significantly during dehydration. This can impair circulation and blood pressure.
Total body water is typically lower in obese individuals primarily because:
- A They habitually drink less water
- B Adipose tissue contains less water
- C They excrete more water via kidneys
- D They have a chronically reduced plasma volume
Correct answer: Adipose tissue contains less water
Adipose tissue has a lower water content than lean tissue. Increased fat mass reduces total body water percentage.
Which statement best describes steady-state fluid balance?
- A Fluid intake exceeds output
- B Fluid output exceeds intake
- C Fluid intake equals fluid output
- D No fluid exchange occurs between compartments
Correct answer: Fluid intake equals fluid output
At steady state, fluid intake matches fluid output. This maintains stable body fluid volumes over time.
Which of the following describes the 'Indicator Dilution Principle' used to measure compartment volumes?
- A Volume = Amount of tracer injected / Final concentration of tracer
- B Volume = Final concentration of tracer / Amount of tracer injected
- C Volume = Rate of tracer excretion * Time
- D Volume = Total Body Water - Plasma Volume
Correct answer: Volume = Amount of tracer injected / Final concentration of tracer
The indicator dilution principle states that volume is equal to the mass of the marker injected (minus any excreted) divided by the equilibrium concentration.
Which tracer is most appropriate for measuring Total Body Water (TBW)?
- A Inulin
- B Evans Blue
- C Radioactive sodium
- D Deuterium oxide
Correct answer: Deuterium oxide
Heavy water (D2O) or tritiated water (THO) distributes evenly across all fluid compartments, making it the gold standard for measuring TBW.
An 80 kg healthy male has an approximate Intracellular Fluid (ICF) volume of:
- A 32 L
- B 16 L
- C 48 L
- D 12 L
Correct answer: 32 L
TBW is ~60% of body weight (48 L). ICF is 2/3 of TBW, which is approximately 32 L.
Which barrier separates the plasma from the interstitial fluid?
- A Cell membrane
- B Capillary wall
- C Nuclear envelope
- D Blood-brain barrier
Correct answer: Capillary wall
The capillary endothelium is the semi-permeable barrier that separates the intravascular compartment (plasma) from the interstitial space.
In the intracellular fluid (ICF), the primary balancing anions for Potassium are:
- A Chloride and Bicarbonate
- B Proteins and Phosphates
- C Bicarbonate and Sulfate
- D Lactate and Citrate
Correct answer: Proteins and Phosphates
While Chloride is the main ECF anion, the ICF relies on negatively charged proteins and organic phosphates (like ATP) to maintain electrical neutrality.
If a person is infused with a hypertonic saline solution, what is the net movement of water?
- A From ECF to ICF
- B From ICF to ECF
- C No net movement occurs
- D Water moves into the transcellular space only
Correct answer: From ICF to ECF
Hypertonic ECF increases osmotic pressure outside the cells, drawing water out of the ICF until osmolarities equalize.
Which of the following constitutes the smallest percentage of the Extracellular Fluid (ECF)?
- A Interstitial tissue fluid
- B Blood plasma
- C Transcellular fluid
- D Lymphatic fluid
Correct answer: Transcellular fluid
Transcellular fluid (CSF, synovial, ocular fluids) usually makes up only 1-2 liters, much less than interstitial fluid or plasma.
Which substance would be best to measure the volume of the Extracellular Fluid (ECF) specifically?
- A Albumin
- B Mannitol
- C Antipyrine
- D Iodinated starch
Correct answer: Mannitol
Mannitol, inulin, and sucrose are large molecules that cross capillaries but cannot cross cell membranes, thus distributing throughout the ECF.
The primary difference in composition between plasma and interstitial fluid is the concentration of:
- A Sodium
- B Potassium
- C Proteins
- D Bicarbonate
Correct answer: Proteins
Plasma contains significantly higher concentrations of proteins (colloids) compared to interstitial fluid, which is largely a protein-free ultrafiltrate.
In a clinical setting, Interstitial Fluid volume is calculated as:
- A ECF volume - Plasma volume
- B TBW - ICF volume
- C Plasma volume + Transcellular fluid
- D Total Body Weight * 0.2
Correct answer: ECF volume - Plasma volume
Since there is no direct marker for the interstitium, it is calculated by subtracting the measured plasma volume from the measured ECF volume.
The 'Gibbs-Donnan Effect' explains why:
- A ICF osmolarity is lower than ECF osmolarity.
- B Plasma has more cations than the interstitium.
- C Water moves toward hypotonic solutions.
- D Sodium is pumped out of the cell.
Correct answer: Plasma has more cations than the interstitium.
Non-diffusible plasma proteins (anions) attract diffusible cations (like Na+), leading to a slightly higher cation concentration in plasma than in the interstitium.
Which of the following individuals would likely have the highest percentage of Total Body Water relative to their weight?
- A A 70-year-old male
- B An obese 30-year-old female
- C A 1-day-old newborn infant
- D An athletic 25-year-old male
Correct answer: A 1-day-old newborn infant
Infants have the highest TBW percentage (~70-75%), which decreases with age and increasing body fat.
What happens to ICF and ECF volumes after a significant loss of 'Isotonic' fluid (e.g., diarrhea)?
- A ECF volume decreases; ICF volume remains unchanged.
- B Both ECF and ICF volumes decrease equally.
- C ECF volume decreases; ICF volume increases.
- D ECF volume remains unchanged; ICF volume decreases.
Correct answer: ECF volume decreases; ICF volume remains unchanged.
Loss of isotonic fluid reduces ECF volume, but because there is no change in ECF osmolarity, there is no osmotic gradient to pull water from the ICF.
Which mechanism is primarily responsible for maintaining the high K+ and low Na+ concentrations in the ICF?
- A Osmosis
- B Na+/K+-ATPase pump
- C Facilitated diffusion
- D Starling forces
Correct answer: Na+/K+-ATPase pump
The sodium-potassium pump actively transports 3 Na+ out and 2 K+ in, maintaining the chemical gradients between ICF and ECF.
Evans Blue dye binds to which component to measure plasma volume?
- A Red blood cells
- B Hemoglobin
- C Fibrinogen
- D Plasma albumin
Correct answer: Plasma albumin
Evans Blue (T-1824) binds tightly to albumin; since albumin stays in the vascular space, the dye tracks the plasma volume.
If a patient's hematocrit is 40% and their plasma volume is 3L, what is their total blood volume?
- A 4.2 L
- B 7.5 L
- C 1.2 L
- D 5.0 L
Correct answer: 5.0 L
Blood Volume = Plasma Volume / (1 - Hematocrit). Thus, 3 / (1 - 0.4) = 3 / 0.6 = 5 L.
Which condition would most likely lead to 'Intracellular Edema' (swelling of cells)?
- A Increase in plasma oncotic pressure
- B Infusion of hypertonic mannitol
- C Severe dehydration (ECF hypertonic)
- D Hyponatremia (low ECF sodium)
Correct answer: Hyponatremia (low ECF sodium)
Hyponatremia lowers ECF osmolarity relative to ICF, causing water to move into the cells via osmosis, leading to swelling.
How does the water content of lean muscle tissue compare to adipose (fat) tissue?
- A Muscle has significantly more water than fat.
- B Fat has significantly more water than muscle.
- C They have roughly the same water content.
- D Neither tissue contains significant water.
Correct answer: Muscle has significantly more water than fat.
Lean tissue is approximately 70-75% water, whereas adipose tissue is only about 10% water. This is why body composition affects TBW.
A patient with SIADH (Syndrome of Inappropriate Antidiuretic Hormone) retains pure water. What is the effect on body fluid osmolarity?
- A Both ECF and ICF osmolarities increase
- B ICF osmolarity increases
- C Both ECF and ICF osmolarities decrease
- D Only ECF osmolarity decreases
Correct answer: Both ECF and ICF osmolarities decrease
Water distributes freely; adding pure water dilutes both compartments, leading to a decrease in osmolarity in both the ECF and ICF.
Which of the following is considered 'Sensible' water loss?
- A Evaporation from the lungs
- B Diffusion through the skin
- C Urine output from the kidneys
- D Loss through sweat during cold weather
Correct answer: Urine output from the kidneys
Sensible loss refers to measurable loss (urine, feces, sweat), while insensible loss refers to unmeasured evaporation from skin and lungs.