Ventilation-Perfusion Ratio Practice Questions
20 free Ventilation-Perfusion Ratio practice questions for the Physiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
In a healthy individual standing upright, which of the following is true regarding the Ventilation-Perfusion (V/Q) ratio at the apex of the lung compared to the base?
- A The V/Q ratio is higher at the apex because perfusion decreases more than ventilation.
- B The V/Q ratio is lower at the apex because perfusion is proportionally higher there.
- C The V/Q ratio stays the same at both locations because of pulmonary autoregulation.
- D The V/Q ratio is lower at the apex because ventilation drops much more than perfusion.
Correct answer: The V/Q ratio is higher at the apex because perfusion decreases more than ventilation.
In the upright lung, gravity causes both ventilation and perfusion to increase from apex to base. However, perfusion increases much more steeply than ventilation, resulting in a higher V/Q ratio (approximately 3.0) at the apex and a lower ratio (approximately 0.6) at the base.
Which of the following describes the gas partial pressures in an alveolar unit with a V/Q ratio of zero (a physiological shunt)?
- A PO2 = 149 mmHg; PCO2 = 0 mmHg
- B PO2 = 100 mmHg; PCO2 = 40 mmHg
- C PO2 = 0 mmHg; PCO2 = 100 mmHg
- D PO2 = 40 mmHg; PCO2 = 45 mmHg
Correct answer: PO2 = 40 mmHg; PCO2 = 45 mmHg
When the V/Q ratio is zero, there is no ventilation to provide oxygen or remove carbon dioxide. Consequently, the alveolar gas partial pressures equilibrate with the mixed venous blood entering the pulmonary capillaries.
What is the primary compensatory response of the pulmonary vasculature to a localized area of low V/Q (hypoventilation)?
- A Hypoxic pulmonary vasoconstriction to divert blood to well-ventilated areas
- B Local vasodilation that increases blood flow into the poorly ventilated area
- C Local release of nitric oxide to improve overall alveolar gas exchange
- D Reflex bronchoconstriction that matches ventilation to the reduced blood flow
Correct answer: Hypoxic pulmonary vasoconstriction to divert blood to well-ventilated areas
Hypoxic pulmonary vasoconstriction (HPV) is a unique mechanism where pulmonary arterioles constrict in response to low alveolar PO2. This reduces blood flow to poorly ventilated 'shunt' areas, minimizing the impact on arterial oxygenation.
A patient is diagnosed with a massive pulmonary embolism. What is the expected V/Q ratio in the affected lung segment?
- A V/Q = Near Zero
- B V/Q = 0.8
- C V/Q = 1.0
- D V/Q = Infinity
Correct answer: V/Q = Infinity
A pulmonary embolism obstructs blood flow (perfusion = 0) to a ventilated area. Since the denominator in the V/Q ratio is zero, the ratio becomes infinite, creating alveolar dead space.
Which of the following characterizes an alveolar unit acting as 'Alveolar Dead Space'?
- A High ventilation with no perfusion
- B High perfusion with no ventilation
- C Perfectly matched ventilation and perfusion
- D Partial obstruction of both airway and capillary
Correct answer: High ventilation with no perfusion
Alveolar dead space refers to inspired air that reaches the alveoli but cannot participate in gas exchange because those alveoli are not perfused. This results in a V/Q ratio of infinity.
In the 'West Zones' of the lung, Zone 1 is defined by which relationship between pressures?
- A Arterial pressure > Venous pressure > Alveolar pressure
- B Arterial pressure > Alveolar pressure > Venous pressure
- C Alveolar pressure > Arterial pressure > Venous pressure
- D Venous pressure > Alveolar pressure > Arterial pressure
Correct answer: Alveolar pressure > Arterial pressure > Venous pressure
In Zone 1, alveolar pressure exceeds arterial pressure, potentially collapsing the pulmonary capillaries and stopping blood flow. While rarely present in healthy people at sea level, it can occur during hemorrhage or positive pressure ventilation.
How does the 'Alveolar-arterial (A-a) oxygen gradient' typically change in a patient with a significant V/Q mismatch?
- A The A-a gradient remains normal.
- B The A-a gradient decreases toward zero.
- C The A-a gradient increases significantly.
- D The A-a gradient changes only with ventilator use.
Correct answer: The A-a gradient increases significantly.
A V/Q mismatch prevents arterial blood from fully equilibrating with alveolar gas. This inefficiency results in a wider gap between the calculated alveolar PO2 and the measured arterial PO2, thus increasing the A-a gradient.
If a patient has a large right-to-left anatomical shunt, how will their hypoxemia respond to 100% supplemental oxygen?
- A The hypoxemia will improve substantially, nearing normal levels.
- B The hypoxemia will show little to no improvement, even on high FiO2.
- C The arterial PO2 will rise moderately, but hypoxemia will persist.
- D The PCO2 will fall to normal levels due to improved gas exchange.
Correct answer: The hypoxemia will show little to no improvement, even on high FiO2.
Shunted blood completely bypasses ventilated alveoli and never 'sees' the high O2. While the O2 content of the non-shunted blood may increase slightly, the dilution effect of the shunted venous blood prevents a significant rise in arterial PO2.
Which gas partial pressures are specifically used in the 'Bohr Equation' to calculate the fraction of dead space?
- A Alveolar oxygen partial pressure
- B Arterial PCO2 and Mixed Expired PCO2
- C Mixed venous oxygen partial pressure
- D Inspired nitrogen concentration level
Correct answer: Arterial PCO2 and Mixed Expired PCO2
The Bohr equation uses the difference between arterial PCO2 (representing alveolar PCO2) and mixed expired PCO2 to determine the volume of the breath that did not participate in gas exchange (dead space).
During heavy exercise, why does V/Q matching typically improve in the lungs?
- A Cardiac output decreases significantly, allowing more time for diffusion.
- B Increased pulmonary arterial pressure recruits capillaries in the upper lobes.
- C The person switches from nose breathing to mouth breathing during exertion.
- D Surfactant production ceases entirely to allow for thinner membranes.
Correct answer: Increased pulmonary arterial pressure recruits capillaries in the upper lobes.
During exercise, the increase in pulmonary blood pressure forces open (recruits) and widens (distends) capillaries in the apices (Zone 1 and 2). This makes perfusion more uniform throughout the lung, better matching the ventilation.
An area of the lung with a 'Low V/Q' ratio (but not zero) is most likely to cause:
- A Hypercapnia without any accompanying hypoxemia
- B Hypoxemia that responds well to supplemental oxygen
- C An infinite alveolar-arterial oxygen gradient
- D Immediate onset of acute respiratory alkalosis
Correct answer: Hypoxemia that responds well to supplemental oxygen
Low V/Q units contribute deoxygenated blood to the systemic circulation, causing hypoxemia. Unlike a true shunt (V/Q=0), these units still have some ventilation, so increasing the inspired O2 concentration can significantly improve the oxygenation of blood leaving these units.
Which of the following local effects occurs in the bronchioles when an area of the lung has a very high V/Q ratio (dead space)?
- A Bronchodilation to increase oxygen uptake
- B Bronchoconstriction due to low local PCO2
- C Increased mucus secretion to trap pathogens
- D Release of histamine from mast cells
Correct answer: Bronchoconstriction due to low local PCO2
In dead space, there is no CO2 being delivered from the blood. Low PCO2 in the small airways causes local bronchiolar constriction, which helps divert ventilation away from the non-perfused 'wasted' area toward better-perfused alveoli.
The ventilation-perfusion ratio of the entire lung in a healthy resting adult is approximately:
- A 0.2
- B 0.8
- C 2.0
- D 5.0
Correct answer: 0.8
With a typical alveolar ventilation of about 4 L/min and a cardiac output (pulmonary blood flow) of 5 L/min, the global V/Q ratio is approximately 0.8.
What happens to the V/Q ratio in the base of the lung when a person moves from a standing to a supine position?
- A It becomes infinite due to complete loss of perfusion.
- B It stays fixed at 0.6 regardless of body position.
- C It increases toward 1.0 as the gravity gradient is reduced.
- D It decreases toward 0.1 because of abdominal pressure.
Correct answer: It increases toward 1.0 as the gravity gradient is reduced.
In the supine position, the gravitational difference between the 'top' and 'bottom' of the lung is minimized. Perfusion and ventilation become more evenly distributed from the apex to the base, bringing the V/Q ratios closer to 1.0.
Which of the following clinical conditions is most associated with a 'Low V/Q' mismatch?
- A Pulmonary embolism with vascular obstruction
- B Hemorrhagic shock from acute blood loss
- C High altitude pulmonary edema (HAPE)
- D Chronic Obstructive Pulmonary Disease (COPD)
Correct answer: Chronic Obstructive Pulmonary Disease (COPD)
COPD involves airway obstruction and mucus plugging, which leads to many regions of the lung being poorly ventilated despite remaining perfused. This creates a widespread low V/Q mismatch.
If an alveolar unit has a V/Q ratio of 1.0, the PCO2 in that alveolus will be approximately:
- A 0 mmHg
- B 45 mmHg
- C 100 mmHg
- D 40 mmHg
Correct answer: 40 mmHg
When ventilation and perfusion are perfectly matched (V/Q = 1.0), the alveolar gas partial pressures remain at the standard values (PO2 ~100 mmHg and PCO2 ~40 mmHg) provided by normal breathing.
Why does a V/Q mismatch typically cause hypoxemia more readily than hypercapnia?
- A Hemoglobin is already O2-saturated; hyperventilation clears excess CO2.
- B Oxygen is much more soluble in blood plasma than carbon dioxide is.
- C The body lacks any sensors that detect carbon dioxide levels.
- D Peripheral chemoreceptors respond only to changes in oxygen levels.
Correct answer: Hemoglobin is already O2-saturated; hyperventilation clears excess CO2.
The blood leaving high V/Q units cannot 'pick up' extra O2 to compensate for low V/Q units because hemoglobin is already saturated. However, high V/Q units can 'dump' extra CO2 because the CO2 dissociation curve is more linear, allowing for total CO2 normalization through hyperventilation.
A V/Q ratio of 0.5 indicates that:
- A Ventilation is twice the rate of perfusion.
- B There is no gas exchange occurring.
- C The unit is in West Zone 1.
- D Perfusion is twice the rate of ventilation.
Correct answer: Perfusion is twice the rate of ventilation.
A ratio of 0.5 (1/2) means that for every 1 unit of ventilation, there are 2 units of perfusion. This characterizes a 'low V/Q' unit where perfusion is in excess of ventilation.
Which of the following gas compositions is most likely found in an apex alveolar unit of a standing person?
- A PO2 = 40; PCO2 = 45
- B PO2 = 100; PCO2 = 40
- C PO2 = 132; PCO2 = 28
- D PO2 = 159; PCO2 = 0
Correct answer: PO2 = 132; PCO2 = 28
At the apex (high V/Q), ventilation is high relative to perfusion. Because less CO2 is delivered and more O2 is 'left behind' by the sparse blood flow, the gas composition shifts toward inspired air (higher PO2, lower PCO2).
The fraction of cardiac output that bypasses gas exchange is calculated using which formula?
- A V = RR x (TV - DS) formula
- B The Fick Principle method
- C The Starling's Equation model
- D The Shunt Equation (Qs/Qt)
Correct answer: The Shunt Equation (Qs/Qt)
The Shunt Equation ($Q_s/Q_t = [C_{c'}O_2 - C_aO_2] / [C_{c'}O_2 - C_vO_2]$) is used to calculate the portion of the cardiac output that does not participate in gas exchange.