Gas exchange and transport Practice Questions
20 free Gas exchange and transport practice questions for the Physiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following factors would cause a rightward shift in the oxyhemoglobin dissociation curve?
- A Decreased temperature of the surrounding tissue
- B Decreased 2,3-bisphosphoglycerate (2,3-BPG) levels
- C Increased blood pH due to systemic alkalosis
- D Increased partial pressure of carbon dioxide (PCO2)
Correct answer: Increased partial pressure of carbon dioxide (PCO2)
An increase in PCO2, temperature, H+ concentration (lower pH), or 2,3-BPG reduces hemoglobin's affinity for oxygen. This 'right shift' facilitates the unloading of oxygen into the tissues where metabolic activity is high.
The majority of carbon dioxide is transported in the blood in which of the following forms?
- A Dissolved in plasma
- B As bicarbonate ions (HCO3-)
- C Bound to the globin part of hemoglobin (carbaminohemoglobin)
- D As carbonic acid (H2CO3) in the erythrocytes
Correct answer: As bicarbonate ions (HCO3-)
Approximately 70% of CO2 is transported as bicarbonate. Carbonic anhydrase in red blood cells catalyzes the conversion of CO2 and water into carbonic acid, which then dissociates into bicarbonate and hydrogen ions.
According to Fick's Law of Diffusion, the rate of gas transfer across the respiratory membrane is inversely proportional to:
- A The surface area of the membrane
- B The partial pressure gradient of the gas
- C The thickness of the respiratory membrane
- D The solubility of the gas in the membrane
Correct answer: The thickness of the respiratory membrane
Fick's Law states that diffusion rate is directly proportional to surface area and concentration gradient, but inversely proportional to the distance or thickness the gas must travel. Conditions like pulmonary edema or fibrosis increase thickness and impair gas exchange.
The 'Chloride Shift' (Hamburger phenomenon) occurs in systemic capillaries to maintain electrical neutrality. This involves the exchange of bicarbonate for which ion?
- A Sodium (Na+) moving into the erythrocyte
- B Chloride (Cl-) moving into the erythrocyte
- C Potassium (K+) moving out of the erythrocyte
- D Chloride (Cl-) moving out of the erythrocyte
Correct answer: Chloride (Cl-) moving into the erythrocyte
As bicarbonate ions build up inside the red blood cell and diffuse out into the plasma, chloride ions move into the cell to balance the loss of negative charge. This process is reversed in the lungs.
What is the physiological significance of the 'Haldane Effect'?
- A Low oxygen levels in the tissues promote the loading of oxygen onto hemoglobin
- B The binding of oxygen to hemoglobin promotes the release of carbon dioxide
- C High CO2 levels in the tissues cause the release of oxygen from hemoglobin
- D Increased pH in the lungs increases the affinity of hemoglobin for oxygen
Correct answer: The binding of oxygen to hemoglobin promotes the release of carbon dioxide
The Haldane Effect describes how oxygenation of blood in the lungs displaces CO2 from hemoglobin. Conversely, in the tissues, deoxygenated hemoglobin has a higher affinity for CO2 and H+, aiding in CO2 transport.
Which of the following describes 'Cooperativity' in hemoglobin-oxygen binding?
- A Binding one oxygen molecule increases affinity for further oxygen molecules
- B Hemoglobin binds to both CO2 and O2 at the exact same binding site simultaneously
- C Iron atoms in the heme group repel oxygen unless CO2 is already bound
- D Myoglobin and hemoglobin compete for the same oxygen molecule in muscle tissue
Correct answer: Binding one oxygen molecule increases affinity for further oxygen molecules
Hemoglobin undergoes a conformational change (from T-state to R-state) when oxygen binds. This cooperative binding results in the characteristic sigmoidal (S-shaped) oxyhemoglobin dissociation curve.
Carbon monoxide (CO) is dangerous primarily because:
- A It causes a massive rightward shift in the dissociation curve
- B It has an affinity for hemoglobin over 200 times greater than oxygen
- C It dissolves readily in plasma and prevents the chloride shift
- D It stimulates the central chemoreceptors to stop breathing entirely
Correct answer: It has an affinity for hemoglobin over 200 times greater than oxygen
CO competes for the same binding sites as O2 but binds much more tightly. Not only does it reduce the O2-carrying capacity, but it also shifts the curve to the left, making it harder for any bound oxygen to be released to tissues.
The partial pressure of oxygen (PO2) in mixed venous blood returning to the right atrium is typically approximately:
- A 100 mmHg
- B 40 mmHg
- C 46 mmHg
- D 20 mmHg
Correct answer: 40 mmHg
Resting tissues typically extract about 25% of the oxygen from arterial blood (PO2 ~100 mmHg). This leaves the mixed venous blood with a PO2 of roughly 40 mmHg and an oxygen saturation of about 75%.
Henry's Law states that the amount of gas dissolved in a liquid is proportional to its partial pressure and its:
- A Molecular weight
- B Solubility coefficient
- C Temperature in Celsius
- D Vapor pressure
Correct answer: Solubility coefficient
Henry's Law explains why CO2 dissolves much more readily in blood than O2. Even though the pressure gradient for CO2 is small, its high solubility ensures it moves efficiently in and out of solution.
In the lungs, why does the alveolar PO2 (approx. 104 mmHg) remain lower than the atmospheric PO2 (approx. 159 mmHg)?
- A Due to the active transport of nitrogen into the alveolar space
- B The high resistance of the trachea to oxygen molecule diffusion
- C Oxygen is converted to ozone in the upper respiratory tract lining
- D Constant diffusion of O2 into the blood and humidification of air
Correct answer: Constant diffusion of O2 into the blood and humidification of air
As air is inhaled, it is humidified (adding water vapor pressure) and mixed with 'old' air in the functional residual capacity. Additionally, oxygen is constantly leaving the alveoli to enter the pulmonary capillaries.
Which enzyme, located on the luminal surface of pulmonary and systemic capillaries, is essential for the rapid interconversion of CO2 and HCO3-?
- A Angiotensin-converting enzyme
- B Lactate dehydrogenase
- C Alveolar elastase
- D Carbonic anhydrase
Correct answer: Carbonic anhydrase
While carbonic anhydrase is highly concentrated inside red blood cells, it is also present on capillary endothelium. It ensures that the equilibrium between CO2 and bicarbonate is reached quickly during the brief time blood spends in the capillaries.
What is the 'Bohr Effect'?
- A Increased CO2 and H+ concentration decrease hemoglobin's affinity for oxygen
- B Increased O2 concentration increases the affinity of hemoglobin for CO2
- C The effect of dissolved nitrogen on the solubility of oxygen in blood plasma
- D The relationship between total lung volume and airway resistance during breathing
Correct answer: Increased CO2 and H+ concentration decrease hemoglobin's affinity for oxygen
The Bohr effect specifically refers to the shift in the oxygen-hemoglobin dissociation curve caused by changes in PCO2 and pH. This allows blood to release more oxygen in metabolically active tissues where CO2 and acidity are higher.
A 'Shunt' (V/Q = 0) in the lungs refers to a condition where:
- A There is ventilation but no perfusion
- B Ventilation and perfusion are perfectly matched
- C The partial pressure of oxygen in the alveoli is zero
- D There is perfusion but no ventilation
Correct answer: There is perfusion but no ventilation
A physiological shunt occurs when blood bypasses ventilated alveoli (e.g., due to an obstructed airway or fluid-filled alveoli). This blood remains 'venous' in composition as it returns to the left heart, lowering arterial PO2.
The 'Alveolar-arterial (A-a) oxygen gradient' is used clinically to determine if hypoxemia is caused by:
- A Hypoventilation or lung parenchymal disease
- B Anemia or iron deficiency causing reduced hemoglobin
- C Carbon monoxide poisoning affecting hemoglobin binding
- D Heart failure or reduced cardiac output
Correct answer: Hypoventilation or lung parenchymal disease
A normal A-a gradient with low arterial PO2 suggests the problem is extrinsic to the lungs (like hypoventilation). An elevated A-a gradient suggests a gas exchange problem within the lungs, such as a V/Q mismatch or diffusion defect.
Which molecule binds to the central cavity of the hemoglobin tetramer to stabilize the 'Tense' (T) deoxygenated state?
- A Glucose-6-phosphate (G6P) molecule
- B Nitric oxide (NO) molecule
- C Bicarbonate ion (HCO3-) molecule
- D 2,3-bisphosphoglycerate (2,3-BPG)
Correct answer: 2,3-bisphosphoglycerate (2,3-BPG)
2,3-BPG is a byproduct of glycolysis in RBCs. By stabilizing the T-state, it promotes the release of oxygen; levels of 2,3-BPG increase during chronic hypoxia or high-altitude adaptation.
In a person standing upright, how does the Ventilation-Perfusion (V/Q) ratio change from the base to the apex of the lung?
- A The V/Q ratio is lowest at the apex
- B The V/Q ratio is uniform throughout the lung
- C The V/Q ratio depends mainly on airway diameter
- D The V/Q ratio is highest at the apex
Correct answer: The V/Q ratio is highest at the apex
Both ventilation and perfusion decrease from the base to the apex due to gravity, but perfusion decreases more sharply. Consequently, the apex is 'over-ventilated' relative to its perfusion, resulting in a high V/Q ratio.
Diffusion of which gas is usually 'perfusion-limited' in a healthy lung, meaning it reaches equilibrium between the alveolus and capillary very quickly?
- A Carbon monoxide (CO) gas
- B Oxygen during heavy exercise
- C Nitrous oxide (N2O) gas
- D Oxygen with severe pulmonary fibrosis
Correct answer: Nitrous oxide (N2O) gas
Nitrous oxide does not bind to hemoglobin, so its partial pressure in the blood rises instantly to match the alveolus. Oxygen is also typically perfusion-limited in healthy individuals at rest, equilibrating within one-third of the capillary length.
Methemoglobinemia is a condition where the iron in hemoglobin is in the ________ state, which cannot bind oxygen.
- A Ferrous (Fe2+)
- B Metallic (Fe0)
- C Ferric (Fe3+)
- D Oxidized (Fe4+)
Correct answer: Ferric (Fe3+)
Iron must be in the reduced ferrous (Fe2+) state to bind O2. Methemoglobin (Fe3+) cannot bind oxygen and also causes the remaining normal hemes to hold onto O2 more tightly, further starving tissues.
Which of the following describes the partial pressure of CO2 (PCO2) in the alveoli of a person who is hyperventilating?
- A It will increase above 45 mmHg
- B It will remain exactly at 40 mmHg
- C It will decrease below 40 mmHg
- D It will become equal to the PO2
Correct answer: It will decrease below 40 mmHg
Alveolar PCO2 is inversely proportional to alveolar ventilation. Hyperventilation 'washes out' CO2 from the lungs faster than the tissues can produce it, leading to hypocapnia (low PCO2).
What is the primary role of myoglobin in muscle cells?
- A To act as an oxygen reservoir and aid O2 diffusion
- B To transport CO2 from the muscle to the blood
- C To regulate the pH of the sarcoplasm during exercise
- D To pump calcium ions into the sarcoplasmic reticulum
Correct answer: To act as an oxygen reservoir and aid O2 diffusion
Myoglobin has a much higher affinity for oxygen than hemoglobin and lacks cooperativity (monomeric). It holds onto oxygen at low PO2 levels where hemoglobin would release it, providing a local reserve for mitochondria during muscle contraction.