IB Biology Quiz: Understand Gas Exchange
6 questions · exam conditions
0:00
Understand Gas ExchangeQuestion 1 of 6

Analysis of a person's exhaled air shows a higher concentration of water vapour and carbon dioxide, and a lower concentration of oxygen compared to inhaled air. What accounts for the fact that the percentage of nitrogen is almost unchanged?

Nitrogen is used and produced by the body at equal rates, resulting in no net change in its concentration in the lungs.
Nitrogen has a very low partial pressure and solubility, so it does not readily cross the alveolar membrane into the blood.
Haemoglobin binds nitrogen with high affinity, but it is immediately released back into the alveoli before exhalation.
The body's cells require nitrogen for protein synthesis, but they obtain it from diet, not from inhaled atmospheric gas.
← Back to quizzes

IB Biology Quiz

IB Biology Quiz: Understand Gas Exchange

Practice Understand Gas Exchange in IB Biology 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 Understand Gas Exchange, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

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

Analysis of a person's exhaled air shows a higher concentration of water vapour and carbon dioxide, and a lower concentration of oxygen compared to inhaled air. What accounts for the fact that the percentage of nitrogen is almost unchanged?

  1. Nitrogen is used and produced by the body at equal rates, resulting in no net change in its concentration in the lungs.
  2. Nitrogen has a very low partial pressure and solubility, so it does not readily cross the alveolar membrane into the blood. (correct answer)
  3. Haemoglobin binds nitrogen with high affinity, but it is immediately released back into the alveoli before exhalation.
  4. The body's cells require nitrogen for protein synthesis, but they obtain it from diet, not from inhaled atmospheric gas.
Explanation: Although nitrogen constitutes about 78% of the air we breathe, it has very low solubility in blood plasma under normal atmospheric pressure. Unlike oxygen and carbon dioxide, there is no physiological mechanism (like a carrier molecule) to transport it, and it does not participate in metabolic reactions. Therefore, the amount of nitrogen that dissolves in the blood is negligible, and almost all the nitrogen inhaled is subsequently exhaled.

Question 2

Which statement correctly distinguishes the roles of the external and internal intercostal muscles during the human breathing cycle?

  1. External intercostals contract for quiet inhalation; internal intercostals contract for quiet exhalation.
  2. Both sets of muscles contract during forceful inhalation and relax during forceful exhalation.
  3. External intercostals contract to lift the rib cage during inhalation; internal intercostals contract to lower the rib cage during forced exhalation. (correct answer)
  4. Internal intercostals are the primary muscles for inhalation; external intercostals are only used for coughing and sneezing.
Explanation: The external intercostal muscles are oriented to lift the rib cage up and out upon contraction, increasing thoracic volume for inhalation. Quiet exhalation is passive. However, during forced exhalation (e.g., during exercise or coughing), the internal intercostal muscles contract, pulling the rib cage down and inward, actively decreasing thoracic volume to expel air more rapidly.

Question 3

Carbon monoxide poisoning is life-threatening because it severely impairs oxygen transport. What is the specific molecular mechanism by which CO exerts this effect?

  1. CO binds to the same heme iron sites as oxygen with much higher affinity and also increases the affinity of the remaining sites for oxygen. (correct answer)
  2. CO dissolves in the blood plasma, increasing its viscosity and preventing red blood cells from reaching the capillaries.
  3. CO causes a massive rightward shift in the oxygen-haemoglobin dissociation curve, preventing oxygen from binding in the lungs.
  4. CO directly inhibits the enzymes of cellular respiration within the mitochondria, preventing the use of oxygen by cells.
Explanation: Carbon monoxide's danger comes from a two-fold effect on haemoglobin. First, it binds to the iron atom in the heme group, the same site as oxygen, but with an affinity that is over 200 times greater, effectively outcompeting oxygen. Second, when CO binds to one site on a haemoglobin molecule, it allosterically increases the binding affinity of the remaining sites for oxygen. This shifts the dissociation curve to the left, meaning that any oxygen that is bound is not easily released to the tissues.

Question 4

During moderate, sustained exercise, alveolar P(O₂) remains relatively constant despite a large increase in oxygen consumption by the tissues. What is the primary reason for this stability?

  1. Oxygen diffusion across the alveolar membrane reaches a maximum rate and cannot increase further, creating a bottleneck.
  2. The affinity of haemoglobin for oxygen decreases, causing oxygen to be released from the blood back into the alveoli.
  3. The body begins to rely more on anaerobic respiration, reducing the overall demand for oxygen from the lungs.
  4. The increase in pulmonary blood flow is precisely matched by an increase in alveolar ventilation, maintaining equilibrium. (correct answer)
Explanation: The respiratory control centre ensures that as oxygen is consumed more rapidly by the tissues (and more CO₂ is produced), alveolar ventilation (the amount of fresh air reaching the alveoli per minute) increases in direct proportion. This tight coupling of ventilation to perfusion and metabolic rate means that oxygen is supplied to the alveoli as fast as it is removed by the blood, keeping the alveolar partial pressure of oxygen stable.

Question 5

Which of the following describes a key feature of the gas exchange surface in the alveoli of a human lung?

  1. A thick, muscular wall that allows for contraction to actively expel air during exhalation.
  2. A dry surface maintained by ciliated epithelium to prevent fluid from interfering with diffusion.
  3. A very small total surface area that is compensated for by a high partial pressure of oxygen.
  4. A rich blood supply from the pulmonary artery providing deoxygenated blood to a dense capillary network. (correct answer)
Explanation: For efficient gas exchange, a respiratory surface must have a steep concentration gradient. This is maintained in the lungs by a massive blood supply. The pulmonary artery delivers deoxygenated blood, rich in CO₂, to a vast network of capillaries surrounding each alveolus. This ensures that blood with low O₂ and high CO₂ is constantly flowing past the alveolar air, maximizing the gradients for O₂ to enter the blood and CO₂ to leave it.

Question 6

A pneumothorax occurs when air enters the pleural space, causing the lung to collapse. What underlying physical principle is disrupted, leading directly to this collapse?

  1. The elastic recoil of the lung tissue is overcome by the high pressure of the air entering the pleural space.
  2. The pressure within the pleural space equalizes with atmospheric pressure, eliminating the transpulmonary pressure that keeps the lung inflated. (correct answer)
  3. The surface tension of the pleural fluid is broken, preventing the lung from adhering to the thoracic wall.
  4. The diaphragm is forced into a permanent state of relaxation, preventing any further attempts at inhalation.
Explanation: Normally, the pleural space has a negative pressure relative to both the atmosphere and the alveoli. This pressure difference, called the transpulmonary pressure (alveolar pressure - intrapleural pressure), acts as a distending force that pulls the lungs open. When air enters the pleural space, the intrapleural pressure rises to equal atmospheric pressure. This eliminates the transpulmonary pressure, and the natural elastic recoil of the lung tissue causes it to collapse inward.