- Carbon monoxide poisoning is life-threatening because it severely impairs oxygen transport. What is the specific molecular mechanism by which CO exerts this effect?
Answer: 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.
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.
- Which of the following describes a key feature of the gas exchange surface in the alveoli of a human lung?
Answer: A rich blood supply from the pulmonary artery providing deoxygenated blood to a dense capillary network.
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.
- 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?
Answer: The increase in pulmonary blood flow is precisely matched by an increase in alveolar ventilation, maintaining equilibrium.
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.
- 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?
Answer: The pressure within the pleural space equalizes with atmospheric pressure, eliminating the transpulmonary pressure that keeps the lung inflated.
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.
- Which statement correctly distinguishes the roles of the external and internal intercostal muscles during the human breathing cycle?
Answer: External intercostals contract to lift the rib cage during inhalation; internal intercostals contract to lower the rib cage during forced exhalation.
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.
- 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?
Answer: Nitrogen has a very low partial pressure and solubility, so it does not readily cross the alveolar membrane into the blood.
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.
