Questions: Gas Transport and Regulation of Ventilation

Question 1

A healthy individual is breath-holding as long as possible. What primarily forces the resumption of breathing?

• A. Blood oxygen falls to a critically low level that activates peripheral chemoreceptors, signaling the medulla to restart breathing
• B. Rising blood CO₂ lowers CSF pH, which central chemoreceptors in the medulla detect, generating an irresistible drive to breathe
• C. The diaphragm muscles fatigue and involuntarily contract, initiating inspiration
• D. Falling blood pH from lactic acid accumulation during breath-holding activates peripheral chemoreceptors

Answer: (B)

In healthy individuals, the primary ventilatory drive is rising CO₂, not falling O₂. Central chemoreceptors in the medulla monitor CSF pH, which tracks arterial PCO₂ (CO₂ diffuses into CSF; H⁺ does not). Even a 1 mmHg rise in PCO₂ produces noticeable increase in ventilatory drive. During breath-holding, CO₂ accumulates while O₂ remains above the ~60 mmHg threshold for peripheral chemoreceptor activation. Breath-holding terminates from CO₂ accumulation long before O₂ becomes critically low — which is why 'hyperventilating' before a breath hold (blowing off CO₂) paradoxically extends it.

Question 2

A highly active muscle is producing high CO₂, low pH, and elevated temperature. How does the Bohr effect optimize oxygen delivery to this tissue?

• A. The O₂-Hb dissociation curve shifts left, increasing hemoglobin's affinity for O₂ and ensuring a steady supply to the stressed tissue
• B. Hemoglobin fully unloads its O₂ in the lungs before reaching active tissue, maximizing delivery
• C. The curve shifts right, decreasing hemoglobin's O₂ affinity at the tissue's PO₂, releasing more O₂ precisely where metabolic demand is highest
• D. Chemoreceptors detect low pH and increase ventilation rate, raising arterial PO₂ to compensate

Answer: (C)

The Bohr effect is the rightward shift of the O₂-Hb dissociation curve in response to high CO₂, low pH, and elevated temperature — exactly the conditions created by active metabolism. A rightward shift means hemoglobin has LOWER affinity for O₂ at any given PO₂, releasing more O₂ at the tissue's partial pressure. This is self-matching: metabolically active tissue creates the conditions that cause hemoglobin to unload oxygen right there, without any separate regulatory signal. A leftward shift (option A) would increase affinity — which happens in the lungs where conditions are reversed, aiding reloading.

Question 3

The majority of carbon dioxide in the blood is transported as bicarbonate ions rather than as dissolved CO₂ or carbaminohemoglobin.

• T. True
• F. False

Answer: True

Approximately 70% of CO₂ is transported as HCO₃⁻ (bicarbonate), generated inside red blood cells by carbonic anhydrase: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. The HCO₃⁻ then exits via the chloride shift. Only about 7% dissolves directly in plasma, and about 23% binds to hemoglobin as carbaminohemoglobin (at the protein backbone, not the heme). The bicarbonate route dominates because carbonic anhydrase in red blood cells accelerates CO₂ conversion by orders of magnitude compared to the uncatalyzed reaction in plasma.

Question 4

In healthy individuals at rest, low blood oxygen is the primary stimulus that drives the urge to breathe.

• T. True
• F. False

Answer: False

This is the most common misconception about respiratory control. In healthy individuals, the primary ventilatory drive is rising CO₂, detected as falling CSF pH by central chemoreceptors. Peripheral chemoreceptors (carotid and aortic bodies) respond to low PO₂ but require it to fall below ~60 mmHg before contributing meaningfully — a level rarely reached at rest. Only in patients with chronic hypercapnia does the central chemoreceptor system adapt to chronically elevated CO₂ and become less responsive, shifting the primary drive to hypoxic (low O₂) stimulation.

Question 5

Why is the oxygen-hemoglobin dissociation curve sigmoidal rather than linear, and what are the physiological consequences of its shape?

Model answer: The sigmoidal shape arises from cooperative binding: each O₂ molecule that binds to one hemoglobin subunit changes the protein's conformation, making the remaining subunits more receptive (T→R state transition). The flat upper plateau (high PO₂, as in the lungs at ~95 mmHg) means hemoglobin loads efficiently even if PO₂ drops somewhat — saturation stays near 100%. The steep lower portion (low PO₂, as in active tissues at ~40 mmHg) means small drops in PO₂ cause large O₂ release. Normal physiology exploits both zones: loading in the plateau, unloading in the steep region.

A linear O₂-binding curve would fail to achieve both efficient loading and efficient unloading simultaneously. The sigmoidal shape is why hemoglobin can carry ~20 mL O₂ per 100 mL blood, compared to ~0.3 mL dissolved in plasma — a 65-fold difference that makes aerobic metabolism possible in large organisms. The Bohr effect then fine-tunes unloading by shifting the steep portion further right in active tissues.