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Oxygen is transported primarily bound to hemoglobin (98.5%) as oxyhemoglobin, with the remainder dissolved in plasma. The oxygen-hemoglobin dissociation curve is sigmoidal and shifts right (releasing more O₂) in tissues with high CO₂, low pH, elevated temperature, or 2,3-BPG — a phenomenon called the Bohr effect. Carbon dioxide is transported as dissolved CO₂ (7%), carbaminohemoglobin (23%), and bicarbonate ions (70%), the last via the chloride shift in red blood cells. Ventilation is controlled by the medullary respiratory centers, with central chemoreceptors monitoring CSF pH (a proxy for PCO₂) and peripheral chemoreceptors responding to low PO₂, high PCO₂, and low pH.
Sketch the O₂-Hb dissociation curve and practice shifting it left/right under different conditions. Work through blood gas scenarios (e.g., respiratory acidosis vs. alkalosis) to understand the feedback between breathing rate and blood pH.
From your study of gas exchange and Fick's laws, you know that O₂ and CO₂ move across the alveolar membrane by diffusion down partial pressure gradients. But diffusion alone delivers a trivially small amount of oxygen — blood plasma can dissolve only about 0.3 mL O₂ per 100 mL at normal PO₂. The body's solution is hemoglobin, which binds oxygen cooperatively through four heme subunits. The result is the oxygen-hemoglobin dissociation curve, a sigmoid shape that encodes two physiologically crucial zones: a flat upper plateau where hemoglobin loads O₂ efficiently in the lungs (even if PO₂ drops somewhat, saturation stays high), and a steep lower slope where small drops in PO₂ in the tissues cause large O₂ release. Evolution has positioned normal arterial PO₂ (≈95 mmHg) on the plateau and tissue PO₂ (≈40 mmHg) on the steep portion — perfect for loading in the lungs and unloading in active tissues.
The Bohr effect describes how local tissue conditions shift this curve rightward, meaning hemoglobin releases more O₂ at the same PO₂. High CO₂, low pH, elevated temperature, and 2,3-BPG all shift the curve right. Think of it as a chemical signal: when a tissue is metabolically active, it produces exactly the conditions that cause hemoglobin to let go of O₂ right there. The shift is self-matching — harder-working muscle receives more oxygen automatically, without any conscious regulation. The reverse happens in the lungs: CO₂ is blown off, pH rises, temperature is slightly lower, and the curve shifts left, helping hemoglobin reload oxygen.
CO₂ travels by three mechanisms. About 7% dissolves directly in plasma. Another 23% binds to hemoglobin as carbaminohemoglobin (at the protein backbone, not the heme group — this is why it doesn't interfere with O₂ binding in a simple way). The dominant route (70%) is conversion to bicarbonate: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻, catalyzed by carbonic anhydrase inside red blood cells. The HCO₃⁻ then exits via the chloride shift (Hamburger shift), where Cl⁻ enters red cells in exchange. In the lungs, the whole process reverses — bicarbonate re-enters, recombines, and CO₂ is exhaled.
Ventilation is regulated by a feedback loop you'll recognize from your homeostasis prerequisite. Central chemoreceptors in the medulla monitor the pH of cerebrospinal fluid, which reflects arterial PCO₂ (CO₂ diffuses into CSF; H⁺ cannot). Rising PCO₂ → falling CSF pH → increased respiratory drive. This is the primary drive in healthy individuals. Peripheral chemoreceptors in the carotid and aortic bodies add sensitivity to severe hypoxia (PO₂ below ≈60 mmHg), high PCO₂, and low pH simultaneously. The system is exquisitely sensitive to CO₂ — even a 1 mmHg rise in PCO₂ noticeably increases minute ventilation — which is why breath-holding terminates from CO₂ accumulation long before O₂ becomes critically low.