Respiratory System

Gas Exchange and Fick's Law

• Diffusion across the alveolar membrane follows $V_{gas} \propto \frac{A \cdot D \cdot (P_1 - P_2)}{T}$ Emphysema destroys surface area $A$; fibrosis and edema thicken $T$; supplemental oxygen raises the pressure gradient to compensate.
• Alveolar $P_{O_2}$ is about 100 mmHg vs. 40 mmHg in venous blood, driving oxygen loading.

Oxygen and Carbon Dioxide Transport

• About 98.5 percent of $O_2$ rides hemoglobin; cooperative binding makes the dissociation curve sigmoidal — a flat loading plateau in the lungs and a steep unloading region in tissues ($P_{50} \approx 26$ mmHg).
• Bohr effect: high $CO_2$, low pH, heat, and 2,3-BPG right-shift the curve, releasing $O_2$ where metabolism is high; fetal hemoglobin and CO shift it left.
• $CO_2$ travels mostly as bicarbonate via carbonic anhydrase: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$ with the chloride shift preserving charge balance in RBCs.

Control of Ventilation

• Central chemoreceptors in the medulla sense CSF pH as a proxy for arterial $CO_2$ — the primary drive; peripheral chemoreceptors respond to $P_{O_2}$ below about 60 mmHg. Hyperventilation causes respiratory alkalosis; hypoventilation causes acidosis.

Key Takeaways

• Fick's law variables map directly onto lung diseases.
• Right shift releases oxygen; left shift hoards it.
• Bicarbonate is the main $CO_2$ vehicle, linked to acid-base balance.
• $CO_2$, not $O_2$, is the master regulator of breathing.