Two experiments, deliberately separated
The upper scene is a fixed-pressure snapshot across an alveolar–capillary interface. The lower scene is a whole-body metabolic ratio. It is not a coupled cardiopulmonary simulator. Decorative vehicles, cells, and particles are schematic and do not represent measured volumes or transit times.
Pressure gradients and Fick’s law
ΔPO₂ = PAO₂ − PincomingO₂
ΔPCO₂ = PincomingCO₂ − PACO₂
V̇gas ∝ Dgas × (A/T) × ΔP
For each gas, diffusion conductance depends on its own diffusivity D, exchange area A, and barrier thickness T. Positive displayed O₂ gradients point toward blood; positive displayed CO₂ gradients point toward air. Negative gradients reverse those directions. At zero gradient, random exchange continues in both directions with no net transfer.
The normal teaching snapshot uses alveolar PO₂ 100 and PCO₂ 40 mmHg; incoming capillary PO₂ 40 and PCO₂ 46 mmHg. These are rounded illustrative values, not reference intervals. The oxygen slider spans 0–115 mmHg, matching the preceding oxygen-curve notebook; this is an interface range, not the full physiological range.
M = (selected area / 100%) / relative thickness
O₂ transfer index = 100 × M × |ΔPO₂| / 60
CO₂ transfer index = 100 × M × |ΔPCO₂| / 6
Each index compares one gas with its own normal example. Within each gas, D is held fixed and cancels from the relative calculation. The two 100% values are not equal molar fluxes. CO₂ has much greater solubility than O₂; neither its physical diffusion coefficient nor its equilibration time can be read from particle speed. The colored bars saturate visually at 200%, while their numerical labels remain unclipped.
The visual net-particle rate is proportional to its gas’s index up to a display cap; a minimum and maximum crossing duration keep the motion visible. Gas directions, displayed pressures, and index calculations are not altered for styling. Smaller active area localizes the visible crossings to the remaining interface. At zero area there is no transmembrane passage, although particles continue moving on either side.
RQ and its inputs
RQ = V̇CO₂ / V̇O₂
V̇CO₂ = RQ × V̇O₂
Default: 0.80 × 250 = 200 mL/min
Inputs use consistent gas-volume units (conceptually STPD). RQ is dimensionless. Presets are fat ≈0.70, a mixed-substrate example of 0.80, and carbohydrate 1.00. This model restricts RQ to 0.70–1.00 to teach common oxidative-substrate examples; it does not assert that every physiological or measured ratio must fall within those bounds.
The token diagram is normalized to 10 O₂ volume units. CO₂ units = 10 × RQ. Changing V̇O₂ changes calculated V̇CO₂ and the conceptual metabolic flow animation, but not that normalized ratio. RQ is not the ratio of gas partial pressures, inhaled/exhaled total volumes, or local snapshot diffusion indices.
The gas-exchange ratio measured at the mouth is RER. In a suitable metabolic and gas-store steady state, RER approximates tissue RQ. Transient ventilation, bicarbonate buffering, and CO₂ storage changes can separate them. The page does not infer diet, nutrition adequacy, or clinical status from an RQ value.
Where the analogy stops
The membrane is not an open loading gate; gases diffuse through a thin tissue–fluid barrier. Hemoglobin carries most blood oxygen, whereas much blood CO₂ is carried chemically as bicarbonate. Molecular CO₂, not a bicarbonate ion, diffuses into alveolar air. The cell and vehicle drawings do not calculate hemoglobin saturation or gas content.
No alveolar gas equation, blood-content curve, capillary equilibration profile, shunt, V̇/Q̇ distribution, perfusion adjustment, pH, temperature, buffering kinetics, or tissue oxygen delivery is computed. Alveolar and incoming-blood pressures are held at the selected values; therefore the animation does not “use up” the gradient and does not predict exit or systemic arterial blood gases. The thick-wall and reduced-area buttons isolate membrane effects rather than simulate all effects of edema, fibrosis, or emphysema.
Sources and further reading
References support the physiology. The illustration, user interface, normalized indices, and demonstration presets are original teaching constructions, not clinically validated models.
- National Heart, Lung, and Blood Institute. How the Lungs Work: What Breathing Does for the Body. Overview of oxygen delivery and carbon dioxide removal.
- National Library of Medicine. Gas exchange. Alveoli, capillary blood flow, and gas exchange.
- OpenStax. Anatomy and Physiology 2e, §22.4: Gas Exchange. Partial pressures, diffusion, and relative gas solubility. §22.5: Transport of Gases. Hemoglobin and bicarbonate transport.
- Price, E. R., & Mager, E. M. (2020). Respiratory quotient: Effects of fatty acid composition. Journal of Experimental Zoology Part A, 333(9), 613–618. doi:10.1002/jez.2422. Substrate dependence and the approximate nature of fat RQ.
- Ramos-Jiménez, A., et al. (2008). The respiratory exchange ratio is associated with fitness indicators both in trained and untrained men. Clinical Medicine: Circulatory, Respiratory and Pulmonary Medicine, 2, 1–9. doi:10.4137/ccrpm.s449.
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