BIO 304 · Human Anatomy & Physiology
Gas Exchange & Transport
Respiratory System · Module 13
A reference for the Gas Exchange video. Oxygen and CO₂ cross at two places: alveoli and tissues. Both moves are passive diffusion down partial pressure gradients. Hemoglobin is the carrier that makes it work.
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- Explain alveolar gas exchange using partial pressures.
- Describe how oxygen is transported and identify what shifts the oxyhemoglobin dissociation curve.
- Trace CO₂ transport in the three main forms.
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Gas Exchange & O₂ Transport
Partial pressures (key values)
- Air at sea levelPO₂ ~160 mmHg, PCO₂ ~0.3 mmHg
- Alveolar gasPO₂ ~104 mmHg, PCO₂ ~40 mmHg
- Mixed venous bloodPO₂ ~40 mmHg, PCO₂ ~46 mmHg
- Arterial bloodPO₂ ~100 mmHg, PCO₂ ~40 mmHg
- TissuePO₂ ~40 mmHg, PCO₂ ~45 mmHg
External respiration (alveolus)
- O₂ diffusesalveolus → capillary (down 64 mmHg gradient)
- CO₂ diffusescapillary → alveolus (down 6 mmHg gradient; CO₂ more soluble)
- Respiratory membranethin (~0.5 µm); large surface area for fast diffusion
O₂ transport in blood
- Bound to hemoglobin (~98.5%)each Hb binds up to 4 O₂; oxyhemoglobin
- Dissolved in plasma (~1.5%)tiny; what PaO₂ measures directly
- Hb saturation (SaO₂)percent of Hb sites with O₂ bound; pulse oximetry reads it
Oxyhemoglobin curve
- Sigmoid shapecooperative binding; small PO₂ drop in tissue releases a lot of O₂
- Right shift = unload O₂high CO₂, low pH (Bohr effect), high temperature, high 2,3-BPG
- Left shift = hold O₂low CO₂, high pH, low temperature, low 2,3-BPG
- Fetal Hb (HbF)higher O₂ affinity than adult Hb; pulls O₂ across placenta
CO₂ Transport & Control
CO₂ transport (three forms)
- Bicarbonate (~70%)CO₂ + H₂O → H₂CO₃ → H+ + HCO₃-; carbonic anhydrase catalyzes
- Bound to hemoglobin (~20%)carbamino-Hb; binds globin chains
- Dissolved in plasma (~7-10%)CO₂ more soluble than O₂
Chloride shift
- In RBC at tissuesHCO₃- moves out into plasma; Cl- moves in to maintain neutrality
- Reversed at lungsHCO₃- moves back in; reforms CO₂ for exhalation
Haldane effect
- Deoxygenated Hbcarries CO₂ more easily
- Oxygenated Hb at lungsreleases CO₂ for expiration
Control of breathing
- Medullary respiratory centersventral respiratory group (inspiration) + dorsal respiratory group
- Pontine centersfine-tune rhythm
- Central chemoreceptorsin medulla; sense CSF pH (which reflects CO₂); main driver of ventilation
- Peripheral chemoreceptorscarotid & aortic bodies; sense O₂, CO₂, pH; respond to severe hypoxia
When things go wrong
- Hypoxemialow PaO₂; many causes
- Hypercapniahigh PaCO₂; ventilation failure
- CO poisoningbinds Hb 200x stronger than O₂; pulse ox falsely normal
- MethemoglobinemiaFe in Hb oxidized to Fe³⁺; cannot bind O₂
- Respiratory acidosisCO₂ retention → H+ rises → pH falls
- Respiratory alkalosishyperventilation → CO₂ blown off → pH rises
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