Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 42.7 Adaptations for Gas Exchange Include Pigments That Bind and Transport Gases
Efficient gas exchange depends on respiratory pigments like hemoglobin that dramatically increase the oxygen-carrying capacity of blood. These pigments, along with CO₂ transport mechanisms and specialized adaptations in diving mammals, ensure proper oxygen delivery even under high demand or low-oxygen conditions.
1. Coordination of Circulation and Gas Exchange
- In alveoli, O₂ diffuses into blood, CO₂ diffuses out
- In systemic capillaries, O₂ diffuses into tissues, CO₂ diffuses into blood
- Blood returning to lungs has low PO₂ and high PCO₂, restoring balance via alveolar exchange
2. Respiratory Pigments
- O₂ solubility in plasma is low, so most O₂ is carried by respiratory pigments
- These pigments (e.g. hemoglobin) increase O₂ capacity from ~4.5 to ~200 mL/L
- Hemocyanin (with copper) is used by mollusks and arthropods
- Hemoglobin, found in vertebrate red blood cells, has 4 heme groups, each with an iron atom that binds one O₂ molecule
3. Hemoglobin and Cooperativity
- Hemoglobin binds O₂ reversibly
- Shows cooperative binding: binding of one O₂ increases affinity for others
- Likewise, release of one O₂ lowers affinity of others → efficient unloading in tissues
- Curve of O₂ saturation is steep in tissue range: small PO₂ drop = large O₂ release
4. Bohr Shift and pH
- CO₂ from cellular respiration forms carbonic acid → lowers pH
- Lower pH reduces hemoglobin’s affinity for O₂
- This Bohr shift promotes O₂ unloading where CO₂ production is high (active tissues)
5. Carbon Dioxide Transport
- About 7% of CO₂ is dissolved in plasma
- Most CO₂ enters red blood cells, reacts with water (via carbonic anhydrase) to form H₂CO₃
- H₂CO₃ dissociates to H⁺ and HCO₃⁻; H⁺ binds hemoglobin, buffering pH
- Most HCO₃⁻ exits RBCs and is carried in plasma
- In lungs, reactions reverse and CO₂ diffuses out into alveoli for exhalation
6. Diving Mammal Adaptations
- Weddell seals and whales can dive for 20–120+ minutes
- They store O₂ in high blood volume and myoglobin-rich muscles
- Blood flow during dives is restricted to vital organs; muscles switch to fermentation when O₂ runs low
- Diving reflex (slowed heart rate, restricted peripheral circulation) is present even in humans
- These traits likely evolved through natural selection for underwater foraging
In a Nutshell
Gas exchange efficiency is boosted by respiratory pigments like hemoglobin, which use cooperative binding to deliver O₂ where it’s needed most. CO₂ is carried mostly as bicarbonate, helping buffer blood pH. Diving mammals maximize O₂ storage and strategically conserve it, illustrating remarkable evolutionary adaptations to extreme conditions.