Gas Exchange

$\text{Rate of diffusion} \propto \frac{\text{surface area} \times \text{concentration difference}}{\text{thickness of exchange surface}}$

Efficient gas exchange requires: large SA, thin surface, steep concentration gradient.

Path: trachea → bronchi → bronchioles → alveoli

Alveolar Adaptations

• Millions of alveoli → huge surface area
• One cell thick → short diffusion distance
• Dense capillary network → maintains gradient
• Moist lining → gases dissolve
• Ventilation → maintains concentration gradient

Ventilation

• Inspiration: diaphragm contracts/flattens, external intercostals contract → volume ↑ → pressure ↓ → air in
• Expiration: diaphragm relaxes/domes, internal intercostals contract → volume ↓ → pressure ↑ → air out

• Water flows over gill filaments (lamellae)
• Countercurrent system: blood flows opposite direction to water
• Maintains concentration gradient along ENTIRE length of lamella
• More efficient than parallel flow (up to 80% O₂ extraction)

• Spiracles → tracheae → tracheoles
• Air delivered directly to cells (no blood involved)
• Diffusion is the main mechanism
• Active ventilation in larger insects (abdominal pumping)
• Water in tracheole tips removed during activity → air reaches closer to cells

• Stomata: gas exchange with atmosphere
• Spongy mesophyll: large air spaces → large internal surface area
• Guard cells: open/close stomata (balance CO₂ uptake vs water loss)

• Fick's law: rate ∝ (SA × Δconc) / thickness
• Lungs: alveoli; large SA, thin, good blood supply, moist, ventilated
• Fish gills: countercurrent flow → steep gradient maintained
• Insects: tracheal system; air directly to cells
• Plants: stomata and spongy mesophyll