Physical Considerations

Surface area to volume ratio

In a geometric object, SA/V decreases as volume increases

Important for life; many processes decline in efficiency as SA/V decreases

Organisms aren't geometric shapes (cubes or spheres), as they have many mechanisms to do this

Fick's Law of Diffusion

Diffusion is a slow process

Controlled primarily by 3 factors

Pressure gradient (P1-P2)

Surface area (A) #

Path length (L)

Higher pressure gradient increases diffusion

Air inside of lungs is at a higher pressure than the blood

Greater surface area increases diffusion

Gills, lungs, and trachea accomplish this

Ventilation increases P1

Perfusion decreases P2

Larger path length decreases diffusion

Circulatory system decreases the path length; gases only have to travel a short distance

Countercurrent flow in fish

Oxygenated water and deoxygenated blood run in the opposite direction

Water flow maximizes P2

Lamellae minimize L

Bird lungs

Unidirectional air flow so there are always two breaths in the respiratory system

This maximizes the pressure gradient