Lecture 14: Big Ideas I: Physical Considerations

Surface Area:Volume Ratio

As size increases, surface area:volume decreases

Reasons why this ratio is important

Gas exchange

Regulation of body temperature

Water balance

Absorption of nutrients and disposal of wastes

Fick's Law of Diffusion: Q=DA(P1-P2)/L

A=Cross-sectional area on which diffusion occurs

P1 & P2: Partial Pressures of the gas at two locations

D=Diffusion Coefficient- depends on diffusing substance, medium, temperature

L=Path Length between locations

Q=Rate of diffusion

Partial Pressure Gradient: (P1-P2)/L

In order to facilitate gas exchange, a large surface area, large partial pressure difference, and small L is optimal.

Surface Area: Gills, Lungs, Tracheae (Insects)

Partial Pressure Difference: Ventilation & Perfusion (P's)

Reducing L: respiratory/circulatory systems

Ventilation: Active movement of respiratory medium over gas exchange surfaces

Perfusion: Active circulation of blood over gas exchange surfaces

Countercurrent Flow & unidirectional flow: Used by bony fish to maximize gas exchange

Gills made of gill filaments covered by lamellae, thin folds & site of gas exchange

Countercurrent flow allows there to be a gradient across the entire region of gas exchange

Birds use unidirectional air flow to maintain partial pressure gradient.

Anterior & Posterior air sacs

Temperature

Big organisms must remove heat from metabolism; small organisms must conserve heat

Rate of biochemical reactions doubles with 10 degree Celsius increase in temperature

Organisms either regulate body temperature so environment doesn't affect them, or live under more narrow conditions & accept environmental influence