ABSTRACT
Convection is a highly effective means of transport, and most processes could not occur efficiently without it. Try to imagine how different your life would be were it not for convective transport within your body. A quick calculation of O2 diffusion (penetration time) through your body assuming a maximum body thickness, 2∆, of 18 cm and a fairly high diffusivity of 1 × 10−8 m2/s:
t Dab
= = ×
=− ∆2
2 83 643 0 0
3 643 1 10 61
( . ) ( . 9)
( . ) ( ) ,000 s (10.1)
shows that sustaining a vigorous life in three dimensions would be impossible. That is why organisms exploiting O2 diffusion alone to sustain respiration are often no larger than a small insect.
