Physiology of Finfish and Shellfish (2+1)

2.1.7. Respiration of water

Some of the simplest arrangements for respiratory gas exchange are found in aquatic animals. Many small organism obtain oxygen by diffusion through their body surface, (protozoms) without having any specialized respiratory organs

$$FO_2 \ = \ {VC_2r^2 \over 6k}$$

Where FO₂ concentration of O₂ at surface in fractions of an atmosphere

VC₂ rate of oxygen consumption Cm³ of tissue)

r = radius of the organism

k = Cm³ O₂ diffused /Cm ² area at / atm / Cm^-1

and without circulating blood. Larger and most complex animals often have specialized surfaces for gas exchange and also a blood system to transport oxygen more rapidly than diffusion alone can provide.

Aquatic mollusks: extenedium

Birds human: ventilation lungs