Although this topic examines gas exchange the exchange of heat and water have the same relationship with surface area and volume. Heat/water loss is affected by surface area : volume. In large organisms heat/water loss is less than in small organisms. This is because the organism has :

• a low surface area : volume
• longer diffusion pathways
• longer distances in general
• probably more insulation so it is harder for the heat to escape.

Conversely, in smaller organisms heat/water loss is greater than in large organisms. The organism has much shorter pathways; all its internal organs are closer to the surface and have less insulation.