Surface Area to Volume Ratio


Surface area: volume ratio crops up in many exam questions. They can be questions relating to trees, plants, fish or mammals. The question will be about the size/shape of the particular organism or how its size/shape is adapted to its usually adverse surroundings.


Exchange In Organisms

A small organism, like an amoeba, has a large surface area: volume ratio and so it can take all the oxygen it needs by diffusion across the body surface. However, a large organism, like a mammal, has a much smaller surface area: volume ratio, so it cannot get all the oxygen it needs in this way. (A large surface area: volume ratio is preferable for carrying out exchange of substances). Such large organisms need special respiratory organs such as lungs for taking in oxygen.




Heat and water loss

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 longer pathways and longer distances, 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.


Calculating the ratio


Large Mammals have difficulties regulating body temperature in hot climates due to:


Blood vessels near the surface of the skin help to regulate body temperature by:


The importance of a larger body size and mass to mammals in colder climates are:



Fish Gas Exchange

Structure of Respiratory Surfaces


Fish Ventilation


Ventilation in Mammals

Very small organisms such as those consisting of a single cell, have no special tissues, organs or systems for gaseous exchange. Mammals are large, multi cellular organisms and they have a complex system for gaseous exchange. Mammals needs such a system single celled organism does not.

Single celled organisms

  • Large surface area to volume (ratio) for diffusion;
  • short diffusion pathway ( to all parts of organism)
  • oxygen/ carbon dioxide diffuse in and out.
  • Small surface area to volume
  • long diffusion pathway
  • waterproof/ gastight skinneed internal gas exchange surface which is moist with a large s/a

Maintenance of Breathing: At Rest (Nervous System)

During Exercise

Breathing In: