The volume of an object increases as the cube of its linear dimensions, while the surface area increases only as the square of the linear dimensions. It is important that as organisms increase in size they alter their diffusion mechanisms to compensate for the reduction in surface area to volume ratio.

In this diagram for example the distance from a particular point on the surface to the centre has been doubled by the increase in size so one would expect diffusion to take twice as long to bring material to the centre, but in the smaller object each unit of material on the inside is effectively being supplied by six units of equivalent surface area, whereas in the large object the surface area per unit of volume is only 3 units, this must further limit the rate of diffusion to the central regions of the object. Also remember that if a waste material is being produced within the organism then it will take longer to diffuse out and will occur in much higher concentrations in the deeper layers of the structure.

In cases like this the surface area may be increased by folding of the surface, or by flattening the shape. This maintains the volume but will greatly increase the surface area for the exchange of materials. In the example above a spherical object may have its surface area increased by progressive flattening. Naturally diffusion cannot make the molecules run uphill, that is from a lower concentration to a higher one, and as the concentrations in adjacent areas become more and more equal the net rate of movement of the molecules between the adjacent areas will rapidly decrease to zero.

In the Platyhelminthes (flatworms) the body is flattened and the animal glides over the surface of the substrate using cilia. In other groups such as the true worms, external body foldings are used to increase the surface area for exchange. In the sea cucumber these foldings are positioned in the anus so that they can be retracted from harm if they are attacked. (Soft frilly bits tend to attract the attention of predators.)

Once the body assumes a cylindrical shape of more than a millimetre in radius, the rate of diffusion becomes too slow to adequately supply the tissues with oxygen and remove their wastes. As a result the evolution of transport systems to carry materials to and from the exchange surfaces occurred. The exchange surfaces themselves are not static, they require a flow of external medium over their surface to ensure rapid diffusion into the organism.

Consider an organism absorbing material from the exterior at a rapid rate, the external solution soon becomes depleted of materials in the region next to the body surface and this can only be avoided if the animal moves constantly into a new area, or if the surface is moved to cause the external medium to flow over it. Ventilation movements of these kinds are common features of exchange surfaces. The other problem of a large organism is to transfer materials from one area to another, consider how long it would take for nutrient molecules to diffuse to your big toe from the small intestine. Clearly a transport system is required to move materials from one region of the organisms to another.