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1. Thin walls for short diffusion path 2. Narrow, so have a large surface area to volume ratio 

 - An electrical impulse travels down the axon of a pre-synaptic neurone. - The movement of this electrical impulse initiates the release of chemical messengers which are...

Natural selection is when certain characteristic are 'selected' for. Each species have simiar characteristics but differences in genes means that their characteristics can vary for example individual bird...

Diffusion is the spreading out of particles of any substance in solution or of gas resulting in a net movement of an area of high concentration to an area of low concentration. Diffusion is key to the sur...

Four characteristics of an efficient respiratory surface are: thin epithelium, large surface area, abundant capillaries, ventilation mechanism.