1.1 Understand why many animals have a heart and circulation (mass transport to overcome limitations of diffusion in meeting the requirements of organisms).

diffusion: the movement of particles down a concentration gradient.

the larger the surface area to volume ratio the more rapidly particles will diffuse. smaller organisms have a large surface area to volume ratio, as well as a shorter distance for particles to travel this makes diffusion a sufficient transport method to obtain nutrients and excrete waste products.

ref. Ficks law

advantages of a double circulatory system:

blood can pass through areas of the body at different speeds. slowly through regions of gaseous exchange; maximising transfer. Then vigorously around the body; enabling organism to be more active.

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