Red blood cells are very small which allows them to pass through narrow capillaries, they have a biconcave shape which increases surface area allowing oxygen to be diffused more effectively, they contain haemoglobin which binds with the oxygen, and they do not contain a nucleus which leaves more space inside to hold more haemoglobin.
Nerve cells are very long to enable them to deliver messages long distances, they have synapses at the end of each cell to make sure the message can go in only one direction, and they have a myelin sheath which increases the thickness of the cell and makes the electrical impulse "jump" thus increasing the speed the messages are sent at.
Hope this helps :-)
