Amphibians and mammals both have circulatory systems to transport oxygen and nutrients to tissues. However, their circulatory systems differ due to their metabolic demands and physiology.

Compare and contrast the circulatory systems of amphibians and mammals.

Octopamine is a key signaling molecule in insects. It is produced by neurosecretory cells in the central nervous system but acts as a hormone to stimulate metabolic activity in distant flight muscles during flight.

Explain how it is possible for octopamine to have an effect on flight muscle cells located far from where it is released.

Crabs possess a copper-containing respiratory pigment called haemocyanin in their hemolymph. Under conditions of active exercise, increased carbon dioxide levels in the hemolymph reduce the affinity of haemocyanin for oxygen.

Suggest a mechanism by which carbon dioxide could reduce the oxygen affinity of haemocyanin.