Changes led to the development (in lungfish) of a double circulatory system from a single circulatory system. These changes provided a more efficient circulatory and respiratory system to fuel the ...

Red blood cell Carry oxygen. Contain haemoglobin (rich in iron) to carry oxygen. Biconcave shape to provide a large surface area for the diffusion of oxygen. No nucleus to provide more space for ...

The heart is a muscular organ that pumps blood around your circulatory system. The circulatory system is the heart and all the blood vessels in the body which carry cells and substances to all its ...