Open and closed vascular systems: As animals increased in size and complexity during the Cambrian, specialized circulatory systems evolved. Open systems allowed fluid to bathe tissues directly, while closed systems developed vessels and true blood, enabling efficient long-distance transport of oxygen and nutrients.

Blood, a specialized fluid carrying nutrients and waste, evolved around 520 million years ago in multicellular organisms. It evolved to feed internal cells and carry away waste. As oxygen levels rose and multicellular life emerged, blood evolved to transport oxygen and nutrients to cells.