Can you help me to answer these questions about Biology ?

1. To surive, cells must have continuous supply of O2, nutrients & other useful substances. Unicellular cells are small in size, they are very close to the environment, they can absorb and remove materials by osmosis, active transport, and diffusion easily.

But for multicellular organisms, diffusion and other method of transport is not fast enough to provide sufficient materials for maintaining life and metabolism. So circulatory system is necessary.

2. It mainly consist of blood, blood vessels and the heart.