Why is a keystone species' role in the ecosystem compared to the keystone brick's role in the arch?

The picture represents a structure of the respiratory system. which is the function of this structure? to bring air into the body to exchange oxygen with carbon dioxide to carry air to the lungs to release oxygen from the body

This is a situation in which genes are attached to an organism's sex chromosomes; the sex of an organism influences the expression of a gene.

Why do the circulatory systems of land vertebrates have separate circuits to the lungs and to the rest of the body?