What is a cycle? in biology

a biological life cycle is a series of changes in form that an organism undergoes, returning to the starting state.

The ways in which an element or compound such as water moves between its various living and nonliving forms and locations in the biosphere is called a biogeochemical cycle. Biogeochemical cycles important to living organisms include the water, carbon, nitrogen, phosphorus, and sulfur cycles.

I wasn’t for sure what you meant but I hope this helps you :)

its 42

i say its d?

they deliver oxygen to the blood