Why does blood return from the lungs
to the heart?

Fossils can only be formed when? if mud, silt or tar is in the area to preserve the fossil ierosion does not occur and allows the fossil to form oxygen and microorganisms cannot decay the bones the area is dry and does not contain mud or silt

Depending on the organism the number of in a cell may change

What is responsible for the uneven heating between the poles and the equator on any given day