Elastin is a fibrous protein that coils and recoils like a spring within the elastic fibers of connective tissue and accounts for the elasticity of structures such the skin, blood vessels, heart, lungs, intestines, tendons, and ligaments. Elastin, also called elasticin, functions in connective tissue together with collagen, to which it is structurally related. Whereas elastin provides elasticity, collagen provides rigidity to connective tissue. Elastin contains about 30% each of glycine and leucine, and about 15% of proline. It is normally no longer made after puberty and aging begins.