Elastin

• Random network of polypeptide chains, found where living things need to stretch/adapt their shape; coiling/cross-linking make it strong and extensible (skin can stretch around our bones and muscles because of elastin)
• Allow our alveoli to inflate and deflate, the bladder expand to hold urine, and our blood vessels stretch and recoil as blood is pumped through them (help maintain the pressure wave of blood as it passes through)