Collagen is made of protein. It provides the connective tissue, the physical framework and the principal support structure of our body. Collagen combined with calcium defines the shape and strength of the bones. Flexible collagen forms ligaments and joints. Close to 35% of all the protein in the body consists of collagen. Translucent collagen forms the cornea, the lens of the eye.
It defines the strength and flexibility of our skin and all the tubes in our body including our blood vessels and gut. Our muscle cells do the contracting which makes the muscle shorter and thicker (bulging biceps), but collagen runs right through the muscle from the tendon at one end to the tendon at the other and enables the muscle to work the joints.
Like all proteins, collagen is made of amino acids linked together. If you boil up collagen in hot water long enough the water breaks down the long fibrils of amino acids into shorter chains. That's what we know as gellatin.
The best way to provide the body with replacement collagen is by drinking bone broth. Here's a recipe: