Structure of collagen. Illustration of the structure of the protein collagen, as found in connective tissue. Across top, the smallest unit shown here is the triple alpha helix of the protein (diameter of 1.5 nanometres). These helices combine to form a collagen molecule (across centre), which in turn combines to form a collagen fibril (across bottom, diameter of 50 nanometres). Collagen is the most abundant protein in the human body and has a tough fibrous tensile strength. It is found throughout the body's connective tissues (skin, cartilage, bone, ligaments and tendons) providing structural support. For this artwork with labels, see image C023/8713.