Tendons are primarily made of which substance?

Tendons are primarily made of collagen, which is a fibrous protein that provides strength and structure. Collagen fibers give tendons their tensile strength, allowing them to withstand the pulling forces generated during movement. This characteristic is essential for the proper functioning of muscles and joints, as tendons connect muscles to bones and transmit forces generated by muscle contractions.

In tendons, the collagen is organized in a manner that allows for efficient distribution of mechanical stress, enabling them to effectively function in a wide range of activities, from walking to jumping. The other substances mentioned do play roles in the body—actin is primarily involved in muscle contraction, elastin provides elasticity in tissues that need to stretch, and keratin is a structural protein found in hair, skin, and nails. However, none of them serve as the primary structural component of tendons, making collagen the correct choice.