The longest bone in a human's arm is the humerus. It extends from the shoulder to the elbow and plays a crucial role in the arm's structure and mobility. The humerus articulates with the shoulder joint at the proximal end and with the radius and ulna at the elbow joint. It is responsible for a wide range of movements, including lifting, throwing, and supporting the arm's weight. Injuries to the humerus can result in significant impairment of arm function, highlighting its importance in daily activities and physical tasks.