The given options are "cardi", "lung", "heart", "skin", and "hair". Out of these options, "heart" is the correct answer because it is the only option that is directly related to the cardiovascular system. The term "cardi" is a prefix meaning heart, "lung" refers to the respiratory system, "skin" is related to the integumentary system, and "hair" is a part of the integumentary system as well. Therefore, "heart" is the most appropriate and relevant answer.