The word "呼吁" is formed by combining two characters, each containing the mouth radical (口) to indicate relation to vocal expression. "呼" consists of 口 and the phonetic component "乎", meaning to call out, while "吁" consists of 口 and the phonetic component "于", meaning to sigh or appeal. Together, they create a compound word where both characters contribute semantically to convey the meaning of urging or pleading through voice.