The reading most learners meet first is yù, the appeal word in 呼吁. This one is xū and is about breath, not speech. If the line is about sighing or panting, it is xū.
usage
Written language, and bound: it turns up in 长吁 (to heave a sigh), 长吁短叹 and 气喘吁吁 (panting). For everyday sighing, say 叹气.
Left mouth radical, the indexing element. The sigh named here is breath pushed out through the mouth, which puts 吁 with 叹 (sigh), 呼 (breathe out) and 嘘 (to shush).
Right 于 supplies the sound, yú giving xū in this reading, the same pairing seen in 盱. 于 was drawn as breath rising and curving, which suits a character for a long exhalation.
