This is the 7004th most frequent Chinese word.
生命线
Lifeline; something essential for survival or success.
In this sentence, '生命线' metaphorically describes transportation as crucial for maintaining the city's function and vitality.
交通是城市的生命线。
Transportation is the lifeline of a city.
'生命线' here can refer to an actual physical element or metaphorically to the crucial factor supporting the patient's health.
通过手术，医生成功修复了患者的生命线。
Through surgery, the doctors successfully repaired the patient's lifeline.
In this case, '生命线' metaphorically represents something essential for the sustainability and success of the person's career.