The doctor asked him to take a deep breath and then exhale slowly.

exhale 是动词，由前缀 ex-（表示"向外"）和词根 hale（来自拉丁语 halare，意为"呼吸"）组成，字面意思是"向外呼气"。exhale 指"呼出气体"的动作，与 inhale（吸气）相对。