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• Use “ “ for phrases o [ “pediatric abdominal pain” ] • Use – to remove results with certain terms o [ “abdominal pain” –pediatric ] • Use OR to account for alternate terms o [teenager OR adolescent ]
胎儿的某些血管包含在胎盘绒毛中,绒毛延伸如子宫壁。母体的血液进入到绒毛周围的空间(绒毛间隙)内。在绒毛间隙中的母血仅由一层薄膜(绒毛膜)与绒毛中的胎儿血分隔开。母亲血液中的药物可以透过这层薄膜进入绒毛中的绒毛毛细血管,再经脐带进入胎儿体内。