The Balkans is one of the few areas of Europe where free flowing natural river systems still exist.

巴尔干半岛是欧洲少数几个仍留存着自由流动的天然河流系统的地区之一。

In one building, a network of air flow systems has been put in place to keep the air smelling as fresh as possible.

在一栋建筑中，一个气流系统网络已经就位，以尽可能保持空气清新。

In one experiment, the scientists increased the levels of GDF11 in aging mice by surgically connecting the blood flow systems of young mice to the old mice.

在一个实验中，科学家用外科手术将年轻小白鼠和老年小白鼠的血管系统连接起来，增加了老年小白鼠血管的GDF11含量。

In one experiment, the scientists increased the levels of GDF11 in aging mice by surgically connecting (or infusing) the blood flow systems of young mice to the old mice.

在一个实验中，科学家用外科手术将年轻小白鼠和老年小白鼠的血管系统连接起来，增加了老年小白鼠血管的 GDF11含量。