What is the role of the recurring phrase "Ah! Yellow River" in the poem "Ode to the Yellow River"?

The sentence "Ah! Yellow River!" appears three times in the poem, changing the main part of the poem from "Ah! Yellow River! You are the cradle of the Chinese nation" to "will be carried forward under your nurture / "Growth" is clearly divided into three levels. The three levels are led by "Ah! Yellow River!", respectively praising the Yellow River for nurturing the Chinese nation, defending the Chinese nation, and inspiring the Chinese nation.