Analogy: Land : River :: Body : Water

The answer is water.

Land is to river as body is to water.

Reasoning: Land is the solid substrate through which a river (a body of flowing water) courses. Similarly, the body is the physical structure that contains water — which makes up ~60% of human body weight, filling cells, blood, and interstitial spaces. Both relationships describe a container/contained dynamic between solid structure and essential fluid.