Interdependency is when several things are reliant upon each other to function. For example, in a car, the engine and battery are interdependent; as the motor charges the battery, the battery sparks the engine.
In the human body, the respiratory, circulatory, digestive, and excretory systems are interdependent. The respiratory system provides necessary oxygen to the rest of the systems that they cannot get on their own. This keeps the cells alive. The circulatory system is needed to transport nutrients from the digestive system and respiratory system about the rest of the body, but without the nutrients, the heart could not beat. The digestive system absorbs sugars and nutrients, breaking them down for the body to use, but can only get oxygen from the circulatory system. All of these systems produce waste products, and the remaining system, the excretory system, processes and removes these toxins and wastes. This prevents other systems from breaking down due to poisons.
None of these systems can function alone, and if they could, you might expect them to leave the body on their own!