You Have to Have a System
The human body is like a complex organization that has an important job to get done on a tight deadline. In order to get everything done perfectly and on time, it has to use a system. Actually, the human body uses many systems that work side by side.
Some of the body's systems are directly connected to the heart, while others are not. Of course, the heart is like the president of the organization. Even if it is not directly involved in the system, it still plays a part. Obviously, if the heart isn't working, nothing else is working either. The heart actively participates in the circulatory system, while it just keeps an eye on the respiratory and excretory systems.