How do all systems in the human body work together?
The brain is hardwired at birth by chemical and physical laws to a series of feedback systems that regulate all the various systems of the body—circulatory, digestive, reproductive, nervous, lymphatic, muscular, etc. etc. These feedback systems regulate bodily functions in three steps: first, a set point is established, the ideal strength of a given element; second, a detector that measures the quantity of that given element, constantly monitors the level; third, a correctional system returns the element to its acceptable level. Some of these elements are body temperature, glucose level, pH level (acidic or basic), fluid levels, etc. So, for example, if the blood is carrying too much sugar (the accepted level of glucose is exceeded), the pancreas excretes more insulin, until the level is back to the set point, then shuts off; if the body temperature exceeds its set point (in most, 98.6 degrees), the sweat glands secrete moisture, and the body cools because evaporation is a cooling process. The various systems of the body, then, work together to maintain acceptable levels of chemical, hormonal, etc. elements through the co-ordinated efforts of the regulatory feedback system. Since each system is organically and chemically linked to all the others (in fact, the separation of the body's mechanisms is a taxonomy for our convenience, not a "reality"), the regulatory system keeps all organs, vessels, etc. working together.