The body does not perform as a cluster of separate mechanisms, but rather a cascade of events that all starts with proper control by the nervous system.
The nerves that travel through and control every function of your body originate at the spinal cord and their transmission may be disrupted if the joints of the surrounding spinal column are not moving properly. This disruption in biomechanical integrity combined with altered physiological function is what chiropractors call a subluxation. Below is a chart that illustrates the relationship of the spinal nerves exiting the vertebra branching off to the various organ systems. You can see why it is not uncommon for a chiropractor to treat a patient with mid-back pain who also suffers from irritable bowel system, a patient with a subluxated sacrum who has been unsuccessfully trying to become pregnant, or a patient with an upper back fixation and acid reflux.