Homeostasis is the epitome of regulation. Is there anything more that needs to be said? Keeping trillions of cells in an animal working at the same time is an amazing feat. Regulation may be difficult, and it requires lots of energy, but animal life could not exist without it. Sensors throughout the body are like a natural twitter feed. Their entire job is providing perpetual status updates on the levels of things throughout the body, from iron levels to water levels. All of the tweets are processed by the brain, which decides if anything must be adjusted to maintain the perfect balance. If even the smallest solute level is disrupted it can wreak serious havoc on the cells. In addition to cellular level regulation, animals have entire systems that they must coordinate, and the only way to bring them all together is through extensive coordination and regulation. Simply put, animals must regulate everything if they are going to thrive as large multi-system organisms.
Two words: sinoatrial node. It is the pacer for the entire body. If there was ever a master regulator of the entire body, this is it. If the heart beats too fast or too slowly it can be problematic. Most of all, it is problematic if it doesn't beat at all.
The regulation of gases in the blood falls under the category of homeostasis, and, like circulation, it is a chief point of regulation. It is so important, in fact, that an animal's brain doesn't even trust them to do it. It is taken care of subconsciously by the medulla and the pons. If you have ever tried to take over your own breathing, it can be a daunting task. Try for one minute to breathe regularly but consciously. It can be difficult to keep as even a rhythm as your brain can. By the end of the minute, your brain is probably dying to take over again like someone who has let someone else drive his or her car. Regulation of respiration is just too important to be handled by anyone but the brain.