Equilibrium is maintained by the hypothalamus, which is a part of the brain. The hypothalamus receives signals via sensory and interneurons from throughout the body. The hypothalamus then evaluates if the levels of different hormones are at the correct level. If not, then the hypothalamus sends signals through interneurons into the pituitary gland, telling the pituitary gland (both portions of it) to produce more or less of a chosen hormone.
If equilibrium was lost then there would be a hormone imbalance. The person with the imbalance would have many side effects such as: minor moodiness, depression, intense feelings, etc. To regulate the levels, there are a few options, such as, hormone therapy.
The cholesterol equilibrium is maintained through positive feedback from organs to the brain. The brain receives the signals and tries to release more of the cholesterol from the body systems. These signals travel through neurons to several portions of the brain.
If equilibrium is lost, there may be a build up cholesterol in the arteries and other vessels used for the transport of blood. To correct the issue, a lower dietary amount of cholesterol should be maintained. Sometimes surgery is required.
There's a balance between oxygen and carbon dioxide in all of the blood cells in the body, and this is the equilibrium that is maintained in the heart.