Many organs in the body constantly adjust to compensate for your specific needs at any one time. Your heart speeds up and slows down as needed. The sugar and fat in your blood increase when you need a boost of energy. Your lungs work harder when your body needs more oxygen. More blood is directed to the muscles when exercising. You develop sexually once you've reached a certain age and have stored up enough energy and raw material to do so. When something threatens you or your family, a surge of energy flows through you, supplying you with an increased strength to combat the threat. How does all this happen so flawlessly and automatically. The answer lies in the endocrine system, a series of glands that produce hormones (mainly proteins) that travel through the blood to stimulate certain organs to act a certain way. Very small amounts of hormones released into the blood produce dramatic effects.
These messengers are not as fast as nerve impulses, but they are fast enough. Think of the amount of time it takes your heart rate to increase when something scary startles you. Because of the potency of hormones, if a certain hormone remains in the blood for too long or too much of it is produced, the consequences could be disastrous. The hormones are kept in balance by many systems of feedback. When an organ stimulates the production of a hormone, this hormone is released into the blood stream to cause the desired effect. The hormone eventually reaches the organ that produced the stimulus, which caused the hormone to be produced in the first place. The organ detects the hormone in the blood and as a result stops stimulating the production of the hormone. This way the hormones are kept in check and thus the body is kept in balance. Hormone production is stimulated by other hormones from other endocrine glands, or by the hypothalamus, a portion of the brain that detects hormones in the blood and receives nerve impulses from the brain.
Your body is extremely sensitive to hormones. It has developed a complex system of feedback to control the levels of each and every hormone in the body. This keeps the body in a balanced state, and only allows the hormone to be used for a limited period of time. When you introduce anabolic steroids, you're introducing a hormone the body is not prepared for. The body has no way of turning off the stimulus for the production of anabolic steroids, because the user keeps injecting them into his/her body. The body thus has no protection against the harmful side effects caused by anabolic steroids.
An entertaining analogy: Your hormones are similar to politicians. They have a term limit. They can help produce dramatic changes throughout the land that is your body. But their power is always limited by feedback from other organs, as the power of politicians is limited by feedback from different branches of government and by the law under which they operate. Taking anabolic steroids is like introducing a politician with an extremist personal agenda, no concern for others, complete immunity to the law and the powers of superman. The drug, like a crooked indestructible politician, causes dramatic events throughout the land that is your body, and your body has no way of controlling it. Although the analogy is ridiculous, the result can be devastating... (see Anabolic Steroids under Drugs)
The liver is the only internal organ that will completely regenerate itself if partly damaged. Up to 25% can be removed and within a short period of time it will grow back to its original size and shape. That is, as long as the remaining tissue is healthy.