AS induced growth is relegated mainly to skeletal muscle cells. It is also resultant typically due to break down of that muscle. Thus, it would not effect muscles like the heart and diaphragm as they are made up of skeletal muscle mass, nor are you breaking them down by weight resistant excercise. The following summarizes the mechanism through which AS works.
"One of these primary metabolic changes is increased protein synthesis and nitrogen retention, leading to increased size and strength of the skeletal muscle cell. The connection of a steroid molecule and receptor site is fleeting: once the steroid molecule delivers its cellular command, it moves on to other receptor sites, delivering its message over and over until it is converted into another compound or is excreted in the urine. This capacity of androgens to be converted into other substances by chemical reactions and enzymes within certain tissues of the body will be extremely significant later when we discuss potential side effects (see Health Risks section).
Anabolics also enhance size and strength through a different mechanism. Intense training causes the body to produce increased levels of cortisol, a catabolic substance that causes muscle tissue breakdown. Anabolics have an anticatabolic effect; that is, they prevent the breakdown of muscle tissue by displacing cortisol from its receptors. Consequently, recovery processes are speeded up and injuries, including the microscopic muscle damage incurred from heavy training, heal faster. Some authorities believe that the anticatabolic properties of steroids may be of equal significance to the anabolic properties. "