It'll make your heart work harder, possibly thicken the cardiac muscle but it won't be nearly as effective on your lung capacity and heart stroke volume (which will increase with aerobic work) and lower your heart rate.<br />
I'm not a doctor or fitness professional so i may be completely off point. But that's how I understand it.
It's better than doing nothing of course, but better still is a standard cardio workout. Best is both combines. <br />
Of chief concern is bulking up. Increasing body mass makes your systems work harder to support you. Intense bulking without strengthening your cardio can result in complications.
Increased mass means more work for your heart to pump blood, which can result in heart failure if you put your cardiovascular system under stress. Natural body building is unlikely to result in significant problems unless it is very extreme, since the act of training helps strengthen the heart to compensate. The use of steroids to achieve greater mass is for more dangerous since it applies the growth before the heart is ready. Weight gain from fat has similar effects.
Yes, the increased blood flow needed to hold heavier ob<x>jects makes the heart work harder, therefore it exercises the heart quite effectively.