As a general rule, the lung is visible thru the heart when there is minimal atelectasis.
X-rays have to travel thru (from front (ventrum) to back (dorsum)) in this order :
patient front > skin > muscle > bone > lung > heart muscle > blood in heart >
heart muscle > lung > bone > muscle > skin > patient back > to x-ray plate.
The more atelectatic, the more denser the pulmonary tissue, and therefore the x-ray
has higher potential for being deviated / obliterated / absorbed. The less atelectatic,
the less chance of deviation / obliterated / absorption.