When using terms such as medial, distal, etc, you must have another objest to compare the object to. For example, the heart is medial to the axilary region (tip of shoulder). It is quite likely that the heart is "medial."
Medial basically refers to being in the middle. Lateral basically refers to on or at the sides. The lungs are located on either side of the heart, so they are lateral to the heart.