The heart and lungs of mammals are closely associated. You can find the lungs located on either side of the heart and often toward the dorsal side of the thoracic cavity. The lungs are soft, spongy tissue that are attached to the heart by the pulmonary artery and veins. Blood leaving the heart from the right ventricle will travel to the lungs where it receives oxygen before returning to the heart via the pulmonary vein, which enters at the left atrium.
The diaphragm is a thin sheet of muscle located beneath the lungs and separates the abdominal cavity from the thoracic cavity. When the diaphragm lowers, the chest cavity expands creating less pressure inside than outside, this causes the lungs to inflate. On fresh specimens, a straw placed in the trachea can be used to inflate the lungs.
The image below shows a rat specimen.