Final answer:
The lungs are not perfectly bilateral because the heart takes up space on the left side of the thoracic cavity, leading to the left lung being smaller with only two lobes compared to the right lung's three lobes. The pleurae, surrounding the lungs, ensure smooth gliding and protection during breathing.
Step-by-step explanation:
The human lungs are not perfectly bilateral due to the positioning of the heart within the thoracic cavity. The right lung is larger and contains three lobes, while the smaller left lung has only two lobes to accommodate the heart. This asymmetrical arrangement allows the heart to fit snugly beside the left lung, creating the cardiac notch — an indention in the surface of the left lung. The lungs connect to the trachea via the right and left bronchi and are surrounded by pleurae, which prevent friction and protect the lungs during breathing. The pleurae are essential as they ensure the smooth gliding of the lungs against the thoracic wall and other structures while breathing.
Moreover, the shape and size differences between the right and left lungs are accounted for by evolutionary adaptations and the constraints of other organs sharing the thoracic cavity. The right lung is shorter and wider than the left lung, reflecting the need for space to house the liver, which resides just beneath it. The diaphragm, a muscular structure involved in breathing, lies inferior to the lungs and marks the end of the thoracic cavity.
Overall, the complex structure of the lungs including the bronchi and alveoli, as well as the surrounding pleural membrane, are crucial for respiration and accommodate the anatomical requirements of the heart and other thoracic organs.