Final answer:
The hilum is the concave structure on the mediastinal surface of the lungs where blood vessels, bronchi, lymphatic vessels, and nerves enter or exit the lung.
Step-by-step explanation:
The structure found on the mediastinal surface that serves as the site for entry and exit of blood vessels, bronchi, lymphatic vessels, and nerves in the lungs is the hilum. The hilum is a concave structure located on the medial aspect of each lung and is the point through which the mentioned structures either enter or leave the lung.
Other structures, such as the pleura, surround and protect the lungs, whereas the alveoli are the tiny air sacs within the lungs where gas exchange occurs. The diaphragm is a large, flat muscle that aids in breathing and forms the floor of the thoracic cavity but is not the point of entry for the vessels and nerves.