Final answer:
Vessels enter and leave the lungs at the hilum, a concave structure on the mediastinal surface of the lungs where blood vessels, lymphatic vessels, nerves, and a bronchus reside.
Step-by-step explanation:
Vessels enter and leave the lungs at the hilum, which is a concave structure on the mediastinal surface of the lungs. The hilum serves as the pivotal point through which blood vessels, lymphatic vessels, nerves, and a bronchus enter and exit the lung tissue. This structure facilitates the connection of the lungs with the rest of the body's circulatory and nervous systems. The pulmonary capillaries surrounding the alveoli within the lungs are specifically the sites where gas exchange occurs, allowing carbon dioxide to exit the blood and oxygen to enter.