Final answer:
The structure composed of light, spongy tissue with air-filled spaces is the lungs, specifically the alveoli within them, which are pivotal for gas exchange.
Step-by-step explanation:
The tissue that consists of light, spongy tissue whose volume is primarily occupied by air-filled spaces is the lungs. The lungs contain a multitude of tiny air sacs called alveoli, where gas exchange occurs. These alveoli are the functional units of the lungs, providing a massive surface area for oxygen to be absorbed into the blood, and for carbon dioxide to be released from the blood. This exchange enables breathing and is crucial for maintaining life. The vast network of capillaries surrounding each alveolus ensures that the exchange of gases is efficient.