Your lungs are sponge-like organs that transfer air molecules to and from blood cells. They expand as you inhale, and contract as you exhale. A normal person inhales a few pints of air with every breath, and this is why your buoyancy shifts as you breathe.

As you breathe, the air is pulled into millions of tiny air sacs called alveoli. These microscopic sacs are thin enough to allow air to pass to and from the blood cells that circulate through the lungs.

As you can see, your lungs are a major air space in your body. Fortunately, this air space equalizes automatically as you breathe. However, your lungs are delicate and can easily rupture if they become a closed air space. For this reason, you must never hold your breath when scuba diving.