A healthy human body normally has two lungs: a right lung and a left lung.
The lungs are the primary organs of the respiratory system. They sit inside the chest, one on each side of the heart, and exchange gases between the air and bloodstream. Oxygen passes from inhaled air into the blood, while carbon dioxide moves from the blood into the lungs to be exhaled. The diaphragm is the main muscle driving breathing.
The two lungs are not identical. The right lung is larger and divided into three lobes, while the left lung has two lobes because it shares space with the heart. Each lung is surrounded by a pleural sac that allows it to move smoothly during breathing.
A useful neighbouring fact is that the lungs do not pump blood around the body; the heart does that. Instead, the right side of the heart sends oxygen-poor blood to the lungs, and the left side receives the newly oxygenated blood. Inside the lungs, gas exchange occurs in millions of tiny air sacs called alveoli.