The lungs are large, triangular organs located in the thoracic cavity of the body on each side of the heart. They are responsible for the transportation of oxygen into the body, which is necessary for the conduction of cellular respiration within the cells of the body.

The primary bronchi lead to each lung and divide into the smaller bronchioles, which divide continually to the terminal, and finally the respiratory bronchioles. The respiratory bronchioles lead to the alveoli, small air sacs which act as the site of gas exchange between the air outside and the blood in the body. The larger bronchioles of the lungs are supported by cartilage tissue, while the alveoli are made of squamous epithelial tissue. Everything is held together by the stroma, a connective tissue.

The lungs are covered by what is called the pleura, a protective membrane that attaches the lungs to the thorax wall, and secreted a fluid that allows it to expand and deflate smoothly. Inhalation and exhalation are controlled by differences in air pressure; when the intercostal muscles of the ribs contract, the thoracic cavity’s size increases, pulling on the attached lungs and causing the space in the lungs to increase. The air pressure inside the lung becomes lower than the air pressure outside, and air rushes into the lung to equalize the pressure. When the intercostal muscles relax, the lungs volume decreases and a higher air pressure than the outside atmosphere is created, which forces air out of the lungs to equalize the pressure.

Oxygen and carbon dioxide are transported between the lungs and the cells of the body. Cellular respiration produces CO₂, which diffuses into the blood and combines with water to form carbonic acid. It is transported to the lungs, where the carbonic acid combines with hydrogen, splits into water and CO₂, and is diffuses into the alveoli. Here, oxygen diffuses from the area of high concentration in the alveoli, to the area of low concentration in the blood, where it binds to hemoglobin to form oxyhemoglobin. Blood travels to the tissues, where blood diffuses into the cells. This entire process is called respiration.