Lungs are the organs of respiration, consisting of a pair of cone-shaped sacs with numerous branching air passages. They are located in the thorax and expand to fill most of its cavity when we inhale. The lungs play a vital role in supplying oxygen to the blood and removing carbon dioxide, a waste product of metabolism.
The right lung is larger than the left lung because it has three lobes, while the left lung only has two. The lobes are separated by fissures, which allow air to travel from the hilus (opening) to all parts of the lungs. Each lobe is then divided into smaller units called bronchopulmonary segments. There are about 10 segments in each lobe (5 in the right lung and 3+2 in the left). Segments are further divided into smaller units called bronchioles.
The primary function of the lungs is gas exchange: Oxygen enters through inhalation and diffuses across alveolar membranes into pulmonary capillaries where it binds to hemoglobin molecules in erythrocytes (red blood cells). Carbon dioxide diffuses from erythrocytes across alveolar membranes and is exhaled. In addition, other functions of the lungs include: phonation (production of sound), cough reflex, protection against aspiration, filtration, drainage, secretion and absorption.