Air travels to the lungs through a series of air tubes and passages. It enters the body through the nostrils or the mouth, passing down the throat to the larynx, or voice box, and then to the trachea, or windpipe. In the chest cavity the trachea divides into two branches, called the right and left bronchi or bronchial tubes, that enter the lungs.
A scanning electron micrograph reveals the tiny sacs known as alveoli within a section of human lung tissue. Human beings have a thin layer of about 700 million alveoli within their lungs. This layer is crucial in the process called respiration, exchanging oxygen and carbon dioxide with the surrounding blood capillaries
In the adult human, each lung is 25 to 30 cm (10 to 12 in) long and roughly conical. The left lung is divided into two sections, or lobes: the superior and the inferior. The right lung is somewhat larger than the left lung and is divided into three lobes: the superior, middle, and inferior. The two lungs are separated by a structure called the mediastinum, which contains the heart, trachea, esophagus, and blood vessels. Both right and left lungs are covered by an external membrane called the pleura. The outer layer of the pleura forms the lining of the chest cavity.
The branches of the bronchi eventually narrow down to tubes of less than 1.02 mm (less than 0.04 in) in diameter. These tubes, called bronchioles, divide into even narrower tubes, called alveolar ducts. Each alveolar duct ends in a grapelike cluster of thin-walled sacs, called alveoli (a single sac is called an alveolus). From 300 million to 400 million alveoli are contained in each lung. The air sacs of both lungs have a total surface area of about 93 sq m (about 1000 sq ft), nearly 50 times the total surface area of the skin.
In addition to the network of air tubes, the lungs also contain a vast network of blood vessels. Each alveolus is surrounded by many tiny capillaries, which receive blood from arteries and empty into veins. The arteries join to form the pulmonary arteries, and the veins join to form the pulmonary veins. These large blood vessels connect the lungs with the heart.