The human trachea (windpipe) divides into two main bronchi (also mainstem bronchi), the left and the right, at the level of the sternal angle and of the fifth thoracic vertebra or up to two vertebrae higher or lower, depending on breathing, at the anatomical point known as the carina. The right main bronchus is wider, shorter, and more vertical than the left main bronchus. The right main bronchus subdivides into three lobar bronchi, while the left main bronchus divides into two. The lobar bronchi divide into tertiary bronchi, also known as segmental bronchi, each of which supplies a bronchopulmonary segment. A bronchopulmonary segment is a division of a lung separated from the rest of the lung by a connective tissue septum. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. There are ten segments per lung, but due to anatomic development, several segmental bronchi in the left lung fuse, giving rise to eight. The segmental bronchi divide into many primary bronchioles which divide into terminal bronchioles, each of which then gives rise to several respiratory bronchioles, which go on to divide into two to 11 alveolar ducts. There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the basic anatomical unit of gas exchange in the lung. So, the bronchi in no way aid in, or bring about exchange of air, as the alveolar air sacs do this. There are many diseases of the bronchi which can lead to breathing problems so this is an important function of the bronchi. In fact, the main etiology of asthma is the fact that the bronchi become restricted, that is, their lumen decreases. So due to this, insufficient air reaches the lungs and so, the person starts wheezing. This is especially seen in chronic bronchial asthma. Therefore, when the bronchi function properly, it ensures that the airway remains patent, so that air can pass freely. Besides allowing air to pass, the bronchi performs another function. The bronchi are lined by various types of cells such as ciliated cells and goblet or mucus cells. When air enters from the nose, the nose hair and cilia try to prevent and trap pathogens from coming in. However, the humidified air passes through trachea and the bronchi. The cilia and mucus present in the bronchi further prevent pathogens from entering any further into the lungs. If pathogens enter into the lungs, it could lead to various respiratory system diseases. This is especially seen when there is malfunctioning of cilia in the bronchi, as is seen in bronchitis, where there is inflammation of the bronchi. The hyaline cartilage forms an incomplete ring in the bronchi, giving them a "D"-shaped appearance in the larger bronchi (as compared with the "O"-shaped complete cartilaginous rings of the trachea), and as small plates and islands in the smaller bronchi. Smooth muscle is present continuously around the bronchi. In the mediastinum, at the level of the fourth thoracic vertebra, the trachea divides into the right and left primary bronchi. The bronchi branch into smaller and smaller passageways until they terminate in tiny air sacs called alveoli. The cartilage and mucous membrane of the primary bronchi are similar to those in the trachea. As the branching continues through the bronchial tree, the amount of hyaline cartilage in the walls decreases until it is absent in the smallest bronchioles. As the cartilage decreases, the amount of smooth muscle increases. The mucous membrane also undergoes a transition from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium. The alveolar ducts and alveoli consist primarily of simple squamous epithelium, which permits rapid diffusion of oxygen and carbon dioxide. Exchange of gases between the air in the lungs and the blood in the capillaries occurs across the walls of the alveolar ducts and alveoli.