page_title: Respiratory tract
section_title: Lower respiratory tract
hierarchical_section_title: Respiratory tract / Structure / Lower respiratory tract
context_page_description: The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory mucosa or respiratory epithelium.
  Air is breathed in through the nose to the nasal cavity, where a layer of nasal mucosa acts as a filter and traps pollutants and other harmful substances found in the air. Next, air moves into the pharynx, a passage that contains the intersection between the oesophagus and the larynx. The opening of the larynx has a special flap of cartilage, the epiglottis, that opens to allow air to pass through but closes to prevent food from moving into the airway.
  From the larynx, air moves into the trachea and down to the intersection known as the carina that branches to form the right and left primary bronchi. Each of these bronchi branches into a secondary bronchus that branches into tertiary bronchi, that branch into smaller airways called bronchioles that eventually connect with tiny specialized structures called alveoli that function in gas exchange.
  The lungs which are located in the thoracic cavity, are protected from physical damage by the rib cage.
context_section_description: The lower respiratory tract or lower airway is derived from the developing foregut and consists of the trachea, bronchi (primary, secondary and tertiary), bronchioles (including terminal and respiratory), and lungs (including alveoli). It also sometimes includes the larynx. 
  The lower respiratory tract is also called the respiratory tree or tracheobronchial tree, to describe the branching structure of airways supplying air to the lungs, and includes the trachea, bronchi and bronchioles.
  trachea
  main bronchus (diameter approximately 1 – 1.4 cm in adults)
  lobar bronchus (diameter approximately 1 cm)
  segmental bronchus (diameter 4.5 to 13 mm)
  subsegmental bronchus (diameter 1 to 6 mm)
  conducting bronchiole
  terminal bronchiole
  respiratory bronchiole
  alveolar duct
  alveolar sac
  alveolus
  At each division point or generation, one airway branches into two or more smaller airways. The human respiratory tree may consist on average of 23 generations, while the respiratory tree of the mouse has up to 13 generations.  Proximal divisions (those closest to the top of the tree, such as the bronchi) mainly function to transmit air to the lower airways. Later divisions including the respiratory bronchiole, alveolar ducts and alveoli, are specialized for gas exchange.
  The trachea is the largest tube in the respiratory tract and consists of tracheal rings of hyaline cartilage. It branches off into two bronchial tubes, a left and a right main bronchus. The bronchi branch off into smaller sections inside the lungs, called bronchioles. These bronchioles give rise to the air sacs in the lungs called the alveoli.
  The lungs are the largest organs in the lower respiratory tract. The lungs are suspended within the pleural cavity of the thorax. The pleurae are two thin membranes, one cell layer thick, which surround the lungs. The inner (visceral pleura) covers the lungs and the outer (parietal pleura) lines the inner surface of the chest wall. This membrane secretes a small amount of fluid, allowing the lungs to move freely within the pleural cavity while expanding and contracting during breathing. The lungs are divided into different lobes. The right lung is larger in size than the left, because of the heart's being situated to the left of the midline.  The right lung has three lobes – upper, middle, and lower (or superior, middle and inferior), and the left lung has two – upper and lower (or superior and inferior), plus a small tongue-shaped portion of the upper lobe known as the lingula. Each lobe is further divided up into segments called bronchopulmonary segments.  Each lung has a costal surface, which is adjacent to the ribcage; a diaphragmatic surface, which faces downward toward the diaphragm; and a mediastinal surface, which faces toward the center of the chest, and lies against the heart, great vessels, and the carina where the two mainstem bronchi branch off from the base of the trachea.
  The alveoli are tiny air sacs in the lungs where gas exchange takes place.  The mean number of alveoli in a human lung is 480 million. When the diaphragm contracts, a negative pressure is generated in the thorax and air rushes in to fill the cavity. When that happens, these sacs fill with air, making the lung expand. The alveoli are rich with capillaries, called alveolor capillaries. Here the red blood cells absorb oxygen from the air and then carry it back in the form of oxyhaemaglobin, to nourish the cells. The red blood cells also carry carbon dioxide (CO
  2) away from the cells in the form of carbaminohemoglobin and releases it into the alveoli through the alveolor capillaries. When the diaphragm relaxes, a positive pressure is generated in the thorax and air rushes out of the alveoli expelling the carbon dioxide.
caption: TracheaMainstem bronchusLobar bronchusSegmental bronchusBronchioleAlveolar ductAlveolus
exif: {"Image Orientation": "Horizontal (normal)", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop 7.0", "Image DateTime": "2005:07:27 15:16:48", "Image ExifOffset": "156", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "294", "Thumbnail JPEGInterchangeFormatLength": "3733", "EXIF ColorSpace": "Uncalibrated", "EXIF ExifImageWidth": "270", "EXIF ExifImageLength": "350"}