lower tract

• trachea is also called windpipe
• trachea is composed of smooth muscle embedded with C-shaped rings of cartilage, which provide rigidity to keep the air passage open
• the trachea divides into two branches called bronchi
• bronchus is the singular term for bronchi
• one branch leads to the right lung and the other to the left lung
• the inner walls of the trachea and bronchi are composed of a mucous membrane embedded with cilia
• the mucous membrane traps incoming particles, and the cilia move the entrapped material upward into the pharynx
• as the entrapped material reaches pharynx, it is coughed out, sneezed out, or swallowed
• like the trachea, bronchi contain C-shaped rings of cartilage
• each bronchus divides into smaller and smaller branches, eventually forming bronchioles
• alveoli are tiny air sacs formed where bronchioles terminate
• alveolus is the singular term for alveoli
• an alveolus resembles a small balloon because it expands and contracts with inflow and outflow of air
• pulmonary capillaries lie adjacent to the thin tissue membranes of the alveoli
• carbon dioxide diffuses from the blood within the pulmonary capillaries and enters the alveolar spaces
• oxygen from the alveoli diffuses into the blood
• after the exchange of gases, freshly oxygenated blood returns to the heart
• the lungs are divided into lobes: three lobes in the right lung and two lobes in the left lung
• mediastinum is the space between the right and left lungs
• mediastinum contains the:

  • heart
  • aorta
  • esophagus
  • bronchi
• pleura is a serous membrane that envelops the lobes of the lungs, and folds over to line the walls of the thoracic cavity
• visceral pleura is the innermost membrane that lies next to the lung
• parietal pleura is the outermost membrane that lines the thoracic cavity
• between the two pleura membranes is the pleural cavity
• pleural cavity contains a small amount of lubricating fluid, which permits the visceral pleura to glide smoothly over the parietal pleura during breathing
• ventilation depends on a pressure differential between the atmosphere and chest cavity
• diaphragm is a large muscular partition that lies between the chest and abdominal cavities
• the diaphragm assists in changing the volume of the thoracic cavity to produce the needed pressure differential for ventilation
• when the diaphragm contracts, it partially descends into the abdominal cavity, thus decreasing the pressure within the chest and drawing air into the lungs
• when the diaphragm relaxes, it slowly re-enters the thoracic cavity, thus increasing the pressure within the chest (inspiration)
• as the pressure within the chest increases, air leaves the lungs (expiration)
• intercostal muscles assist the diaphragm in changing the volume of the thoracic cavity by elevating and lowering the rib cage