The upper respiratory tract consists of the nose, pharynx and larynx.
The lower respiratory system is contained within the thoracic cavity which is made up of the spine, ribcage and its muscles. It is lined at the bottom with the diaphragm. The thoracic cavity is lined with pareital pleura and the lungs inside are covered with visceral pleura. These two membranes help to form the pleural cavity which is filled with pleural fluid. This reduces friction between the expanding and contracting lungs.
Each lung is supplied with air from one single tube -the trachea which shares the pharynx with the digestive system. The trachea divides into two separate tubes, the bronchi which lead into the root of the lungs. The bronchi continue to divide, forming a vast networks of tiny tubules called bronchioles. Each bronchiole eventually supplies the main gas exchangers of the lungs, the ‘grape like‘ alveolar sacs. Each alveolar sac is comprised of many individual air sacs called alvioli. The alvioli are completely encased by pulmonary capillaries which provide a huge area for gas exchange.
Short distance, pressure gradient, surfectant and healthy blood supply = gaseous exchange
Due to the structure of the ribcage – expansion in every direction is possible when the respiratory muscles contract.
Expansion creates more volume and thus the internal pressure of the cavity falls below that of the atmosphere.
As soon as this pressure gradient exists, air rushes down the nasal cavity, pharynx, trachea, bronchi and bronchioles into the already expanding lungs.
Then respiratory muscles relax – the thoracic cavity returns to rest.