The respiratory zone

 The respiratory zone.

The respiratory zone or bronchioles begins from the terminal bronchioles and then leads into alveolar duct… {whose walls consists of diffusely arranged rings of smooth muscle cells & C/tissues} So, the alveolar duct leads into alveoli “or alveolar sacs”{i.e. a thin walled air sacs}, it is the site of gas exchange, there by, each lung consists of “300million” of gas-filled alveoli. 

Two cell types form the alveolar wall; *the squamous epithelium type:1.  &  a cuboidal shaped type: II. The alveolar type I cell, comprises 95% the alveolar surface & forms a thin barrier that separates the air in alveoli from the blood in pulmonary capillaries (ie, exteriorly).. as the other alveolar type II cells(cuboidal epithelial) secrete an agent substance so called: *surfactant” I.e. an active substance keeps or maintain the alveolar patent..

NB: Surfactant composed of: dipalmitoylphosphatidylcholine…(oily substance)                                  DPPC (40%), & cholesterol (50%), & Proteins(10%)