37. Respiratory bronchioles

Respiratory bronchioles are areas of transition (hybrids) between the conducting and respiratory portions of the airways. In addi tion to the typical bronchiolar epithelium of the terminal bron chioles, these passageways contain outpouchings of alveoli, which comprise the respiratory portion of this system.

Terminal bronchioles give rise to respiratory bronchioles.

Respiratory bronchioles branch to form two to three alveolar ducts, which are long sinuous tubes.

Alveolar sacs are spaces formed by two or more conjoined alveoli. They are lined by the simple squamous alveolar epithe lium. Alveoli are the terminal, thin—walled sacs of the respiratory tree that are responsible for gas exchange. There are approximately 300 million alveoli per lung, each one 200–300 mm in diameter. Blood—air interface. Oxygen in the alveoli is separated from hemoglobin in the red blood cells of alveolar capillaries by five layers of membrane and cells: the alveolar epithelial cell (api cal and basal membranes) and its basal lamina, the basal lami na of the capillary and its endothelial cell (basal and apical membranes), and the erythrocyte membrane. The total thick ness of all these layers can be as thin as 0,5 mm.

Alveolar epithelium contains two cell types. Type I cells completely cover the alveolar luminal surface and provide a thin surface for gas exchange. This simple squamous epithelium is so thin (-25 nm) that its details are beyond the resolution of the light microscope.

Type II cells are rounded, plump, cuboidal—like cells that sit on the basal lamina of the epithelium and contain mem brane—bound granules of phospholipid and protein (lamel lar bodies). The contents of these lamellar bodies are secreted onto the alveolar surface to provide a coating of surfactant that reduces alveolar surface tension.

Alveolar macrophages (dust cells) are found on the surface of the alveoli.

Derived from monocytes that extravasate from alveolar capillaries, alveotar macrophages are part of the mononu – clear phagocyte system. Dust cells, as their name implies, continuously remove parti cles and other irritants in the alveoli by phagocytosis.

New words

respiratory bronchioles – дыхательные бронхиолы

hybrids – гибриды

respiratory portions – дыхательные части

airways – воздушные трассы

bronchiolar – бронхиолярный

terminal bron chioles – предельные бронхиолы

passageway – проходы

tocomprise – включить

ducts – трубочки

sinuous tubes – извилистые трубы

thin—walled – окруженный тонкой стеной

sacs – мешочки

respiratory tree – дыхательное дерево

hemoglobin – гемоглобин

apical – апикальный