a-short-review-of-histology

Alveoli:

Alveoli are thin walled sacs which are the terminal portions of the respiratory system in the lungs
They are the anatomical and functional units in which gaseous exchange takes place
The presence of individual alveoli forming outpouchings of the bronchiolar wall are what defines the respiratory bronchioles of the bronchial tree
In the main, however, alveoli are grouped together in alveolar sacs
Alveolar sacs are collections of alveoli which arise from a short alveolar duct
The alveolar duct is the terminal division of a respiratory bronchiole
Air passes into and out of the alveolus through the same opening
The alveolar walls are shared with either a neighbouring alveolus arising from the same alveolar duct or with an alveolus of an adjacent alveolar sac
In the latter case, pores are present to allow air to pass from one alveolar sac to the next
In their absence, the thin alveolar walls would rupture if there was a difference in the air pressure between adjacent alveolar sacs

Question:

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Micrograph of Alveoli:

In this micrograph the thin walled alveoli are illustrated

The detailed structure of the walls cannot be distinguished at this magnification
However, by implication, you should appreciate that the distances that respiratory gasses have to pass in order to get from the air within the alveolar sacs into the blood within the lumina of the blood capillaries within the alveolar walls must be very short
This short distance facilitates the exchange of gasses which occurs in both directions