External respiration is the process by which gases diffuses into and out of the
blood across the
air-blood barrier in the
lungs. It is the second stage in
respiration; after
pulmonary ventilation and before
respiratory gas transport in the
blood.
Respiratory membrane (air-blood barrier)
The
respiratory membrane of the
alveoli is the site of
external respiration. It consists of hollow ball-shaped chambers of
simple squamous epithelium that make up the
alveolar sacs.
Alveolar pores are holes that connect neighbouring sacs together. The clusters of alveolar sacs are covered in
pulmonary capillaries that will exchange gases with the air inside the
alveoli. The
tissue of the
respiratory membrane also contains some
elastic fibers to help the
lungs to recoil during
expiration and alveolar
macrophages ("dust cells") that clean up debris and
bacteria.
cuboidal cells found in the
respiratory membrane produce a
surfactant; a lipid that keeps the walls of the
alveoli from sticking together.
External respiration
External respiration occurs at the points of contact between the
respiratory membrane and the
pulmonary capillaries. Carbon dioxide diffuses into the
alveoli and
oxygen diffuses into the
blood, each following its concentration gradient.
Figure 12.3.1 - Alveolus diagram By LadyofHats [Public domain], via Wikimedia Commons
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1 - Anatomy and Physiology Basics