Alveoli

Definition
Pulmonary alveoli are air spaces in the lungs where CO2 leaves the blood and O2 enters the blood
Approximately 300,000,000
At the end of bronchioles
Form clusters called alveolar sacs

Structure
The wall of each alveolus is lined by thin flat cells (Type I cells)
The wall contains numerous capillaries
It also contains a mesh of elastic and collagenous fibres (collagen - supports; elastic - expansion and contraction)
It has pneumocytes (Type II cells)
Pneumocytes secrete surfactant - a film of fatty substance (lowers surface tension to prevent collapse of the alveolus)
another type of cell - alveolar macrophage - engulf foreign particles like dust, bacteria, carbon particles, blood cells and carry them away

Function
Exchange of gases between the blood and the atomospheric air
Brings the blood in the capillaries into close contact with the inspired air, only two layer of epithelium separating the  two.
Oxygen is absorbed and carbon di oxide is released by the haemoglobin in the blood which get excreted by the expired air



































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