And this consists of an extracellular matrix we call the basement membrane. Now the basement membrane actually connects the epithelial cells of the alveolis to the endothelial cells of our blood vessels. So these cells shown in blue are the endothelial cells. So these cells are the endothelial cells of our pulmonary arterial, and these cells are the endothelial cells of our pulmonary Venuel. Okay, so now that we know what the structure of our alveolus actually looks like, let's discuss how gas exchange actually takes place and why. Oxygen is taken up by the capillaries, but carbon dioxide is released by the capillaries.