Alveoli
Alveoli exchange oxygen with red blood cells when the heart's Sinoatrial and Atrioventricular nodes send electric signals to the hearts ventricles. The right side of the heart then contracts twice,[source?] sending blood shooting into the pulmonary circulatory system, where capillary beds are located in the lungs. The Alveoli then exchange oxygen and discard carbon dioxide with the red blood cells, which then return to the heart's left atrium.
The singular of alveoli is one alveolus. Alveoli are tiny grape-like sacs where gas exchange takes place in the lungs (oxygen and carbon dioxide). When you breathe out, the body delivers carbon dioxide to the alveoli, and you release it in your exhalation. When you breathe in, oxygen fills the alveoli and then enters the blood, so it can be delivered to the rest of the body. In asthma there is no damage to the alveoli, which is different from another common lung disease which is called: chronic obstructive pulmonary disease, in which alveoli are damaged. A network of blood capillaries surround walls of each alveolus.The walls are extremely thin(one cell thick)and moist,thus allowing gaseous diffussion through them. There are over 300 million alveoli in each lung. If you were to spread one person's alveoli across a tennis court, they would cover over half the court!
