Physiologic dead space

The figure depicts dead space. The volume of gas in the airways only (ie, gas proximal to the respiratory bronchiole including mouth, nose, and ventilator tubing) represents anatomic dead space (no gas exchange is possible). Alveolus 1 (eg, alveoli in apices) is not receiving any blood flow because the normal pulmonary artery pressure is not sufficiently high to pump blood to the apices. The two lower alveoli receive blood flow, but blood flow to one of them is blocked (alveolus 3). Thus, only one alveolus is receiving ventilation and perfusion (alveolus 2); alveolus 1 and 3 are being ventilated but not perfused (ie, alveolar dead space). The physiological dead space is the combination of anatomic and alveolar dead space.