How does anatomical dead space reduce the amount of each breath that reaches the alveoli?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

Anatomical dead space refers to the volume of the conducting airways—including the trachea, bronchi, and bronchioles—that do not participate in gas exchange.12

Because ventilation is a cyclic process, a portion of each inspired breath must fill these conducting structures before reaching the alveoli, where gas exchange actually occurs. In a typical resting adult, this anatomical dead space is approximately 150 milliliters. Consequently, out of a standard tidal volume of 500 milliliters, only about 350 milliliters of air reaches the gas-exchange sites to oxygenate the blood and remove carbon dioxide. This effectively reduces the amount of fresh air available for respiration during each cycle.31