Mammals breathe in and out of their lungs, wasting that part of the inhalation which remains in the conducting airways where no gas exchange can occur.
Anatomical dead space is the volume of the conducting airways (from the nose, mouth and trachea to the terminal bronchioles).
[4] Despite the flexibility of the trachea and smaller conducting airways, their overall volume (i.e. the anatomic dead space) changes little with bronchoconstriction or when breathing hard during exercise.
[4][6] As birds have a longer and wider trachea than mammals the same size, they have a disproportionately large anatomic dead space, reducing the airway resistance.
This adaptation does not impact gas exchange because birds flow air through their lungs - they do not breathe in and out like mammals.
Alveolar dead space is negligible in healthy individuals, but it can increase dramatically in some lung diseases due to ventilation-perfusion mismatch.
By quantifying this dilution, it is possible to measure physiological dead space, employing the concept of mass balance, as expressed by the Bohr equation.
When mechanically ventilated using a mandatory mode, the patient breathes at a rate and tidal volume that is dictated by the machine.
Although the amount of gas per minute is the same (5 L/min), a large proportion of the shallow breaths is dead space, which does not help oxygen to get into the blood.
The buildup of carbon dioxide is usually the more noticeable effect unless the breathing gas is hypoxic as occurs at high altitude.