Dead space is the volume of air that is inhaled that does not take part in the gas exchange, because it either remains in the conducting airways or reaches alveoli that are not perfused or poorly perfused. It means that not all the air in each breath is available for the exchange of oxygen and carbon dioxide. 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.
Components Total dead space (also known as physiological dead space) is the sum of the anatomical dead space and the alveolar dead space. Benefits do accrue to a seemingly wasteful design for ventilation that includes dead space.
Carbon dioxide is retained, making a bicarbonate-buffered blood and interstitium possible. Inspired air is brought to body temperature, increasing the affinity of hemoglobin for oxygen, improving O2 uptake. Particulate matter is trapped on the mucus that lines the conducting airways, allowing its removal by mucociliary transport. Inspired air is humidified, improving the quality of airway mucus. In humans, about a third of every resting breath has no change in O2 and CO2 levels. In adults, it is usually in the range of 150 mL. Dead space can be increased (and better envisioned) by breathing through a long tube, such as a snorkel. Although one end of the snorkel is open to the air, when the wearer breathes in, they inhale a significant quantity of air that remained in the snorkel from the previous exhalation. Therefore, a snorkel increases the person's dead space by adding even more airway that does not participate in gas exchange.
Anatomical dead space Anatomical dead space is the volume of the conducting airways (from the nose, mouth and trachea to the terminal bronchioles). These conduct gas to the alveoli but no gas exchange occurs here. In healthy lungs where the alveolar dead space is small, Fowler's method accurately measures the anatomic dead space using a single breath nitrogen washout technique. The normal value for dead space volume (in mL) is approximately the lean mass of the body (in pounds), and averages about a third of the resting tidal volume (450-500 mL). In Fowler's original study, the anatomic dead space was 156 ± 28 mL (n=45 males) or 26% of their tidal volume. 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. 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 Alveolar dead space is defined as the difference between the physiologic dead space and the anatomic dead space. It is contributed to by all the terminal respiratory units that are over-ventilated relative to their perfusion. Therefore it includes, firstly those units that are ventilated but not perfused, and secondly those units which have a ventilation-perfusion ratio greater than one. Alveolar dead space is negligible in healthy individuals, but it can increase dramatically in some lung diseases due to ventilation-perfusion mismatch.
Calculating Just as dead space wastes a fraction of the inhaled breath, dead space dilutes alveolar air during exhalation. By quantifying this dilution, it is possible to measure physiological dead space, employing the concept of mass balance, as expressed by the Bohr equation.
V d V t = P a CO 2 − P e CO 2 P a CO 2 {\displaystyle {\frac {V_{d}}{V_{t}}}={\frac {P_{a\,{\ce {CO2}}}-P_{e\,{\ce {CO2}}}}{P_{a\,{\ce {CO2}}}}}}
where V d {\displaystyle V_{d}} is the dead space volume and V t {\displaystyle V_{t}} is the tidal volume;
P a CO 2 {\displaystyle P_{a\,{\ce {CO2}}}} is the partial pressure of carbon dioxide in the arterial blood, and
P e CO 2 {\displaystyle P_{e\,{\ce {CO2}}}} is the partial pressure of carbon dioxide in the mixed expired (exhaled) air.
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