Dead space (physiology)
| PaO2 | Arterial oxygen tension, or partial pressure |
|---|---|
| PAO2 | Alveolar oxygen tension, or partial pressure |
| PaCO2 | Arterial carbon dioxide tension, or partial pressure |
| PACO2 | Alveolar carbon dioxide tension, or partial pressure |
| PvO2 | Oxygen tension of mixed venous blood |
| P(A-a)O2 | Alveolar-arterial oxygen tension difference. The term formerly used (A-a DO2) is discouraged. |
| P(a/A)O2 | Alveolar-arterial tension ratio; PaO2:PAO2 The term oxygen exchange index describes this ratio. |
| C(a-v)O2 | Arteriovenous oxygen content difference |
| SaO2 | Oxygen saturation of the hemoglobin of arterial blood |
| SpO2 | Oxygen saturation as measured by pulse oximetry |
| CaO2 | Oxygen content of arterial blood |
| pH | Symbol relating the hydrogen ion concentration or activity of a solution to that of a standard solution; approximately equal to the negative logarithm of the hydrogen ion concentration. pH is an indicator of the relative acidity or alkalinity of a solution |
In physiology, dead space is the volume of air which is inhaled that does not take part in the gas exchange, either because it (1) remains in the conducting airways, or (2) reaches alveoli that are not perfused or poorly perfused. In other words, 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 inspiration which remains in the conducting airways where no gas exchange can occur.
Benefits do accrue to a seemingly (but not actually) "wasteful" design for ventilation that includes dead space.
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. Even though 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. Thus, a snorkel increases the person's dead space by adding even more "airway" that doesn't participate in gas exchange.
The total dead space (also known as physiological dead space) is the sum of the anatomical dead space plus the alveolar dead space.
Anatomical dead space is that portion of the airways (such as the mouth and trachea to the bronchioles) which conducts gas to the alveoli. No gas exchange is possible in these spaces. In healthy lungs where the alveolar dead space is small, Fowler's method accurately measures the anatomic dead space by a 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.
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