What is dead space 2 marks
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Here is a concise 2-mark answer on Dead Space:
Dead Space
Dead space is the volume of the airways and lungs that does not participate in gas exchange.
Types
1. Anatomic Dead Space
- The volume of the conducting airways (nose/mouth, trachea, bronchi, bronchioles) - approximately 150 mL in adults.
- Does NOT include respiratory bronchioles and alveoli.
- When a tidal volume of 500 mL is inhaled, only 350 mL reaches the alveoli; 150 mL remains in the conducting airways and does not exchange gases.
2. Physiologic (Functional) Dead Space
- The total lung volume that does not participate in gas exchange.
- = Anatomic dead space + functional dead space (ventilated alveoli that are not perfused).
- The most important cause is a ventilation/perfusion (V/Q) mismatch - alveoli that are ventilated but not perfused by pulmonary capillary blood.
- In healthy individuals, physiologic dead space ≈ anatomic dead space (since V/Q matching is good).
- In disease (e.g. pulmonary embolism), physiologic dead space increases significantly.
Bohr Equation (for measuring physiologic dead space)
VD = VT × (PaCO₂ - PEco₂) / PaCO₂
Where:
- VD = physiologic dead space
- VT = tidal volume
- PaCO₂ = arterial PCO₂
- PEco₂ = mixed expired air PCO₂
The fraction represents dilution of alveolar CO₂ by dead space air (which contains no CO₂).
Source: Costanzo Physiology 7th Edition, p. 199-200
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