Question

Define the term "physiological dead space". Provide an example of the resulting alveolar partial pressures for both O2 and CO2 (mmHg) and describe why these partial pressures are observed.

Answer 1

Answer :

Physiological dead space :

It 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. In other words, not all the air in each breath is available for the exchange of oxygen and carbon dioxide.

Example of alveolar partial pressure of o2 and co2:

Partial pressure of oxygen decreased through several disease processes. The primary processes include decreased inhaled oxygen, hypoventilation, diffusion limitations, and ventilation/perfusion mismatching (V/Q mismatch).

Changes in environmental pressure cause a change in the available oxygen for diffusion into the body. At sea level, the atmospheric pressure is 760 mm Hg. However, as elevation increases, atmospheric pressure decreases. For example, at the summit of Mt. Everest, the atmospheric pressure is as low as 260 mm Hg. When this pressure is used to calculate the alveolar partial pressure of oxygen in the environment, there is approximately 54.6 mm Hg oxygen available for diffusion. This is almost half of what is available at sea level.Essentially, any pathology that decreases ventilation of the alveoli will lead to a hypoventilation defect. These can include:Central nervous system (CNS) depression or malformation from a neurologic deficit, Guillain-Barre, ALS, or drug overdose where the respiratory drive is decreased

Obesity hypoventilation where the excess weight of the chest does not allow for proper inflation,Muscular weakness,Poor chest elasticity secondary to rib fracture or kyphoscoliosis

The result of hypoventilation on oxygenation is that air exchange between the alveolar space and the environment does not effectively occur. This decreases the partial pressure of oxygen in the alveolar space resulting in a lessened diffusion gradient which reduces the partial pressure of oxygen in the blood.