Question

Explain the effect of anatomical dead space on alveolar ventilation and on the composition of alveolar and expired air.

Answer 1

Dead space is defined as the part of the respiratory tract, where gaseous exchange does not take place. Air present in the dead space is called dead space air.
Anatomical Dead Space
Anatomical dead space extends from nose up to termi­ nal bronchiole. It includes nose, pharynx, trachea, bronchi and branches of bronchi up to terminal bronchioles. These structures serve only as the passage for air movement. Gaseous exchange does not take place in these structures.

Normal value of alveolar ventilation is 4,200 mL (4.2 L)/ minute.
It is calculated by the formula:
Alveolar ventilation
= (Tidal volume – Dead space) x Respiratory rate = (500 – 150) mL × 12/minute
= 4,200 mL (4.2 L)/minute.

Volume of normal dead space is 150 mL. Under normal conditions, physiological dead space is equal to anatomical dead space. It is because, all the alveoli are functioning and all the alveoli receive adequate blood flow in normal conditions.