These muscles contract and raise the ribs during inspiration, increasing the diameter and volume of the chest cavity. The external intercostal muscles are also involved. During maximal inspiration, the diaphragm forces the abdominal contents ventrally and caudally.

The contraction of the inspiratory muscles, mainly the diaphragm, causes the chest cavity to expand, creating negative pressure. The movement of air into and out of the lungs is called ventilation. Any increase in alveolar ventilation will increase anesthetic uptake into the pulmonary blood (Fraser 2003).

From an anesthetic point of view, alveolar ventilation is very important because it will control the amount of volatile or gaseous anesthetic agent that can diffuse into the bloodstream. Vasodilation or vasoconstriction, caused by disease or anesthetic drugs, enhances V/Q mismatching and hypoxemia. Both ventilation and perfusion increase toward the dependent regions of the lung, but since blood is heavier than lung parenchyma, perfusion increases at a faster rate than ventilation. In the normal lung, ventilation (V) and perfusion (Q) are not evenly matched (known as V/Q mismatch), and this worsens with lung disease and dorsal or lateral recumbencies. It does no good to ventilate an alveolus that is not being perfused ( alveolar dead space) or to perfuse an alveolus that is not being ventilated because of atelectasis (collapse of alveoli). The “matching” of ventilation and perfusion is important to proper lung function.

Anatomical dead space gravity