What relationship must exist between intrathoracic pressure and intrathoracic volume for the lungs to expel air?
a) As intrathoracic pressure decreases, intrathoracic volume decreases.
b) As intrathoracic pressure increases, intrathoracic volume decreases.
c) As intrathoracic pressure decreases, intrathoracic volume increases.
d) As intrathoracic pressure increases, intrathoracic volume increases.
e) There is no relationship between intrathoracic pressure and volume.