Essentially dependent on providing oxygen and maintaining an open alveolus. Increasing the inspired oxygen percentage (FiO2) will saturate the haemoglobin it comes into contact with, but to maximize the total amount of oxygen taken up you have to open up those alveoli with your peak inspiratory pressure (PIP), delivered for enough time to allow expansion to occur (inspiratory time, Ti) and then keep them open with your positive end expiratory pressure (PEEP).

Carbon dioxide clearance

Essentially dependent on the amount of air you can shift in and out - ie the minute volume (MV). This relates to the tidal volume (TV) and the respiratory rate (RR). PIP is important again, but now PEEP is a disadvantage: you want to empty the lungs as much as possible. The rate and the inspiratory time are closely related; you must allow enough time for lungs to expand appropriately during inspiration, and empty adequately during expiration. Else you get air trapping and progressive hyperinflation ("inadvertent PEEP").

Lung compliance and surfactant

The way in which the lung expands in response to pressure depends on its compliance, which is the opposite of stiffness. A certain opening pressure is needed for the lung to start to expand - this may be high at birth, hence the 5 slow inflation breaths used in resusciation. Drop the PEEP too much, and the lung will completely collapse, requiring higher pressures to "recruit" the alveoli again (ie open them up for gas exchange). Disease eg infection will decrease compliance, as does surfactant deficiency related to prematurity. A stiff lung needs more pressure to expand, and also collapses down more quickly during expiration. So in RDS, you may be able to maintain a high respiratory rate without getting any air trapping; whereas in chronic lung disease you may have to use very slow rates to allow time for the lungs to empty. The other important thing that surfactant does is prevent some alveoli (the ones that are not collapsed) being disproportionately ventilated and then becoming hyperinflated with the risk of pneumothorax, whereas others are not recruited at all.

Ventilation-perfusion mismatch

In some circumstances you can get the lungs open and pump the gases in and out, but oxygenation remains poor. This may be due to congenital cardiac disease with a right to left shunt, or else ventilation-perfusion mismatch, where normal autoregulatory processes are disrupted due to prematurity or pulmonary disease. Pulmonary hypertension can mimic a right to left shunt; eg in meconium aspiration, where the lung damage may cause widespread pulmonary vasoconstriction, increasing the pulmonary vascular resistance, so forcing blood across the foramen ovale or any other shunt (sometimes intrapulmonary) into the wrong side of the heart.