Like any aspect of a human, there are examples of efficiency and inefficiency according to the health status of the individual. A smoker's lungs will be inefficient, struggling to keep the air circulated when badly affected - abused - by the intake of toxic substances.
In those people who practise breathing by pushing the stomach muscles out to fill the lungs. Generally lung capacity is said to increase by 20–25% during intake. If the breath is held longer it will pass more oxygen through the blood and the lungs will be functioning more effectively.
However, the 'design' of the human lung can be said to be efficient with its branching airway structure. There is little unevenness of ventilation, only small pressures are required to expand it and not much dead space compared with the total lung volume.
