The diagram shows a section through the thorax. When breathing out (expiring), what happens to the muscles at X and Y and the pressure at Z?

Answer:

6) E

The easiest one to figure out is Y. During expiration, the diaphragm relaxes and curves upwards.

The pressure Z increases to force the air to move out of the thoracic cavity.

The intercostal muscles also relax to allow expiration.