The blowhole itself is the hinged flap of a watertight valve below which are twin valves which separate and direct the incoming air into 2 streams to supply the 2 lungs. These valves also trap any water which has accidentally passed the blowhole and therefore prevents choking or drowning. Offshore, dolphins dive to much deeper depths,deep enough to feed on giant squid in some areas. They dive with lungs filled and cope with the pressure at depths of up to 500 metres by collapsing their chests as the water pressure compresses the air in their lungs. A dolphin's lungs are probably completely collapsed at 100m and all the air has been forced into thick walled windpipe and nasal passages, from where no nitrogen can be absorbed into the blood.
Dolphins also make more efficient use of oxygen than humans.
The gestation period variance is due to species type and environmental factors such as availability of food. The first two threats are not significant in Cardigan Bay, therefore, man provides the main threat. They also contribute underwater noise which disturbs dolphins' echolocation and communication systems. Other marine mammals are also affected by noise from boats. Contaminants - a high fat diet makes dolphins, porpoises and seals especially vulnerable to environmental contaminants which become increasingly concentrated as they pass up the food chain.