Seal adaptations to deep diving
Seals have several cardio-respiratory adaptations that allow them to dive to depths of up to 600 m, for long periods of time.
These include: relatively larger lungs than humans, high content of myoglobin in the muscle relative to humans, high content of hemoglobin and larger blood volume than humans.
Figure 1 - Comparison between human and seal lung anatomy
As you can see in the image above, seal airways are reinforced with muscle and/or cartilage, which allows them to compress, avoiding the bends.
In the image below, you can see a summary of the main adaptations that seals possess that allow them to cope with deep, long dives.
Figure 2 - Seal adaptations to deep diving