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 smaller lungs than humans, high content of myoglobin in the muscle relative to humans and 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