A seal swims just as readily on its back as on its belly, standing upright or upside down. The front flippers serve as paddles; the body and hind fins provide the propulsion.
Seals can dive down to depths of hundreds of meters. During the first few minutes, they swim actively downwards, after which they go into a kind of gliding flight while they sink even deeper. Their body is totally adapted to long and deep dives. Their blood can absorb much more oxygen than human blood. Furthermore, they can lower their heart rate tremendously during the dive, from 40 to less than 1 beat per minute. This means they don’t need to breathe as often. When they ascend from deep depths, they can pump out the inhaled air from the air sacs, preventing deadly nitrogen bubbles from entering the bloodstream. Once above water, the heart beat of a seal increases to 120 beats per minute in order to provide enough oxygen for the organs.