There are many physiological mechanisms that allow marine mammals to pull off such long dives. The m...

1. Marine Mammal Bradycardia: The Diving Response
There are many physiological mechanisms
that allow marine mammals to pull off such
long dives. The most significant mechanism
is bradycardia, the slowing of an animal's
heart rate to conserve oxygen usage and
reduce carbon dioxide accumulation.
Humans and other non-marine mammals
exhibit this same response, though not as
powerfully as marine mammals do. This
process can be tested quite easily:
1) Measure your own resting heart rate by
sitting still and finding your pulse, on your
wrists or under your jaw bone.
2) Count the number of beats in 15
seconds, and multiply that number by 4 to
get your rate per minute. Repeat this three
or four times to get an average heart rate.
3) Do this same procedure while holding
your breath.
4) Do this same procedure while holding your breath and submerging your face in cold (icy) water.
You may need someone else to take your pulse while you do this.
How do the heart rates of the three scenarios compare to each other?
An adult human heart rate lower than 50 beats per minute (bpm) can be classified as 'bradycardia'.
This slower heart rate can be triggered by the holding of one's breath and submergence in water.
For marine mammals, such as the grey seal (Halichoerus grypus) shown in the BBC video below,
there are several other physiological responses that work in conjunction with bradycardia. Their
lungs collapse during a dive as to decrease buoyancy and force air out of the lungs and into the
airways. Blood is directed out of less critical parts of the body and only into those necessary for
thinking and swimming. Air is blown out of the lungs at the start of the dive, not only to help the
animal sink, but also to rid excess nitrogen from the blood stream. These responses work not only to
conserve oxygen usage, but also to prevent the decompression sickness (commonly known as "the
bends") by keeping air in the airways rather than the lungs. In addition, the cold temperatures at
water depths can work to slow the rate of oxygen uptake by major organs.
When the seal begins its ascent to the surface, the lungs will re-expand back to their original size
and the heart rate will steadily quicken back to normal. During this time, the animal will likely feel

2. rapid warmth return to his or her extremities, allowing for a faster muscular response.
It has been observed that younger marine mammals do not exhibit as strong of a bradycardia
response as older members of the same species. This is measured in the heart rate itself, as a lower
heart rate during a dive indicates a stronger bradycardia. With maturity also comes a higher
concentration of myoglobin, an oxygen-bonding protein in the muscle tissue of vertebrates.
Myoglobin exists in high concentration particularly in marine mammals, with a direct correlation to
diving depth of certain mammals.
By regulating both the amount and distribution
of vital oxygen during a dive, the bradycardia
response allows for many marine mammals to
achieve deeper dives, and with it a greater
variety of food sources. This is one of many
remarkable evolutionary mechanisms for life in
the water that marine mammals exhibit.
http://mermikell.hubpages.com/hub/Marine-Mammal-Bradycardia-The-Diving-Response