The ability of certain aquatic mammals to dive for extended periods of time in the absence of external oxygen supply has caused much interest in biologists. However, before we learn about the specific adaptations and physiological control of seals during deep sea diving. Let’s first discuss what occurs when a mammal dives. During diving there is arterial constriction of the vascular beds of muscle, skin, kidney, liver, spleen, and presumably of all vascular beds except those located in the brain and heart (Bron 1966). When a mammal dives, muscular arteries suddenly constrict and narrow their pathway from the aorta. This results in blood flow loss in all involved organs.
Additionally, since diving stops external absorption of air it also no longer allows for external oxygen supply (Bron 1966). Now since the diving mammal has gone through some rapid and significant changes the animal much be able to live on the oxygen current stored in the lungs or blood and be able to arterial constrict preventing oxygen utilization by peripheral tissues. By conserving and utilizing this limited oxygen supply for oxygen-dependent metabolism in the central nervous system, seals are able to deep-sea dive for longer periods of time then expected.
It is estimated that approximately 20% of resting oxygen consumption is utilized by the central nervous system (40 to 50 ml/min). Therefore, given total stores of 1000 ml used at the rate of 50 ml/min in the central nervous system of a seal, a seal could have enough oxygen for 20 minutes without need for an external oxygen source. This explains why most seals upper diving limit is about 20 minutes. (Bron 1966)
However, it is due to the primary adaptive mechanisms I will discuss in more detail on my other webpages that permit prolonged diving. If it were not for these adaptations that use arterial constriction allowing for conservation and use of oxygen for cerebal metabolism, then once oxygen reserves are depleated, either the dive must be over or death will occur.
Picture provided with permission from Gimp-Savvy.com