Image of Weddell Seal from Wikipedia. 

In many mammals exposure to hypoxic conditions results in the release of a vasodilator called nitric oxide, which improves blood flow and protects oxygen delivery to tissues throughout the body. Diving animals appear to be unique in that some of their blood vessels stay constricted while they are diving even though they experience hypoxia.

A new study published in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology explored why this response to hypoxia in diving mammals was different by examining Weddell seals, Leptonychotes weddellii. They found much lower levels of certain enzymes important for vasodilation stimulated by nitric oxide in seals as compared to dogs, sheep, rats and mice. They also found that blood flow to the brain and heart were maintained during diving although blood flow to the kidneys was decreased. This means that the brain and heart maintained responsiveness to nitric oxide vasodilation. This ability to dilate certain blood vessels while constricting others ensures that oxygen can be delivered to the tissues that most need it during diving.

Source:

Hindle AG, Allen KN, Batten AJ, Huckstadt LA, Turner-Maier J, Schulberg SA, Johnson J, Karlsson E, Lindblad-Toh K, Costa DP, Bloch DB, Zapol WM, Buys ES. Low guanylyl cyclase activity in Weddell seals: Implications for peripheral vasoconstriction and perfusion of the brain during diving. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 316(6): R704-R715, 2019. https://doi.org/10.1152/ajpregu.00283.2018