Cardiovascular Adaptations

Perusing the how-to necropsy guides has been an indispensable use of study time, but to keep learning fresh and to gain further insight into marine mammal anatomy I have started to consult other books. Andersen, H.T. (1969) The biology of marine mammals., has proved a very concise and intriguing source, and lists separate anatomical subjects in chapters. As I was covering the cardial tissues last, I skipped to the corresponding section in Andersen regarding cardiovascular adaptations to diving. Much of this chapter seemed to be review, as I have covered diving adaptations in previous classes (Sperm Whale Diving Adaptations, Marine Mammal Biology I; The Weddell Seal, Polar Ecology and Exploration), but I did appreciate Andersen's inclusion of the history of diving adaptation discovery in marine mammals, and his comparisons to terrestrial mammals. I was less grateful for all the descriptions of forcibly submerging and drowning mammals, but who has time to delve into ethical attitudes of the 1960's.

I won't repeat the cardiovascular adaptations here, as my previous papers do a fairly good job of explaining the profound ability of cetaceans and pinnipeds to dive, but I think it is worthy to note several insights the chapter provided. Vasoconstriction is a critical requirement of marathon diving, whereby bloodflow slows and even halts in the peripheral and distal vessels. This lessens the overall oxygen demand, maintaining high levels of oxygen in the organs and tissues most vital to survival and feeding (esophagus, brain, heart). Much of the skeletal muscle, too, is deprived of oxygenated blood during these long dives. While this may seem counterintuitive, as the animal needs to be able to use its muscles to swim, it makes perfect sense in practice. Myoglobin is highly concentrated in muscle tissue and has a higher affinity for oxygen than haemoglobin. If blood flow were not slowed past these muscles, the myoglobin would rob the blood of its oxygen, preventing it from traveling to the aforementioned vital organs. Marine mammals have an incredible tolerance to high blood pH and lactic acid buildup, so their muscles can still operate under anaerobic conditions for the duration of the dive.

What really struck me was how tight this separation between peripheral vessels and muscle was from the main bloodflow. Lactic acid levels were shown to remain at steady levels during the dive, but dramatically rose once the animal had surfaced, before gradually tapering off again. This clearly illustrates the ability of marine mammals to separate parts of their body that are anaerobically respirating from the parts that are reliant on oxygen.

Harrison, R.J. (1971) Functional Anatomy of Marine Mammals., is another source I consulted, but after browsing its chapters, it was clear that Harrison tackles mostly advanced and specialized topics on select marine mammals. This book will come in handy when a certain species requires additional attention, but as a day-to-day guide, there are better options such as Andersen and Dierauf. Speaking of Dierauf, I found a free Internet copy of the Pinniped Forensic, Necropsy and Tissue Collection Guide.