Cool Facts Re Marine Mammal Anatomy

One of the many benefits of studying human anatomy and physiology is the appreciation you will have for the differences seen in other animals. Some of the most interesting variations in structure and function are found in whales, dolphins, and other marine mammals. Consider, for example, these two items contributed by Dr. Lesley Williams, instructor of microbiology at Palomar College, and beginning this term, The Biology of Marine Mammals (BIOL 135): 

#1 Many diving marine mammals have a sphincter around their inferior vena cava that contracts during extended dives. The point is to block the return flow from the lower extremities so that blood in the lower systemic circuit gets clogged. Think of it as the lower systemic circuit becomes like a clogged drain. With the upper systemic circuit and coronary circuitry still "wide open", a larger % of blood is circulated around the coronary circuit and upper systemic circuit (serving the brain). This adjustment allows the marine mammal to conserve oxygen by distributing it to those organs that absolutely must have a steady supply or they fail (e.g. heart, brain) and denying other tissues (such as skeletal muscles) from taking the precious oxygen. 

#2 In almost all mammals, the left ventricular wall is enlarged in muscle mass compared to the right ventricular wall. This anatomical dissimilarity between the right and left side of the heart can be explained by the fact the force generated from the left ventricle must be strong enough to push of blood to the extremities and back again against gravity. In contrast, the right ventricle only needs enough force to pump blood to the adjacent lungs.  Interestingly, whales and dolphins, which live completely aquatic lives, have hearts with equal thickness between the right and left ventricles. Presumably, this is because the left ventricle does not have to deal with the gravitational forces that would be encountered by land mammals. I suspect that lower demand on the heart not only allows for the larger sizes experienced by some whales but also extends the life of the heart.

Very cool!