Like their pinniped cousins, cetaceans are capable of deep dives and long breath holds (apnea). To accomplish this, marine mammals as a group have increased blood volume, increased number of red blood cells, increased hemoglobin, and increased myoglobin in their muscles. All of these elements help them store extra oxygen in their bodies which can be used during a dive. Most marine mammals also undergo lung collapse during a dive which prevents most gas exchange from occurring at depth. During necropsy you can note that this lung collapse is made possible in part by jointed ribs. Also important is a reduction in their heart rate (bradycardia) and overall metabolic rate while diving which helps the oxygen on board go further, thus extending dive time. The capacity for different species to dive for different maximum dive times is a direct function of their oxygen storage capacity and the amount of energy they expend while diving.
Below are images of a harbor porpoise cardiovascular system. Most odontocete (toothed whales) have similar looking cardiovascular systems although they will of course vary considerably in size. Like all mammals, cetaceans have a four chambered heart and the general arrangement of the major blood vessels is similar. One structure that is particularly well developed in porpoises is a RETE MIRABILE on the dorsal surface of the thoracic cavity (see image below). This structure has been hypothesized to have a number of functions including dampening arterial blood pressure and most recently acting as “trap” for nitrogen before the blood reaches the brain. During necropsy be sure to look for this structure as it is quite a remarkable anatomical feature.
Detailed annotated images of the cardiopulmonary system in a harbor porpoise are shown below.
