There are several remarkable parts of the dolphin's circulatory system. One of the most extraordinary features is the presence of several "retia mirabilia" ("wonderful nets"). These are found in some other mammals besides cetaceans. It is thought that these nets of tiny blood vessels serve to protect vital organs from the effects of water pressure, and possibly to trap any bubbles of nitrogen which may form during ascents from deep dives.
Retia in the thorax and around the spine are fed with blood from ateries in the body wall and supply blood directly to the arteries in the spinal canal. It is thought that this arrangement ensures a constant blood supply to the brain, even during deep dives when the water pressure may interrupt the supply of blood.
Another specialized feature in the dolphin's circulatory system is designed to conserve body heat. In the dorsal fin and the flukes, warm blood flowing towards the extremities passes through arteries which are surrounded by veins carrying the returning blood. This minimizes the loss of heat to the environment, and is known as "countercurrent heat exchange". It also helps the animal lose more heat during periods of intense activity. An increase in the flow of outgoing blood expands the arteries, and thus forces the surrounding veins to contract. The blood is forced to return through other veins, closer to the skin, and give up more heat to the environment.