Dolphins
The circulatory systems of marine mammals follow the general mammalian plan, there are most notable for features associated with the diving response. Dolphins have a few very unique features helping them to survive under water. One of these features is the presence of several retia mirabilis ("wonderful nets"). 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, just how humans have to decompress after deep dives to deal with that nitrogen. Dolphins also social blood vassals that ensure a constant supply of blood to the brain, even during deep dives when the water pressure can interrupt with the supply of blood to their brain. Dolphin's circulatory systems are designed to conserve body heat. In the dorsal fin, blood flows towards the extremities while passing through arteries which are surrounded by veins carrying the returning blood. This process is able minimize the total amount of blood that is lost to the surrounding water. This also helps the animal lose more heat during periods of intense activity. As the flow of outgoing blood expands the arteries it 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.