Cetacean Vascular System

The first topic I have selected for my end-of-term review is the vascular system of cetaceans. Note that in my review of pinniped thoracic cavity necropsy I covered the heart, gaining much insight into an organ I wasn't very familiar with. General vascular anatomy is an important topic to study, and I am sure there are cetacean adaptations that are worthy to note. I am consulting Cetacean Dissections (Burne, R.H.) for this review, and will follow the pattern of blood flow described in the book.

All venous blood is deoxygenated except for blood contained in the pulmonary vein (vein denotes any blood vessel whose flow is moving towards from the heart, and is not a characterization of oxygen content). This venous blood converges upon the caval veins (inferior vena cava and superior vena cava). The coronary sinus is located within the vicinity of the entrance of the inferior vena cava into the right atrium, and this collection of vessels drains blood from the myocardium into the right atrium. From the right atrium blood is shuttled through the tricuspid valve, and so forth into the rest of the heart (already detailed in seal heart post).

The retia mirabilia (wonderful net) is a system of closely located arteries and vessels. This system is especially pronounced in cetaceans and serves a variety of purposes. Countercurrent blood flow is created by placing core-warmed arterial blood next to distally-returning venous blood. This sets up a heat gradient that lowers the rate of heat dissipation, allowing the extremities to more gradually heat up or cool down depending on the situation. Burne also hypothesizes that the network of vessels is so elaborate as to allow continuous circulation during diving, as high pressures may restrict some arteries. Scholander (1940) argues that the retia might allow blood to be shunted past the muscles, which are high in myoglobin concentration and would rob arteries of their oxygen content before their blood load arrived to vital areas. The book becomes dated when Burne explains that there is an "observed absence of a diving brachycardia (bradycardia) in the Cetacea." Noren and Cuccurullo (2004) is just one example out of many of observed bradycardia in cetaceans. Although this does not discredit Burne's hypothesis, it does show that this book is indeed from the 1950's. Retia mirabilia appear throughout the body at key areas, such as the thoracic cavity (thoracic retia mirabilia), which is found in between the heads of the ribs and which extend into the spinal canal. This connects the arteries of the chest wall with those that supply the brain.

Finally, a fascinating insight into the formation of the heart in utero is also given by Burne. While the lungs are inactive, the two sides of the heart are still connected. Communication between the right and left side is done through the foramen ovale, a partition between the two atria, thus by-passing the pulmonary artery and the lungs. This blood contains a mixture of arterial blood carried through the umbilical cord and impure blood from the fetal body. Even with the inter-atrial aperture, some blood does pass into the right ventricle and then on into the pulmonary trunk. Were it not for another by-pass between the pulmonary artery and the aorta (ductus arteriosus), this blood would reach the lungs. When the first breath is taken at birth the foramen ovale closes and becomes a slight depression (fossa ovalis), while the ductus arteriosus shrivels into a ligament (ligamentum arteriosum). During the fetal stage, the lungs are supplied with the blood necessary for growth byh bronchial arteries which extend from the aorta, and these arteries continue to nourish the lungs even after birth.

Citations:

Scholander, P.F. (1940) Experimental Investigations on the respiratory functions in Diving Mammals and Birds. Hvalradets skrifter, 22: 1-32. Oslo.

Noren, S.R. Cuccurullo, T. M. (2004) The development of diving bradycardia in bottlenose dolphins (Tursiops truncatus). Journal of Comparative Physiology. Vol. 174. 2. pp. 139-147.