Respiratory, Cardiovascular and Abdominal Anatomy of the Ringed Seal (Phoca hispida)

This study investigated the anatomy the ringed seal (Phoca hispida) cardiovascular system (heart and large vessels) and lower respiratory tract (lung), as well as the topographic anatomy of the seal abdomen. The ringed seal heart is dorsoventrally flattened, with each ventricle residing on its respective side within the thoracic cage. The heart lies horizontal, parallel to the sternum. The right ventricle is long, spacious and thin walled. The right coronary artery continues as the subsinuosal interventricular branch, making it different from domestic carnivores. The aortic bulb is a large structure in comparison to the equivalent structure in terrestrial mammals, which is hardly noticeable. It lies in contact with the cranioventral aspect of the thoracic cavity. The aortic arch gives rise to three major blood vessels; brachiocephalic trunk, left common carotid and left subclavian artery. The paired caudal vena cavae continue the pelvic venous plexus as paired vessels through the caudal abdominal cavity. At the level of the third lumbar vertebral body the two separate limbs converge into a single vena cava. Soon after this junction as the hepatic veins join the vena cava, they expand into a large hepatic sinus. As the caudal vena cava passes through the diaphragm, muscle fibers of the diaphragm form a caval sphincter. The azygos veins are paired and join into a common trunk before emptying into the cranial vena cava as it enters the right atrium. Intercostal arteries perforate the right azygos vein. There are also communications between left and right azygos veins via short anastomosing branches. The pericardial venous plexus is present in the mediastinal pleura as a loop encompassing the apex of the heart. It is comprised of convoluted veins that eventually drain into the caudal vena cava as a single trunk. Pulmonary arteries are paired vessels that divide into caudal and cranial branches at the level of the principal bronchi. Three common pulmonary veins drain the lung lobes into the left atrium. The lungs are subdivided into seven lung lobes. The right lung is divided into cranial, middle, caudal and accessory lung lobes. The left lung is divided into cranial, middle and caudal lobes. The pulmonary trunk upon reaching the atrial face of the heart divides into the left and right pulmonary arteries that lie adjacent to and follow the dichotomy of the bronchial tree. Three common pulmonary veins are formed by merging of the lobar veins. The common left and right veins drain their respective cranial and middle lobes while the caudal common vein drains both left and right caudal lobes and the accessory lobe. The ringed seal has three major tracheobronchial lymph nodes. Microscopically, the lungs are characterized by a thick subpleural collagenous interstitial tissue, which sends septa deep into the lung parenchyma, thus subdividing the lung lobes into lobules. Segmental bronchi, exhibit a strong elastic fibrous support that is clustered into dense bundles. The bronchial gland duct system of the segmental bronchi is lined with pseudostratified ciliated columnar epithelium with goblet cells. Cartilage support for the bronchial tree extends as far distal as the respiratory bronchiole. Occasionally, respiratory bronchioles have a smooth muscle enhancements at their junction with alveolar ducts. Bronchioles are lined with cuboidal to columnar epithelium. Frequently, bronchioles and respiratory bronchioles have venules covered with squamous epithelium protruding into their lumen. The ringed seal has the cupula of the diaphragm extending cranially to the level of the ninth thoracic vertebra, and is predominantly occupied by the liver. The liver extends caudally to the level of the 15th thoracic vertebra. The gall bladder is located at the level of the 12th thoracic vertebra. The cardia of the U-shaped stomach is located at the The body extends caudally and bends back on itself at the level of the first lumbar vertebra, as the major curvature. The pylorus is located at the level of the 12th thoracic vertebra. The spleen is located on the left side, lying between the liver and cardia of the stomach, extending from the level of the 14th thoracic to the second lumbar vertebrae. When viewing CT scans just medial to the spleen’s dorsal border, the silhouette of the pancreas can be identified. The kidneys extend from the level of the second to the fifth lumbar vertebra. The cranial extremity of the right kidney is located more cranial than that of the left kidney