By Leslie Cook and Mark Hatfield (1999)
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Along with the organ systems that provide for the metabolic needs of the body, we will consider the circulatory system. Oxygen and food are carried from the respiratory and digestive organs to all the tissues and cells; carbon dioxide and other excretory products are carried from the tissues to sites of removal; hormones are transported from the endocrine glands to the tissues; and heat is distributed throughout the body by the circulatory system. The system also has other functions. It aids in combating disease, repairing tissues, and it helps to maintain the constancy of the internal environment (homeostasis) in many other ways. Refer to (squalusmain) for an overview of the Squalus’s circulatory system.
Ancestral fishes retain parts, at least, of all six embryonic aortic arches, but the aortic arches are obviously modified in the adults for the interposition of gills and the supply of blood to the head. During the course of embryonic development, six complete aortic arches differentiate from anterior to posterior. The ventral portion of the first aortic arch disappears, but the ventral portions of the remaining five form the afferent branchial arteries (branchart). The dorsal parts of the remaining aortic arches form the four efferent branchial arteries (branchart2). When comparing Squalus to nonmammalian tetrapods (e.g. Necturus), the first two arches are lost. In the case of mammals (e.g. Felis), the first, second, and fifth aortic arches are lost. Necturus and Felis demonstrate the reduction and further specialization of aortic arches over evolutionary time. These important changes that have occurred are due to the transition from an aquatic to a terrestrial environment (Walker and Homberger 1992).
The dogfish heart (sqheart), lies in the pericardial cavity, which is solidly encased by the cartilaginous pectoral girdle, the large basibranchial cartilage, and the hypobranchial musculature. Its location within this rigid box permits it to act alternately as a pressure pump or as a suction pump. The heart in Necturus, like Squalus, lies in the pericardial cavity but is encased by an ossified and cartilaginous pectoral girdle. In amphibians, the importance of the changes in the heart and aortic arches is a relative increase in the blood pressure in the dorsal aorta and in the general efficiency of the circulatory system. A frog, for example, has a systolic pressure in the dorsal aorta of 30mm Hg compared with a systolic pressure in the dorsal aorta of a dogfish of 17mm Hg. Being a much smaller animal, the relative efficiency of the frog’s circulation is even greater than these values imply. In Felis, the heart is totally encased by an ossified pectoral girdle. In humans, for example, the left ventricle develops a very systemic hydrostatic pressure in the dorsal aorta, which is approximately 100mm Hg as compared to Squalus and amphibians. Over evolutionary time, from ancestral fishes to modern mammals the changes in the systolic and hydrostatic pressures and encasement of the heart that occur correlate mostly with the increase in activity and rate of metabolism (Walker and Homberger 1992).
The functional significance of the renal portal system (sqveins) is not clear, but it is known to be a low-pressure system. This system is not present in agnathous fishes such as the lamprey, but is present in later ancestral fishes (e.g. Squalus) and amphibians. The system is lost in mammals. The loss of the renal portal system in mammals may be correlated with an increased blood pressure, for a large volume of blood now enters the kidneys directly from the aorta, and with the evolution of different mechanisms for conserving body water (Walker and Homberger 1992).