Amphibian and Reptile

Amphibian development provides many opportunities for studying and understanding the ontogeny of physiological processes in vertebrates (19). This article will overview the developmental changes in cardiorespiratory and metabolic physiology, especially during the metamorphic climax. It will also reveal ontogenic  changes in physiology that occur broadly in different amphibian families. For example, the larvae of both anurans (Rana spp.) and urodeles (Ambystoma spp.) show a decreasing dependence upon gills and skin for oxygen uptake as development of the lungs proceeds, but the skin remains important for elimination of CO2 throughout the animal's larval and adult life. Similarly, a rise in blood PCO2 and HCO3- concentration with development occurs in anurans and urodeles, and mechanisms for acid-base balance during metamorphosis change to reflect those of terrestrial air-breathing animals.

While some ontogenetic changes in physiology are quite clearly associated with the transition from an aquatic to a terrestrial habitat (e.g. the changing dependence upon the skin for oxygen uptake), others are less so. For example, heart rate falls precipitously following hatching in strictly water-breathing stages of larval Rana catesbeiana, remains constant during much of middle larval development (the period including the transition to air breathing), and falls sharply once again with metamorphosis to the adult. Vagal tone, which lowers heart rate in resting animals, is absent in young larvae, appears during middle larval development and disappears in the adult. Such changes are not obviously correlated with the transition from aquatic to terrestrial habitat, and could be accounted for in part.
Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, round the body and back to the heart in a single loop. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumped through the pulmonary artery to the lungs. Continued contraction then pumps oxygenated blood round the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.

Figure 2. Amphibian hearts have 2 atria and one ventricle. This provides some separation between oxygenated and deoxygenated blood, although the two types of blood mix in the ventricle. In amphibian hearts, we see double loop circulation - one loop circulates between the heart and the lungs, and the other loop circulates between the heart and the rest of the body.