The Blood-Vascular (Circulatory) System

The frog's blood-vascular system has a heart of three chambers - two thin-walled anterior auricles (atria) and one thick-walled posterior ventricle - enclosed in a double-walled pericardium, plus a large, thin, triangular sinus venosus on the heart's dorsal side and a thick, cylindrical truncus arteriosus on its ventral side. The truncus arteriosus arises from the ventricle and splits into a right and left aortic trunk, each further dividing into three aortic arches - carotid (supplying the head), systemic (which join posteriorly to form the dorsal aorta and supply the rest of the body) and pulmo-cutaneous (supplying the lungs and skin). The sinus venosus collects deoxygenated blood from the body through two anterior precaval veins and one posterior postcaval vein and delivers it to the right auricle, while the left auricle simultaneously receives oxygenated blood returning from the lungs and skin through the pulmonary vein; because the single ventricle handles both bloodstreams together, mixing is only partly prevented (by a spiral valve in the truncus arteriosus), which is why a frog's three-chambered heart is less efficient than the fully separated four-chambered heart of birds and mammals. …