The crocodilian heart and central hemodynamics.

The crocodilian heart is completely divided into two atria and two ventricles, resembling the arrangement in birds and mammals. However, in addition to the systemic aorta (right aorta, RAo) which emerges from the left ventricle, there is a second aorta (left aorta, LAo) that leaves the right ventricle beside the common pulmonary artery. The two aortae communicate immediately outside the valves through a small aperture, the foramen of Panizza. During diastole, the blood pressures in the RAo and LAo equalize through the foramen, and the pressure in the LAo therefore remains higher (under most circumstances) than that generated by the right ventricle preventing the LAo valve from opening. Blood flow in the LAo is biphasic, with a reversal of blood flow in systole due to the closure of the foramen of Panizza by the medial cusp of the RAo valve. Under these circumstances net LAo flow is low, and due solely to flow through the foramen. When peak systolic right ventricular pressure rises above that in the LAo, the valve will open, producing a (partial) pulmonary bypass (right-to-left shunt). This may occur during pulmonary vasoconstriction, or when the systemic (and hence the LAo) blood pressure decreases.