Frog Circulatory System

The frog possesses a closed circulatory system, meaning blood is at all times confined within a continuous system of vessels, driven by a muscular pump — the heart — that lies within a thin protective sac, the pericardium, in the front part of the body cavity. Unlike the four-chambered heart of a mammal, the frog's heart has only three chambers: two thin-walled atria (the right atrium and the left atrium) sitting above, and a single, thick-walled, muscular ventricle below, into which both atria empty. A sinus venosus, a thin-walled sac that collects venous blood, opens into the right atrium, while the pulmonary vein, carrying oxygenated blood back from the lungs and skin, opens directly into the left atrium. Leading away from the ventricle is a single vessel, the conus arteriosus, which internally contains a spiral valve — a structural feature that turns out to be central to how the frog manages to separate its blood flow despite having only one ventricle.

Because the two atria both drain into a single, undivided ventricle, the oxygen-rich blood arriving from the lungs and skin (via the left atrium) and the oxygen-poor blood arriving from the rest of the body (via the right atrium, having collected first in the sinus venosus) do come into contact and mix to some degree within the ventricle. However, this mixing is far from complete: differences in the timing of atrial contraction and in the internal geometry of the ventricle mean that when the ventricle in turn contracts, the somewhat more oxygen-poor blood is preferentially channelled by the conus arteriosus's spiral valve into the pulmocutaneous arteries, which carry it to the lungs and skin to be reoxygenated, while the somewhat more oxygen-rich blood is preferentially directed into the systemic arches and carotid arteries, which distribute it to the rest of the body's tissues. …