Like other cartilaginous fish, the circulatory system of mantas is single-circulation. The heart is located in the pericardial cavity and is made up of four chambers: the atrium and ventricle for both deoxygenated and oxygenated blood. Deoxygenated blood from the body enters the heart via the sinus venosus and is then pumped into the ventricle by atrial contraction. The ventricle then contracts, pumping the deoxygenated blood out the conus arteriosus, through the branchial arteries, and into the gills for oxygenation.
The oxygenated blood from the gills enters the heart via the afferent gill arteries, which connect to the dorsal aortae. The dorsal aortae run along the spine, providing oxygen-rich blood to the rest of the body.
Mantas also have a unique set of blood vessels known as the spiracles, which are located just above the eyes. The spiracles are connected to the mouth and are used to take in water for breathing. When the manta opens its mouth, water is drawn in through the spiracles and into the gills. The gills then remove oxygen from the water and send the oxygenated blood to the rest of the body.
Mantas have a relatively slow heart rate, with a resting heart rate of about 40 beats per minute. This slow heart rate helps the manta to conserve energy.