Marine mammals, like all other mammals, have a four-chambered heart with a complete divide. The right atrium receives deoxygenated blood from the veins and pumps it into the right ventricle. The right ventricle pumps it into the pulmonary artery, which delivers it to the lungs, where it is oxygenated.
The blood then goes to the the left atrium, which pumps it into the left ventricle. The left ventricle pumps the blood into the aorta, which distributes the blood into smaller arteries all over the body.
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