Why a rat have pulmonary circuit?

The rat does not have a pulmonary circuit.

The pulmonary circulation is a separate circuit from the systemic circulation that carries deoxygenated blood away from the heart, through the lungs and back to the heart, where the blood is then pumped out to the body.

The pulmonary circulation is designed specifically for the exchange of gases, so that the deoxygenated blood can pick up oxygen from the lungs and then return to the heart.

The rat, however, has a single-circuit circulatory system, also called systemic circuit, in which the heart pumps out oxygenated blood to the body and the deoxygenated blood returns to the heart, without passing through a separate pulmonary circuit.