What is The double circulation?

The blood’s road network is two loops through one pump (Definition 27.5’s two sides, now as circuits):

• the lung circuit: right ventricle $\rightarrow$ lungs (loading oxygen, unloading carbon dioxide) $\rightarrow$ left atrium;
• the body circuit: left ventricle $\rightarrow$ the whole body (delivering, collecting) $\rightarrow$ right atrium.

Every drop alternates circuits: lungs, body, lungs, body — which is why the left ventricle, pushing round the far larger body loop, is the heart’s thickest-walled room.