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How do cephalopods breathe with gills inside the mantle cavity?

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A blue-ringed octopus with bright blue rings on its beige body crawls over a rocky, coral-strewn seabed. — cephalopod Blue-ringed octopus (genus *Hapalochlaena*).

Cephalopods breathe by actively ventilating their mantle cavity, a process driven by the rhythmic contraction and expansion of the muscular mantle wall.123

The mantle cavity serves as a protected chamber where the feathery, lamellate gills are suspended. As the mantle muscles expand, the volume of the cavity increases, drawing water in. Subsequent contraction of these circular muscles closes the mantle edge and forces the water out through a mobile funnel. This continuous flow of water over the gill surfaces allows for highly efficient gas exchange, with cephalopods capable of extracting 60 to 80 percent of the oxygen from the water passing through the chamber.231

The circulatory system further supports this respiration. Deoxygenated blood is returned to the heart via the vena cava and directed to two branchial hearts located at the base of each gill. These hearts contract to increase blood pressure, forcing blood through the gill capillaries where oxygen is absorbed and carbon dioxide is released. This process is facilitated by the respiratory pigment hemocyanin, which transports oxygen throughout the body.12