A molecule can be removed from blood by passing the blood through channels that are surrounded by nanotubes with spaces therebetween. Each channel is wide enough for blood to flow through, and the nanotubes are spaced close enough to each other to retain the blood within the channels. Gas passing through the spaces between the nanotubes outside the channels comes into contact with the blood at the outer boundaries of the channels, and the molecule in the blood diffuses into the gas. In other embodiments, a molecule can be introduced into blood by passing the blood through channels that are surrounded by nanotubes with spaces therebetween. Gas that includes the molecule passes through the spaces between the nanotubes outside the channels. The gas comes into contact with the blood at the outer boundaries of the channels, and the molecule in the gas diffuses into the blood.
