Excess water 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 excess water in the blood evaporates into the gas. In other embodiments, an undesirable molecule (e.g., ammonia) can be removed from blood by passing the blood through channels that are surrounded by nanotubes with spaces therebetween. 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 undesirable molecule in the blood diffuses into the gas.
