A blood oxygenator 10 comprising both a gas passage and a blood passage, each passage extending between respective inlets and outlets via a gas-blood interface 32 in which gas exchange between a supply gas and blood occurs. Also provided is a supply gas distribution arrangement allowing the oxygenation gas supply to be modulated differently for different interface regions of the gas-blood interface, the interface regions being arranged successively in blood flow direction for blood directed through the bas-blood interface. In one arrangement the distribution mechanism comprises one or more partitions 26 dividing a gas inlet zone 28 into a plurality of gas inlet compartments 28A, B, each one of which may be provided with a separate inlet port. In an arrangement whereby hollow fibres are provided to carry the supply gas a mechanism allowing the openings of one or more of the fibres to be individually closed and/or opened may alternatively or additionally be provided.
