The present invention relates to a valve unit for a microfluid system comprising a first and a second fluid reservoir, a flow chamber, a fluid channel array and a conveying means, said first fluid reservoir being, via the flow chamber, in fluidic communication with the second fluid reservoir through the fluid channel array,wherein the valve unit can be brought from a first state to a second state so that, in said first state, a fluid can be conveyed by the conveying means from said first fluid reservoir through the flow chamber in a predetermined direction of flow into the second fluid reservoir, and so that, in said second state, a fluid can be conveyed by the conveying means from said second fluid reservoir through the flow chamber in a predetermined direction of flow into the first fluid reservoir.