1. An intravascular blood pump, comprising a rotatable shaft carrying an impeller and a housing having an opening, wherein the shaft extends through the opening with the impeller positioned outside said housing, the shaft and the housing having surfaces forming a circumferential gap within said opening, wherein the gap has a length and has a width of 2 μm or less over at least a part of said length and wherein at least one of the surfaces forming the gap is made of a material having a thermal conductivity of at least 100 W/mK; wherein the gap converges towards an impeller-side end of the gap and said width of 2 μm or less is located somewhere within 50% of the length of the gap closest to said impeller-side end of the gap.