Rotary pumps. (A) Illustration of an axial pump, where flow is driven past a straightener consisting of stationary vanes, enters the rotor region that pushes blood through the pump, before it exits the diffuser that contains additional stationary guide vanes. The rotor rotates along mechanical bearings in second-generation devices and is magnetically levitated and driven in third-generation devices. (B) Illustration of centrifugal pump, where the impeller rotates along mechanical bearings in second-generation devices or with magnetic levitation in third-generation devices. The rotation of the impeller pulls blood through the inlet port and expels/throws it radially outward based on guidance of vanes/channels, as it subsequently travels through a volute until it reaches the outlet port. An example cross section is shown for the HeartMate III with an image taken from the instruction manual provided by Abbott. (C) Flow simulations through an axial pump, the Heartmate II (HMII) and a retired centrifugal pump, the Heartware ventricular assistance device (HVAD). Flow streamlines (or paths) are shown to demonstrate how flow travels through each type of pump.¹⁵ The color is the speed of the blood as it travels through the pump, with a corresponding legend provided in the figure. A plot is also shown that shows the volume distribution of shear stress, that is, how much volume of fluid in each pump experiences a particular shear stress range (1 Pa = 10 dynes/cm²).¹⁵ The shear stress in the HM2 is comparable to the HVAD. (D) The wall shear stress distribution is shown for both a axial flow HMII and a centrifugal flow HM3.¹⁶ Shear stress relative to fluid volume is also shown for these simulations.¹⁶ Overall shear stress is higher in the axial flow HMII. It becomes very high along the rotor surface (up to 6000 dynes/cm²), but as demonstrated by the volume to shear stress plot, this does not extend far into the fluid.