Fluid Velocity in a Tube

In a medical laboratory, a technician is observing the behavior of a fluid inside a horizontal tube. The tube has a larger diameter on one side and a smaller diameter on the other. The technician notices that when the fluid reaches the narrower section of the tube, it speeds up. This phenomenon can be explained using principles of fluid mechanics.

Which principle best describes the observed increase in fluid velocity as it passes through the narrower section of the tube?