Why does the formula for blood velocity hold true?
The trick here is that 1 mL = 1 cm3 (or cc).  So if you divide cm3/s by cm2 the result will be in cm/s.  What that means is that the blood volume has three dimensions - if you picture the vessel as a simple &quot;box&quot;, then it has widths in two dimensions, which define a cross-sectional area, and then the third dimension is the distance that the blood has moved past, through the vessel, in order to define the three-dimensional &quot;box&quot; of blood that has moved past as the blood moved that distance.
The principle should be familiar from the phrase &quot;still waters run deep&quot;.  A stream that flows rapidly in a shallow cascade over rocks can abruptly become quite slow at a deep hole, as a great depth and breadth of water is sharing the work of moving forward.