In BME $($and for this class$),$ we generally only consider one of the $3$ directions in a

Cartesian coordinate system. Why is this the case, and which of the directions is

nonzero?

Fluid flow in a biomedical context tends to go in one direction $($or else there

might be some biiig problems$)$ and is considered unaffected by gravity

$($negligible at really small vessels like capillaries really$).$ This eliminates the

vertical y coordinate and one of the orthogonal flat directions leaving us with just

z$.$