Since tissue is 3d it is more appropriate to report length density. This is in length per volume. The units of this are 1/mm or sometimes written as mm^(-1). Here I have used the caret symbol to represent exponentiation.
The number of capillaries per unit area is also in units of 1/mm, not mm2. Remember that the counts are not capillary counts, but capillary intercepts. This is an important distinction. A single capillary may be observed as forming more than 1 intercept on a slide. Under certain circumstances the number of intercepts per unit area is proportional to the length density of the capillaries in the tissue.
1. The capillary intercepts must be counted in an unbiased manner - use a counting frame
2. The sections used in the counting must be IUR.
3. The sampled areas used in the counting must be randomly chosen
When these conditions are met then the Lv = 2Qa formula applies.