Why are there so many capillaries in your bodies?

This is really a Biology question.

The general gist of it is that every cell in your body needs nutrients and oxygen, and give off waste products and carbon dioxide. Imagine making a flat sheet of cells (pack them in real tight for efficiency). The area of that sheet would be an estimate for the number of square miles of capillaries needed to feed them all.

Another way of estimating is think of slicing your body into sheets that are as thick as one cell, see how many sheets that would be, and multiplying by the area of that cross-section. This would be a rough estimate (very very rough) but it might be fun.