Tell Me Why...

...I can nick myself with a razor, scrape any part of me on a fence post, or suffer the slightest cut opening a can, and I bleed.  Because I take an aspirin daily, I bleed pretty easily!

But...

Put me in a doctor's office with a nurse armed with a needle, two prominent veins bulging from my arm, a large rubber tube to tie off above my elbow, and my making a strong fist...the nurse can't find blood although she is poking that needle into my arm expressly in search of such.

I don't understand....