Blood vessels inside the nostril are designed to warm the incoming air during inhalation. They are therefore close to the surface of the nostril skin. A blow to the nose or even the pressure built up in the nostril whist blowing the nose can cause them to rupture. It is not uncommon for nose bleeds especially in younger people, but they do subside in latter life as the skin toughens. However frequent and totally unusal bleeds should be checked out, especially if one is not in youth.