Systemic Response to Abnormally Low Blood Pressure

Describe the systemic response to abnormally low blood pressure.

Describe the specific responses of organs and systems (neural and endocrine) to restore normal pressure. Of the same given length between a pair of 2 mm blood vessels or one 4 mm blood vessel, is blood flow the same or higher in one vs. the other? If one is greater, then which one? What is the difference in resistance between a blood vessel of 4mm vs. a blood vessel of 2mm given the same length?