1) In plain language, please explain WHY the juxtameduallary nephron allows birds and mammals to produce hyperosmotic urine relative to their body

1) In plain language, please explain WHY the juxtameduallary nephron allows birds and mammals to produce hyperosmotic urine relative to their body fluids. (Looking for the larger principle behind this, not every detail of how the nephron works.

2) Lidocaine is a frequently used anesthetic to numb a localized area of the body. Lidocaine is known to impair the voltage gated sodium channels in neurons (and muscle cells). Using what you know about neurons, why specifically does lidocaine cause numbness?