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What is The Principal site of absorption of sodium is and explain in detail?
Other than at thin descending limb of the loop of Henle, all other sites of tubule Na+ are reabsorbed by various mechanisms. 
-        Proximal tubule- 60-70%

Na+- H+ exchange
Na+- Glucose co-transport
Na+- amino acid co-transport
Na+ unitransport

-        The thick ascending limb of the loop of Henle - 30%

Na+- 2Cl-- K+ co-transport

-        Early distal tubule- 7%

Na+- Cl- co-transport

-        Late distal tubule and collecting duct - 3%

ENaC channels