The part of nephron involved in active reabsorption of sodium is

${\mathrm{Na}}^{+}$ in the filtrate can enters proximal tubule cells through various membrane proteins. From these cells ${\mathrm{Na}}^{+}$ is pumped out into extracellular fluid by ${\mathrm{Na}}^{+}$ - ${\mathrm{K}}^{+}$- ATPase pump.