Substances that enter the nephron are not all excreted but may exit the tubule and flow back into the blood. This is called resorption (reabsorption).
This often occurs with substances that are particularly beneficial to the body, including electrolytes (Na+, Cl-, K+, Ca++, HCO3-, phosphate), amino acids, peptides, glucose and water.
Resorption occurs in the proximal and distal convoluted tubules and also the loop of Henle. It can occur either passively (i.e. not requiring energy) or can be described as an active process, requiring energy in the form of ATP to get the substances from the nephron back into the blood stream.