It is important to note that this is reabsorption and not absorption since sodium is filtered out in the glomerulus. Also crucial to note that most sodium is actually reabsorbed in the proximal convoluted tubule that is about 67%, while approximately 25 % is absorbed in the loop of Henle and the rest is absorbed in the distal convoluted tubule and collecting duct about 8 %.
The short answer is in the loop of Henle sodium (Na+), together with potassium (K+) and chloride (Cl−) ions are reabsorbed from the urine by secondary active transport by a Na-K-Cl cotransporter (NKCC2).
Loop of Henle, long, U-shaped portion of the tubule that conducts urine within each nephron of the kidney. The principal function of the loop of Henle is the recovery of water and sodium chloride from the urine. This function allows production of urine that is far more concentrated than blood, limiting the amount of water needed as intake for survival.
Each nephron of the kidney contains blood vessels and a special tubule. As the filtrate flows through the tubule of the nephron, it becomes increasingly concentrated into urine. Waste products are transferred from the blood into the filtrate, while nutrients are absorbed from the filtrate into the blood.
The liquid entering the loop is the solution of salt, urea, and other substances passed along by the proximal convoluted tubule, from which most of the dissolved components needed by the body—particularly glucose, amino acids, and sodium bicarbonate—have been reabsorbed into the blood. The first segment of the loop, the descending limb, is permeable to water, and the liquid reaching the bend of the loop is much richer than the blood plasma in salt and urea. As the liquid returns through the ascending limb, sodium chloride diffuses out of the tubule into the surrounding tissue, where its concentration is lower. In the third segment of the loop, the tubule wall can, if necessary, effect further removal of salt, even against the concentration gradient, in an active-transport process requiring the expenditure of energy. In a healthy person the reabsorption of salt from the urine exactly maintains the bodily requirement: during periods of low salt intake, virtually none is allowed to escape in the urine, but, in periods of high salt intake, the excess is excreted