At the junction of the thick ascending limb and the distal convoluted tubule are a subset of 15–25 cells known as the macula densa that are part of renal autoregulation through the mechanism of tubuloglomerular feedback.

[1] The thin ascending limb is impermeable to water; but is permeable to ions allowing for some sodium reabsorption.

[2] Salt moves out of the tubule and into the interstitium due to osmotic pressure created by the countercurrent system.

K+ is passively transported along its concentration gradient through a K+ leak channel in the apical aspect of the cells, back into the lumen of the ascending limb.

This drives more paracellular reabsorption of Na+, as well as other cations such as magnesium (Mg2+) and importantly calcium Ca2+ due to charge repulsion.