Thick ascending limb of loop of Henle

Overview
The thick ascending limb of loop of Henle (or distal straight tubule) is a segment of the nephron in the kidney. It can be divided into two parts: that in the renal medulla, and that in the renal cortex.

Medullary thick ascending limb
The medullary thick ascending limb remains impermeable to water. Sodium, potassium (K+) and chloride (Cl-) ions are reabsorbed by active transport. 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 K+ "leak" generates a positive electrochemical potential difference in the lumen. The electrical gradient drives more reabsorption of Na+, as well as other cations such as magnesium (Mg2+) and importantly calcium Ca2+.

Loop diuretics block the K+/Na+/2Cl- co-transporter.

Urea which remains in the loop creates a solute potential that prevents water completely osmosing out into the interstitial space. This means that while almost all the ions are reabsorbed, there will still be some water in the urine, and hence, the concentration of the filtrate in the loop is decreased here. (If only ions were present, and a certain amount of ions were reabsorbed, one would expect the same amount of the water to be reabsorbed too, and hence the concentration would remain the same, but this is not true.)

This is also the part of the tubule that generates Tamm-Horsfall protein. The function of this protein is not well understood, but is responsible for creating urinary casts.

Cortical thick ascending limb
The difference between the medullary and cortical thick ascending limbs is mainly anatomical. Functionally, they are very similar.

The cortical thick ascending limb drains urine into the distal convoluted tubule.