The
thick ascending limb of loop of Henle (TAL) also known as
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<sup>+</sup>) and
chloride (Cl<sup>-</sup>) ions are reabsorbed by
active transport. K<sup>+</sup> is passively transported along its concentration gradient through a K<sup>+</sup> leak channel in the apical aspect of the cells, back into the lumen of the ascending limb. This K<sup>+</sup> "leak" generates a positive
electrochemical potential difference in the lumen. The electrical gradient drives more reabsorption of Na<sup>+</sup>, as well as other
cations such as
magnesium (Mg<sup>2+</sup>) and importantly
calcium Ca<sup>2+</sup>.
Loop diuretics such as furosemide block the
K<sup>+</sup>/Na<sup>+</sup>/2Cl<sup>-</sup> co-transporter.
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...
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