Final answer:
The permeability of magnesium ions in the thick ascending limb of the loop of Henle is thought to occur due to the active reabsorption of ions like Na+ and Cl- through a cotransport system, allowing Mg2+ ions to pass through despite the impermeability to water.
Step-by-step explanation:
The permeability of magnesium (Mg2+) ions through the tight junctions in the thick ascending limb (TAL) of the human kidney's loop of Henle is thought to be facilitated by the unique properties of these junctions.
The TAL is notable for its impermeability to water, which is due to the absence of aquaporin proteins. However, it actively reabsorbs ions such as Na+ and Cl− through a cotransport system.
This system, while primarily focused on Na+ and Cl−, also creates a pathway by which Mg2+ can be reabsorbed.
This is likely because the tight junctions must allow for some ionic permeability to enable the Na+ and Cl− transport, and Mg2+ can take advantage of this pathway to pass through.
More detailed molecular studies would be required to determine the precise mechanism by which Mg2+ permeates these junctions.