Final answer:
The lumen of the Distal Convoluted Tubule is negatively charged because the sodium-potassium pump transfers a greater number of positively charged ions out of the cell than into it. Additionally, the movement of ions from the lumen to the interstitial space is attracted to this negative charge, promoting reabsorption of key electrolytes.
Step-by-step explanation:
The lumen of the Distal Convoluted Tubule (DCT) is negatively charged due to the activity of the sodium-potassium pump, which moves three sodium ions out of the cell in exchange for two potassium ions into the cell. This exchange causes the interior of the DCT cells to become negatively charged because there is a net movement of positive charge out of the cell.
Furthermore, the movement of positively charged ions like Na+, K+, Ca++, and Mg++ from the lumen into the interstitial space and vasa recta is promoted by the negative charge created by this action. The countercurrent multiplier system in the loop of Henle also contributes to the electrochemical gradient that facilitates the paracellular reabsorption of these cations.
In essence, the DCT cells pump ions against their concentration gradient, a process which requires a large number of mitochondria. Though these cells have fewer microvilli and mitochondria in comparison to those in the proximal convoluted tubule (PCT), they still actively transport ions to maintain homeostasis. The action of the Na+/K+ ATPase, along with paracellular reabsorption across leaky tight junctions, creates an electrochemical gradient conducive to ion reabsorption.