Final answer:
Aldosterone promotes sodium retention and potassium loss from the blood via the sodium-potassium pump in the distal convoluted tubule, by increasing the activity of Na+/K+ ATPase, resulting in the reabsorption of Na+ and the secretion of K+.
Step-by-step explanation:
The substance that promotes sodium retention and potassium loss from the blood across the wall of the distal convoluted tubule (DCT) is a hormone called aldosterone. Aldosterone works by stimulating the sodium-potassium pump in the walls of the DCT and the collecting ducts of the kidneys.
This hormone increases the synthesis and activity of the Na+/K+ ATPase in the basal membrane, leading to the reabsorption of Na+ into the blood and the secretion of K+ into the renal filtrate, which is subsequently excreted from the body.
When aldosterone causes Na+ to be reabsorbed, a negative electrical gradient is created, which also promotes the secretion of K+ and Cl- into the lumen.
Eventually, peritubular capillaries receive the reabsorbed sodium and chloride along with water, due to osmotic forces, and return them to the circulation, increasing blood volume and sodium levels. Therefore, aldosterone plays a crucial role in regulating the balance of these electrolytes and the overall fluid balance in the body.