The concentration of sodium ions does not remain unchanged after leaving Bowman's capsule. In fact, the concentration of sodium ions decreases as the filtrate moves through the nephron. This is because the sodium ions are actively reabsorbed by the cells of the proximal tubule. The reabsorption of sodium ions is driven by the active transport of hydrogen ions from the proximal tubule cells into the filtrate. The hydrogen ions bind to the sodium ions, forming sodium hydrogen phosphate, which is then transported out of the proximal tubule cells and into the blood. The reabsorption of sodium ions is essential for maintaining the body's water balance and electrolyte balance.