Final answer:
The entry of chloride ions into parietal cells from the bloodstream is facilitated by the movement of sodium ions into the cell. Both ions bind simultaneously to symport proteins on the cell surface and are transported in the same direction towards the interstitial space.
Step-by-step explanation:
The entry of chloride ions into parietal cells from the bloodstream is facilitated by the movement of sodium ions into the cell. Both sodium and chloride ions bind simultaneously to symport proteins on the apical surface of the cell, which transport them in the same direction, towards the interstitial space. Sodium moves down its electrochemical and concentration gradient into the cell and takes chloride ions with it. The energy for this process comes from the Na+/K+ ATPase pump that pumps sodium out of the cell at the basal surface.