In the distal ileum, bicarbonate (HCO3) is exchanged for which ion?

In the distal ileum, bicarbonate (HCO3) is exchanged primarily for chloride ions (Cl-). This transport mechanism plays a crucial role in maintaining electrolyte balance and pH homeostasis. The exchange process involves the movement of bicarbonate ions into the lumen of the intestine, which helps neutralize stomach acid and create an optimal pH for digestive enzymes to function effectively. In return, chloride ions are reabsorbed into the cells lining the ileum.

This bicarbonate-chloride exchange is facilitated by specific transport proteins located in the epithelial cells of the ileum. This mechanism is vital for proper digestion and absorption of nutrients, as well as for the regulatory functions of the intestine. The other ions listed, such as sodium, potassium, and calcium, have different roles in the gastrointestinal tract and do not typically participate in this particular exchange process in the distal ileum. Sodium is more commonly involved in the uptake of glucose and amino acids, potassium plays a role in cellular function and balance, and calcium is crucial for bone health and other metabolic functions but is not directly exchanged with bicarbonate in this context.