What does phosphate combine with to buffer blood?

The correct answer is sodium. Phosphate acts as a buffer in the blood by combining with various ions, and when it comes to physiological buffering, sodium ions play a significant role. In the body, phosphate can exist in different forms, such as hydrogen phosphate (HPO4^2-) and dihydrogen phosphate (H2PO4^-), which can either donate or accept protons (H+), thus helping to stabilize the pH of blood when there are fluctuations.

Sodium ions are important because they can help to maintain the balance of electrolytes in the fluid compartments of the body. When phosphate binds with sodium, it creates sodium phosphate, which can contribute to maintaining the pH balance and overall homeostasis.

Other options are less relevant in terms of their role as buffers in the blood, as they do not directly participate in the phosphate buffering system to stabilize pH as effectively as sodium does. Understanding the role of sodium in this context is crucial for comprehending electrolyte balance and physiological buffering mechanisms in the body.