During cell injury caused by hypoxia, sodium and water move into the cell because:

During cell injury due to hypoxia, the primary reason sodium and water move into the cell relates to the role of adenosine triphosphate (ATP) in maintaining cellular homeostasis. Under normal conditions, cells utilize ATP to power the sodium-potassium pump (Na+/K+ ATPase), which actively transports sodium out of the cell and potassium into the cell. This process maintains the appropriate balance of electrolytes and osmotic pressure within the cell.

When hypoxia occurs, the lack of oxygen leads to reduced ATP production via aerobic metabolism. Without sufficient ATP, the sodium-potassium pump cannot function effectively, resulting in sodium accumulation within the cell. As sodium ions increase inside the cell, water follows osmotically to maintain osmotic balance, leading to cellular swelling.

This mechanism explains the link between hypoxia-induced ATP depletion and cellular sodium and water influx. Thus, the correct answer directly addresses the physiological basis of how reduced energy supply impacts the ionic gradients across the cell membrane during hypoxic conditions.