Hyperosmotic dehydration refers to the loss of both water and sodium, but the water deficiency is greater than the sodium deficiency, so the serum sodium is higher than the normal range and the extracellular fluid is hyperosmotic. When the water deficiency is greater than the sodium deficiency, the osmotic pressure of the extracellular fluid increases, the secretion of antidiuretic hormone increases, the reabsorption of water by the renal tubules increases, and the urine volume decreases. Aldosterone secretion increases, and the reabsorption of sodium and water increases to maintain blood volume. If the water deficiency continues, the osmotic pressure of the extracellular fluid increases further, and the intracellular fluid moves to the outside of the cell, and eventually the degree of water deficiency in the cell exceeds the degree of water deficiency in the extracellular fluid. Water deficiency in brain cells will cause brain dysfunction.