Hypertonic dehydration, also known as primary dehydration or hypernatremia with reduced extracellular fluid, is characterized by more water loss than sodium loss, a serum sodium concentration of 150 mmol/L, and a plasma osmolality of 310 mOsm/L. When there is more water deficiency than sodium deficiency, the osmolality of extracellular fluid increases, antidiuretic hormone secretion is increased, renal tubular reabsorption of water is increased, and urine output is decreased. Aldosterone secretion increases, and sodium and water reabsorption increases to maintain blood volume. If dehydration continues, the osmolarity of extracellular fluid increases further, intracellular fluid moves to the extracellular, and eventually the degree of intracellular dehydration exceeds the degree of extracellular fluid dehydration, which can eventually lead to brain cell dehydration which causes brain dysfunction. Hypertonic dehydration refers to the loss of water and sodium at the same time, but the water deficiency is more than sodium deficiency, so the serum sodium is higher than the normal range, and the extracellular fluid is hypertonic. When the water deficiency is more than sodium deficiency, the osmolarity of 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 sodium and water reabsorption increases to maintain blood volume. If water shortage continues, the osmolarity of extracellular fluid increases further, intracellular fluid moves to the extracellular, and eventually the degree of intracellular water shortage exceeds the degree of extracellular fluid shortage, and brain cell water shortage will cause brain dysfunction. In early or mildly ill patients, sodium is still excreted in the urine because the reduction of blood volume is not obvious and aldosterone secretion is not increased, so its concentration may also increase due to increased water reabsorption. In advanced and severe cases, urinary sodium may be reduced due to decreased blood volume and increased secretion of aldosterone. Hyperosmolar dehydration is usually seen in patients with chronic diseases, such as large skin burns, and large amounts of sweat loss. Since sweat is a hypotonic solution, more water is lost than salt, causing an increase in plasma osmolality resulting in this disease. Therefore, preventive measures should pay attention to the above-mentioned patients active rehydration, in the rehydration should pay attention to electrolyte supplementation, can reduce to avoid causing this disease.