Hypertonic dehydration means that water and sodium are lost at the same time, but there is more water deficiency than sodium deficiency, so the serum sodium is higher than the normal range and the extracellular fluid is hypertonic, also known as primary dehydration. When there is more water deficiency 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 the dehydration continues, the osmolarity of extracellular fluid increases further, intracellular fluid moves to the extracellular, and the degree of intracellular dehydration exceeds the degree of extracellular fluid dehydration, which can eventually lead to brain cell dehydration will cause brain dysfunction. Hypotonic dehydration refers to water and sodium deficiency at the same time, but water deficiency is less than sodium deficiency, serum sodium is lower than the normal range, extracellular fluid is hypotonic, the body reduces the secretion of antidiuretic hormone, so that the reabsorption of water in the renal tubules is reduced, and urinary discharge is increased to increase the osmolarity of extracellular fluid. However, the amount of extracellular fluid is counter more reduced. The intertissue fluid enters the blood blood circulation, which partially compensates for the blood volume but makes the reduction of intertissue fluid more than the reduction of plasma. Faced with a significant reduction in circulating blood volume, the body will no longer take into account osmotic pressure and try to maintain blood volume. What tests are done for hypertonic dehydration causing low central venous pressure? The measured central venous pressure blood pressure is less than the normal value of 0.49 kPa. The blood pressure in the right atrium and the large veins in the thorax is called the central venous pressure. The level of central venous pressure depends on the interrelationship between the ejection capacity of the heart and the amount of blood returned to the heart from the veins. If the heart has a high ejection capacity and is able to eject the blood returning to the heart into the arteries in a timely manner, the central venous pressure is lower. Central venous pressure is another indicator of cardiovascular function. Clinically, when treating shock with infusion, besides observing the change of arterial blood pressure, we also need to observe the change of central venous pressure. The normal range of central venous pressure is 4-12cmH2O, if the central venous pressure is low or has a tendency to decrease, it often indicates that the amount of infusion is insufficient.