The loss of body fluids resulting in a decrease in extracellular fluid is called dehydration. When a person works under high temperature conditions, exercises strenuously or has certain diseases (such as violent vomiting or severe diarrhea), a large amount of water and inorganic salts (mainly sodium salts) will be lost, which will lead to a decrease in the body’s extracellular fluid osmolarity and symptoms such as decreased blood pressure, increased heart rate, four chills, and even coma in severe cases. Signs and symptoms of dehydration are the most common and dominant clinical features; patients have thirst, low urine, high specific gravity of urine, dry lips, reduced skin elasticity and sunken eyes, etc. Blood volume and blood pressure changes are lighter and later than normal levels. Typical manifestations are reduced skin elasticity, prolonged skin spreading time, sunken eye sockets and chimneys, dry tongue and oral membrane, dry skin in the axilla and groin, and skin easily “wrinkles. If there is tachycardia, upright hypotension, lower blood pressure, jugular vein atrophy. If the central venous pressure is reduced, the blood volume has been reduced, the effective circulating blood volume has been reduced, and signs of circulatory insufficiency due to dehydration have appeared. Differential diagnosis of extracellular fluid reduction: (1) Water-deficiency dehydration: Water loss is greater than salt loss, resulting in increased osmolarity of extracellular fluid and transfer of water from intracellular to extracellular. (2) Salt-deficient dehydration: Loss of salt is greater than loss of water, resulting in a decrease in the osmolarity of extracellular fluid and drainage of water from the kidneys, causing a decrease in the volume of extracellular fluid.