Crystal osmotic pressure is the adsorption force on water molecules produced by various ions in the plasma. Because human plasma contains various crystalline substances, such as potassium ions, sodium ions, calcium ions, chloride ions, magnesium ions, etc., these ions will produce a certain adsorption force on water. Also plasma contains colloidal substances such as albumin and globulin, which will also exert a certain attraction to water. The crystal osmolality and colloidal osmolality then make up the plasma osmolality, and in the human plasma osmolality, due to the higher concentration of ionic substances. Therefore, the osmolarity of the human body is mainly composed of crystal osmolarity. Crystal osmolarity is essential to maintain the balance of water inside and outside the cells, because the concentration of particles inside and outside the blood vessels does not differ much, that is, there is no significant difference in crystal osmolarity. However, inside and outside the cell, a different crystal osmolarity difference arises due to the large difference in potassium ion concentration. Only by maintaining the crystal osmolarity difference inside and outside the cell within the normal range, the cell volume can maintain its normal shape. If the crystal osmolarity changes, it may cause a situation of cell rupture.