Hemofiltration is designed by imitating the filtration and reabsorption principle of nephron. The arterial blood of patients is introduced into a semipermeable membrane filter with good permeability and glomerular filtration membrane area. When the blood passes through the filter, the water in the plasma is filtered out (similar to glomerular filtration), thus achieving the therapeutic purpose of removing excess water and solute trapped in the blood. Because the blood flow through the filter is only 20 ~ 30ml/min (only 65,438+0/6 ~ 65,438+0/4 of the renal blood flow), it is impossible to filter out enough liquid only by arterial blood pressure, so it is necessary to pressurize the arterial end with a blood pump and generate a certain transmembrane pressure at the opposite side of the semipermeable membrane with a negative pressure pump, which is generally limited to 66.66kPa(500mmHg). The size of blood filtration rate depends on the area of filtration membrane, transmembrane pressure, sieving coefficient (sieving coefficient of a substance = concentration of a substance in filtration/concentration of a substance in blood) and blood flow. The total filtrate volume of each hemofiltration needs to reach about 20L to achieve better therapeutic effect. In order to compensate the filtered liquid and electrolyte and maintain the balance of the internal environment of the body, it is necessary to supplement the corresponding liquid and electrolyte after (before) the filter to replace the reabsorption function of renal tubules.

The main difference between hemofiltration and hemodialysis is that hemodialysis relies on the dispersion of solute concentration difference on both sides of semi-permeable membrane to remove solute, and its removal efficiency depends on the dispersion of molecules, and the greater the molecular weight, the worse the effect. Hemofiltration, on the other hand, promotes solute transport across the membrane by carrying water solute, and can even remove substances with high molecular weight. The clearance rate of normal glomeruli for creatinine, inulin and other substances with different molecular weights is almost the same. Hemofiltration imitates the principle of clearing solute from normal glomerulus, and filters water and solute in blood by convection. Its clearance rate is independent of molecular weight, and the clearance rate of creatinine and inulin is 100 ~ 120 ml/min. Therefore, hemofiltration is superior to hemodialysis in removing middle molecular substances and similar to normal glomeruli.