Veins are the tubes in the cardiovascular system that guide and carry blood back to the heart. Veins begin in the capillaries and terminate in the atria. Small veins begin in the capillaries and gradually merge into medium and large veins during the return process to the heart and finally into the atria. The venous valves in the venous vessels are damaged and do not close properly, resulting in a continuous flow of blood from the veins. The patient becomes sore and weak. Volumetric load test – can be used as a therapeutic reference for patients with high CVP who still have clinical signs of cardiac output insufficiency. If 500 ml of fluid is rapidly infused within 20 min, the CVP does not increase significantly or even decreases; at the same time, the blood pressure increases and the heart rate decreases, which indicates that the patient has absolute or relative volume deficit and the heart has the potential to continue to receive large amounts of fluid; conversely, the infusion must be done with caution. The lumen of microvenous is irregular, with a diameter of 50-200 μm, with or without smooth muscle outside the endothelium and a thin outer membrane. The microvein immediately adjacent to the capillary is called post-capillary microvein, and its wall structure is similar to capillary, but its diameter is slightly thicker and the gap between endothelial cells is larger, so it is more permeable and also has material exchange function. The posterior microvessels in lymphatic tissues and lymphatic organs also have special structures and functions. Except for large veins, all veins with anatomical names are medium veins. The middle vein has a diameter of 2-9 mm, a thin inner membrane, and a poorly developed or inconspicuous inner elastic membrane. The endothelium is much thinner than that of the accompanying middle artery, and the distribution of circumferential smooth muscle is sparse. The outer membrane is generally thicker than the middle membrane, and there is no outer elastic membrane, and it is composed of connective tissue, and some middle veins may have longitudinal smooth muscle bundles in the outer membrane.