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 converge during the return to the heart to form medium veins, large veins, and finally into the atria. What are the symptoms that are easily confused with them? 1.Microvenous lumen is irregular, the diameter is 50~200μm, the smooth muscle outside the endothelium is either present or absent, the outer membrane is thin. The microvein immediately adjacent to capillary is called post-capillary microvein, 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 microvein in lymphatic tissue and lymphatic organs also has special structure and function. 2.Small veins have a diameter of 200μm or more, and the endothelium has a more complete layer of smooth muscle outside. In the larger small veins, there are one to several layers of smooth muscle in the middle membrane. The outer membrane also becomes thicker. 3.Middle vein Except for large veins, all veins with anatomical names are middle veins. The diameter of the middle vein is 2-9mm, the inner membrane is thin and the inner elastic membrane is not developed or not obvious. The inner membrane is much thinner than that of the accompanying middle artery, and the distribution of circular smooth muscle is sparse. The outer membrane is generally thicker than the middle membrane, there is no outer elastic membrane, it is composed of connective tissue, some of the outer membrane of the middle vein can have longitudinal smooth muscle bundle. 4.Large vein The diameter of large vein is more than 10mm, superior vena cava, inferior vena cava, unnamed vein and jugular vein are all belong to this category. The inner membrane of the wall is thin and the middle membrane is very underdeveloped, which is a few layers of loosely arranged circular smooth muscle, sometimes even without smooth muscle. The outer membrane is thicker, and there are often more longitudinal smooth muscle bundles within the connective tissue. 5.Venous valves are often present in veins with a diameter of 2 mm or more. The valves are two semilunar sheets, opposite to each other, the root is connected to the endothelium, and the free edge faces the direction of blood flow. The valve is formed by the endothelium convex into the lumen fold, the center is the connective tissue containing elastic fibers, the surface is covered with endothelium, its role is to prevent blood backflow.