组织学与胚胎学 内分泌系统 双语教学10Circulatory System.pptVIP

Histology, Circulatory system 3. Function of capillary: The place where the material exchanges take place, and the permeability is regulated by many factors, such as histamine, etc. Capillary density is closely related to the local metabolism rate. Larger diameter, thinner wall and fewer smooth muscles than accompanying arteries. Lumen frequently appears irregular (capacitance vessels). Also classified into: Large, Medium-sized and Small veins, Venule and postcapillary venules. Internal and external elastic membranes are absent, The boundaries between 3 layers are obscure. Ⅳ. Veins – General features capacitance vessel 容量血管 1. Venule and postcapillary venules Postcapillary venules are formed by the confluence of 2-4 capillaries, and range in diameter from 10-30um or more. Venules are formed by the union of several postcapillary venules and have a diameter in 50-200um.1-2 layers of smooth muscle cells in the tunica media or absent 2. Small veins Vary in diameter from 0.2-1mm, The tunica media contains mainly one to four layers of circular smooth muscle separated by elastic and collagenous fibers. 3. Medium-sized veins The medium-sized veins, with a diameter in 2-9mm, include all the named veins of gross anatomy apart from the large veins. The tunica media is much thinner than that of corresponding arteries. The tunica advenitia is well-developed and thicker than the tunica media. Some longitudinally-oriented smooth muscle fibers are frequently present. The tunica adventitia is relatively thick. 4. Large veins. Large veins include the venous trunks with a diameter larger than 1cm(颈外静脉、无名静脉、奇静脉、肺静脉、髂外静脉、门静脉和腔静脉等). The tunica media is very thin and contains only a few layers of circular smooth muscle. The tunica adventitia is the thickest of the three layers and consists of longitudinally-oriented smooth muscle cells. 5. Vein valves: Semilunar-like protrusion of tunica intima, which prevents the blood f