Venules, veins, and special vascular systems. A Venules, veins, and special vascular systems. A. Cross-section of a large venule. The arrow indicates the nucleus of an endothelial cell in a large or muscular venule that is packed with red blood cells. The wall contains two or three layers of nuclei, some of which have the corkscrew shape that is characteristic of smooth muscle cells (SM). An arteriole (ar) is present for comparison. B. Section of the wall of a small vein. This is a higher power view of a portion of the vein seen in Figure 7-3B. The arrow indicates an endothelial cell nucleus, which is all that is visible of the tunica intima. The tunica media (TM) contains two or three layers of smooth muscle cells. The tunica adventitia (TA) merges with adjacent adipose tissue. C. Section of the wall of the vena cava. The approximate locations of the borders of the tunica media (TM) are indicated by horizontal lines; a precise boundary is not visible. The tunica intima (TI) is relatively thick, and the extensive tunica adventitia (TA) contains several profiles of small blood vessels, the vasa vasorum, two of which are indicated by an asterisk (*). D. Specialized vascular connections. (1) Arteriovenous anastomoses can bypass adjacent capillary beds. (2) Two separate capillary beds are connected in series in some organs. In the case of the portal system, the connecting vessel is a vein that delivers blood from the small intestine to the liver. The connective vessel is an arteriole in the glomerular system (kidney) or hypothalamus to the pituitary gland (brain). Source: CARDIOVASCULAR SYSTEM, The Big Picture: Histology Citation: Ash R, Morton DA, Scott SA. The Big Picture: Histology; 2017 Available at: https://accessphysiotherapy.mhmedical.com/DownloadImage.aspx?image=/data/books/2058/ashhist_ch7_f005a-c.png&sec=155784299&BookID=2058&ChapterSecID=155784217&imagename= Accessed: October 20, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved