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Post-capillary venules are the smallest of the veins with a diameter of between 10 and 30 micrometres (μm). When the post-capillary venules increase in diameter to 50μm they can incorporate smooth muscle and are known as muscular venules. [1] Veins contain approximately 70% of total blood volume, while about 25% is contained in the venules. [2]
The short distance is important because the capillary diffusion rate decreases when the diffusion distance increases. Then, because of its large number (10-14 million capillaries), there is an incredible amount of surface area for exchange. However, this only has 5% of the total blood volume (250 ml 5000 ml).
The venous system apart from the post-capillary venules is a high volume, low pressure system. Vascular smooth muscle cells control the size of the vein lumens, and thereby help to regulate blood pressure. [31] The post-capillary venules are exchange vessels whose ultra-thin walls allow the ready diffusion of molecules from the capillaries. [10]
Animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at a faster rate (~20 seconds of the average 60-second cycle) and shows the red blood cell deforming as it enters capillaries, as well as the bars changing color as the cell alternates in states of oxygenation along the circulatory system.
Cutaneous vasoconstriction will occur because of the body's exposure to the severe cold. Examples of endogenous factors include the autonomic nervous system , circulating hormones, and intrinsic mechanisms inherent to the vasculature itself (also referred to as the myogenic response).
High endothelial venules (HEV) are specialized post-capillary venules characterized by plump endothelial cells as opposed to the usual flatter endothelial cells found in regular venules. [1] HEVs enable lymphocytes circulating in the blood to directly enter a lymph node (by crossing through the HEV).
Terminations of the vasa recta form the straight venules, branches from the plexuses at the apices of the medullary pyramids. They run outward in a straight course between the tubes of the medullary substance and join the interlobular veins to form venous arcades. These in turn unite and form veins which pass along the sides of the renal pyramids.
The capillaries connect to venules, and the blood then travels back through the network of veins to the venae cavae into the right heart. The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O 2, glucose, and enzyme substrates into the