The dermal microcirculation consists of a network of arterioles, the rete subpapillae, or superficial plexus, from which capillaries loop into the dermal papillae and so approach the base of the epithelium, before passing back to a venous plexus which lies immediately below the rete subpapillae.
This drains into a flat intermediate plexus in the middle of the reticular layer of the skin and this plexus further connects with a deep laminar venous plexus at the junction of the dermis and the superficial plexus. Arteriovenous anastomoses are common in the deeper layer of the dermis. Some (glomera) are surrounded by sphincter-like groups of smooth muscle and pursue a convoluted course.
Capillaries are tubes lined by a single layer of polygonal or lanceolate epithelial cells, and exchange of oxygen and other metabolites takes place through the wall of these cells and across the walls of venules and arterioles.
Blood flow in the microcirculation is altered by contraction and relaxation of smooth muscle in the arterioles and cutaneous veins, which affects both the total blood flow and its distribution by means of the precapillary sphincters and arteriovenous anastomoses. These alterations in arteriolar and venous tone are controlled by the sympathetic nervous system.
Leukocytes and platelets accumulate in the microcirculation during venous congestion, and their continued presence and subsequent activation are now considered responsible for some of the complications of chronic venous hypertension.
The walls of veins are similar to those of arteries, being composed of three coats: an inner endothelium, a muscular media and an outer fibrous adventitia. However, they differ from arteries in a number of important details. The endothelium of the intima, secretes factor VIII, prostacyclins and fibrinolytic activator. Recurrent spontaneous thrombosis occurs in patients with inherited or acquired abnormalities in some of these mechanisms. The media consists of collagen and elastin fibres and non-striated muscle fibres arranged circularly.
The elastin fibres are in a smaller proportion than in arterial walls. The muscle fibres are most well developed in the superficial veins, and their contraction is controlled by postganglionic adrenergic sympathetic nerve fibres. By contrast, the media of the deep veins contains relatively little muscle and these veins act mainly as passive blood conduits.
The adventitia consists of areolar tissue with longitudinal elastic fibres. In the largest veins, it is very much thicker than the tunica media and contains longitudinal muscle fibres.
Most veins contain bicuspid valves which direct flow proximally and from the superficial to the deep veins. Valves consist of collagen fibres covered by a thin layer of endothelium and are stronger than the vein wall. Valves are most numerous in the deep veins of the calf and are fewer in the popliteal and femoral veins. The iliac veins are usually valveless. The great saphenous vein contains between two and 13 valves, and each direct calf perforating vein contains one valve.
The venous drainage of the lower limb is divided into the superficial and deep systems, the drainage areas of which are separated by the deep fascia. Thus the superficial veins, the great and small saphenous veins and tributaries of the perforating veins drain the skin and subcutaneous fat (the so-called superficial fascia), and the deep veins are responsible for venous return from muscle and other structures deep to the deep fascia.
The volume of venous blood passing through the deep system far exceeds that through the superficial system, the function of the latter being mainly temperature regulation. The superficial veins communicate with the deep veins at the saphenopopliteal and saphenofemoral junctions and, by way of the perforating veins, through openings in the deep fascia.
Our website is not responsible for the information contained by this article. Articleinput.com is a free articles resource thus practically any visitor can submit an article. However if you notice any copyrighted material, please contact us and we will remove the article(s) in discussion right away.
Note: This article was sent to us by: Karen Pean at 02182011
1. Effects of continuous hormonal infusions
All articles are property of their respective authors. Please read our Privacy Policy!
© 2009 ArticleInput.com.
Partners: Damenmode