Blood Vessel Architecture
Blood vessels are responsible for carrying blood from the pulonary system to the heart, to the organs and tissues and back to the heart in a circular system that provides oxygen and nutrients to tissues. Blood vessels range in size from the great vessels, the aorta and vena cava (on the order of cm) to the capillaries less than the diameter of a single blood cell (on the order of µm).1
Generally, blood vessels are composed of three distinct layers, tunica intima, tunica media and tunica adventitia. The intima is composed of an anti-thrombogenic inner layer of endothelial cells, a delicate basement membrane, a thin layer of connective tissue or the lamina propria and a layer of elastic fibers or the internal elastic membrane, which separates the intima from the media. The highlighting factor in the media is the presence of smooth muscle cells oriented circumferentially around the vessel. These smooth muscle cells are regulated by endothelial cells in the intima, and can thus respond to changes in blood flow. Increased flow leads to vasodialation while decreased flow leads to vasoconstriction. This effect is most pronounced in the medium arteries and arterioles. The media also consists of various extracellular matrix proteins collogen and elastin among others, which lend structure and elasticity to the vessel. The adventitia is primarily a connective tissue layer that varies in thickness and composition depending on blood vessel type. Veins larger than 2 mm in diameter also contain one way valves. Since pressure in the venous system is low, these valves assist in maintaining directionality to blood flow on its way back to the heart.1
Endothelial cell orientation is affected to various degrees in each vessel as a result of shear stress transduction. In the medium arteries and arterioles, each of which has a well developed smooth muscle layer, shear stress also serves to regulate vasodialation and constriction. As a result, these vessels serve as the primary source of peripheral resitance to blood flow and can direct blood to where it is needed most including: responses to increased physical activity such as exercise.1
--------------------------------------------------------------------------------------------------------------
1. Rod R. Seely, Trent D. Stephens and Phillip Tate. “Anatomy & Physiology.” 6th ed. McGraw-Hill Companies Inc. New York, New York. 2003
2. http://www.med-ed.virginia.edu/courses/cell/handouts/images/BloodVessel_1.gif
This page was last edited 10 June 2008