- Blood vessel
The blood vessels are part of the
circulatory systemand function to transport bloodthroughout the body. The most important vessels in the system are the capillaries, the microscopic vessels which enable the actual exchange of water and chemicals between the blood and the tissues, while the conduit vessels, arteries and veins, carry blood away from the heart and through the capillaries or back towards the heart, respectively.
The arteries and veins have the same basic structure. There are three layers, from inside to outside while the capillaries have only one thick cell:
Tunica intima" (the thinnest layer): a single layer of simple squamous endothelial cells glued by a polysaccharideintercellular matrix, surrounded by a thin layer of subendothelial connective tissueinterlaced with a number of circularly arranged elastic bands called the" internal elastic lamina".
Tunica media" (the thickest layer): circularly arranged elastic fiber, connective tissue, polysaccharide substances, the second and third layer are separated by another thick elastic band called external elastic lamina. The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the caliber of the vessel.
Tunica adventitia": entirely made of connective tissue. It also contains nerves that supply the muscular layer, as well as nutrient capillaries ( vasa vasorum) in the larger blood vessels.
Capillaries consist of little more than a layer of endothelium and occasional connective tissue.
When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis (pl. anastomoses). Anastomoses provide critical alternative routes for blood to flow in case of blockages.
Laid end to end, all the blood vessels in an average human body would encircle the earth twice, a distance of approximately 100,000 kilometers. [ "Heart, How it Works", American Heart Association [http://www.heartsource.org/presenter.jhtml?identifier=4642] ]
There are various kinds of blood vessels:
Aorta(the largest artery, carries blood out of the heart)
**Branches of the aorta, such as the
carotid artery, the subclavian artery, the celiac trunk, the mesenteric arteries, the renal arteryand the iliac artery.
*Capillaries (the smallest blood vessels)
**Large collecting vessels, such as the
subclavian vein, the jugular vein, the renal veinand the iliac vein.
Venae cavae(the 2 largest veins, carry blood into the heart)
They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing "away from" (arterial) or "toward" (venous) the
heart. The term "arterial blood" is nevertheless used to indicate blood high in oxygen, although the pulmonary arterycarries "venous blood" and blood flowing in the pulmonary veinis rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.
Blood vessels do not actively engage in the transport of blood (they have no appreciable
peristalsis), but arteries - and veins to a degree - can regulate their inner diameter by contraction of the muscular layer. This changes the blood flow to downstream organs, and is determined by the autonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as methods of thermoregulation."' Oxygen(bound to hemoglobinin red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95-100%) with oxygen. In all veins apart from the pulmonary vein, the hemoglobinis desaturated at about 75%. (The values are reversed in the pulmonarycirculation.)
blood pressurein blood vessels is traditionally expressed in millimetres of mercury (1 mmHg = 133 Pa). In the arterial system, this is usually around 120 mmHg systolic(high pressure wave due to contraction of the heart) and 80 mmHg diastolic(low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg. Vasoconstrictionis the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the vascular smooth musclein the vessel walls. It is regulated by vasoconstrictors (agents that cause vasoconstriction). These include paracrinefactors (e.g. prostaglandins), a number of hormones (e.g. vasopressin and angiotensin) and neurotransmitters (e.g. epinephrine) from the nervous system. Vasodilationis a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is nitric oxide(termed endothelium-derived relaxing factorfor this reason).
Permeability of the
endotheliumis pivotal in the release of nutrients to the tissue. It is also increased in inflammationin response to histamine, prostaglandins and interleukins, which leads to most of the symptoms of inflammation (swelling, redness and warmth).
Role in disease
Blood vessels play a role in virtually every medical condition.
Cancer, for example, cannot progress unless the tumor causes angiogenesis(formation of new blood vessels) to supply the malignant cells' metabolic demand. Atherosclerosis, the formation of lipidlumps ( atheromas) in the blood vessel wall, is the prime cause of cardiovascular disease, the main cause of death in the Western world.
Blood vessel permeability is increased in
inflammation. Damage, due to trauma or spontaneously, may lead to haemorrhagedue to mechanical damage to the vessel endothelium. In contrast, occlusion of the blood vessel by atherosclerotic plaque, by an embolised blood clotor a foreign body leads to downstream ischemia(insufficient blood supply) and possibly necrosis. Vessel occlusion tends to be a positive feedback system; an occluded vessel creates eddies in the normally laminar flow or plug flowblood currents. These eddies create abnormal fluid velocity gradients which push blood elements such as cholesterol or chylomicronbodies to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage. [Multiphase Flow and Fluidization, Gidaspow et al., Academic Press, 1992] Vasculitisis inflammationof the vessel wall, due to autoimmune diseaseor infection.
capacitance of blood vessels
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