Blood vessels are the tubular structures of acirculatory system transportingblood in animal bodies.[1] Blood vessels transportblood cells, nutrients, and oxygen to most of thetissues of abody, and also transport waste products and carbon dioxide away from the tissues.[2] Some tissues – such ascartilage,epithelium, and thelens andcornea of theeye – are not supplied with blood vessels, so are termedavascular.
There are five types of blood vessels: thearteries, which carry the blood away from theheart; thearterioles; thecapillaries, where the exchange of water and chemicals between the blood and tissues occurs; thevenules; and theveins, which carry blood from the capillaries back towards the heart.
The word,vascular, is derived from the Latinvas, meaningvessel, and is used in reference to blood vessels.
The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:[6]
The inner layer,tunica intima, is the thinnest layer. It is a single layer of flat cells (simple squamous epithelium) glued by apolysaccharide intercellular matrix, surrounded by a thin layer of subendothelialconnective tissue interlaced with a number of circularly arranged elastic bands called theinternal elastic lamina. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel.
The middle layer oftunica media is the thickest layer in arteries. It consists of circularly arranged elastic fiber, connective tissue and polysaccharide substances; the second and third layer are separated by another thick elastic band calledexternal elastic lamina.[7] The tunica media may (especially in arteries) be rich invascular smooth muscle, which controls the caliber of the vessel. Veins do not have the external elastic lamina, but only an internal one. The tunica media is thicker in the arteries rather than the veins.
The outer layer is thetunica adventitia and the thickest layer in veins. It is entirely made of connective tissue. It also containsnerves that supply the vessel as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.
Capillaries consist of a single layer ofendothelial cells with a supporting subendothelium consisting of abasement membrane andconnective tissue. When blood vessels connect to form a region of diffuse vascular supply, it is called ananastomosis. Anastomoses provide alternative routes for blood to flow through in case of blockages. Veins can havevalves that prevent thebackflow of the blood that was being pumped against gravity by the surrounding muscles.[8] In humans, arteries do not have valves except for the two 'arteries' that originate from the heart's ventricles.[9]
Early estimates by Danish physiologistAugust Krogh suggested that the total length of capillaries in human muscles could reach approximately 100,000 kilometres (62,000 mi) (assuming a high muscle mass human body, like that of abodybuilder).[10] However, later studies suggest a more conservative figure of 9,000–19,000 kilometres (5,600–11,800 mi) taking into account updated capillary density and average muscle mass in adults.[11]
Extremely small vessels located within bone marrow, the spleen and the liver.
They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowingaway from (arterial) ortoward (venous) theheart. The term "arterial blood" is nevertheless used to indicate blood high inoxygen, although thepulmonary artery carries "venous blood" and blood flowing in thepulmonary vein is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.[citation needed]
Blood vessels function to transportblood to an animal's body tissues. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and itsorgans, and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout thecirculatory system.Oxygen (bound tohemoglobin inred blood cells) is the most critical nutrient carried by the blood. In all arteries apart from thepulmonary artery, hemoglobin ishighly saturated (95–100%) with oxygen. In all veins, apart from thepulmonary vein, the saturation ofhemoglobin is about 75%.[13][14] (The values are reversed in thepulmonary circulation.) In addition to carrying oxygen, blood also carrieshormones, andnutrients to the cells of a body and removeswaste products.[15]
Blood vessels do not actively engage in the transport of blood (they have no appreciableperistalsis). Blood is propelled through arteries and arterioles through pressure generated by theheartbeat.[16] Blood vessels also transport red blood cells.Hematocrit tests can be performed to calculate the proportion of red blood cells in the blood. Higher proportions result in conditions such as dehydration or heart disease, while lower proportions could lead toanemia and long-term blood loss.[17]
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 theautonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as methods ofthermoregulation.[21]
The size of blood vessels is different for each of them. It ranges from a diameter of about 30–25 millimeters for theaorta[22] to only about 5 micrometers (0,005mm) for the capillaries.[23] Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting thevascular smooth muscle in the vessel walls. It is regulated byvasoconstrictors (agents that cause vasoconstriction). These can includeparacrine factors (e.g.,prostaglandins), a number ofhormones (e.g.,vasopressin andangiotensin[24]) andneurotransmitters (e.g.,epinephrine) from the nervous system.
The circulatory system uses the channel of blood vessels to deliver blood to all parts of the body. This is a result of theleft and right sides of the heart working together to allow blood to flow continuously to the lungs and other parts of the body. Oxygen-poor blood enters the right side of the heart through two large veins. Oxygen-rich blood from the lungs enters through the pulmonary veins on the left side of the heart into the aorta and then reaches the rest of the body. The capillaries are responsible for allowing the blood to receive oxygen through tiny air sacs in the lungs. This is also the site where carbon dioxide exits the blood. This all occurs in the lungs where blood is oxygenated.[26]
The blood pressure in blood vessels is traditionally expressed inmillimetres of mercury (1 mmHg = 133Pa). In the arterial system, this is usually around 120 mmHgsystolic (high pressure wave due to contraction of the heart) and 80 mmHgdiastolic (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg.[27]
Vascular resistance occurs when the vessels away from the heart oppose the flow of blood. Resistance is an accumulation of three different factors: blood viscosity, blood vessel length and vessel radius.[28] Blood viscosity is the thickness of the blood and its resistance to flow as a result of the different components of the blood. Blood is 92% water by weight and the rest of blood is composed of protein, nutrients, electrolytes, wastes, and dissolved gases. Depending on the health of an individual, the blood viscosity can vary (i.e., anemia causing relatively lower concentrations of protein, high blood pressure an increase in dissolved salts or lipids, etc.).[28]
Vessel length is the total length of the vessel measured as the distance away from the heart. As the total length of the vessel increases, the total resistance as a result of friction will increase.[28] Vessel radius also affects the total resistance as a result of contact with the vessel wall. As the radius of the wall gets smaller, the proportion of the blood making contact with the wall will increase. The greater amount of contact with the wall will increase the total resistance against the blood flow.[29]
Blood vessels play a huge role in virtually every medical condition.Cancer, for example, cannot progress unless the tumor causesangiogenesis (formation of new blood vessels) to supply the malignant cells' metabolic demand.[30]Atherosclerosis represents around 85% of all deaths fromcardiovascular diseases due to the buildup ofplaque.[31]Coronary artery disease that often follows after atherosclerosis can causeheart attacks orcardiac arrest, resulting in 370,000 worldwide deaths in 2022.[32] In 2019, around 17.9 million people died from cardiovascular diseases. Of these deaths, around 85% of them were due to heart attack and stroke.[33]
Blood vessel permeability is increased ininflammation. Damage, due totrauma or spontaneously, may lead tohemorrhage due to mechanical damage to the vesselendothelium. In contrast, occlusion of the blood vessel byatherosclerotic plaque, anembolisedblood clot or aforeign body leads to downstreamischemia (insufficient blood supply) and possiblyinfarction (necrosis due tolack of blood supply). Vessel occlusion tends to be a positive feedback system; an occluded vessel createseddies in the normallylaminar flow orplug flow blood currents. These eddies create abnormal fluid velocity gradients which push blood elements, such as cholesterol orchylomicron bodies, to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage.[34]
The most common disease of the blood vessels ishypertension or high blood pressure. This is caused by an increase in the pressure of the blood flowing through the vessels. Hypertension can lead to heart failure and stroke. Aspirin helps prevent blood clots and can also help limit inflammation.[35]Vasculitis is inflammation of the vessel wall due toautoimmune disease orinfection.
^Potter RF, Groom AC (1983). "Capillary diameter and geometry in cardiac and skeletal muscle studied by means of corrosion casts".Microvascular Research.25 (1):68–84.doi:10.1016/0026-2862(83)90044-4.ISSN0026-2862.PMID6835100.
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