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Blood vessel
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Blood vessels are the tubular structures of a circulatory system that transport throughout many bodies. Blood vessels transport , nutrients, and oxygen to most of the tissues of a body. They also take waste and carbon dioxide away from the tissues. Some tissues such as , , and the lens and of the are not supplied with blood vessels and are termed avascular.

There are five types of blood vessels: the , which carry the blood away from the ; the ; the , where the exchange of water and chemicals between the blood and the tissues occurs; the ; and the , which carry blood from the capillaries back towards the heart.

The word vascular, is derived from the Latin vas, meaning vessel, and is mostly used in relation to blood vessels.


Etymology
  • artery – late Middle English; from Latin arteria, from Greek artēria, probably from airein ("raise").
  • vein – Middle English; from Old French veine, from Latin vena.
  • capillary – mid-17th century; from Latin capillaris, from capillus ("hair"), influenced by Old French capillaire.


Structure
The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:
  • The inner layer, , is the thinnest layer. It is a single layer of flat cells (simple squamous epithelium) glued by a intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel.
  • The middle layer of 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 called external elastic lamina. The tunica media may (especially in arteries) be rich in vascular 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 the tunica adventitia and the thickest layer in veins. It is entirely made of connective tissue. It also contains that supply the vessel as well as nutrient capillaries () in the larger blood vessels.

consist of a single layer of with a supporting subendothelium consisting of a basement membrane and connective tissue. When blood vessels connect to form a region of diffuse vascular supply, it is called an anastomosis. Anastomoses provide alternative routes for blood to flow through in case of blockages. Veins can have valves that prevent the backflow of the blood that was being pumped against gravity by the surrounding muscles. In humans, arteries do not have valves except for the two 'arteries' that originate from the heart's ventricles.The exception is the and the .

Early estimates by Danish physiologist suggested that the total length of capillaries in human muscles could reach approximately (assuming a high muscle mass human body, like that of a ). However, later studies suggest a more conservative figure of taking into account updated capillary density and average muscle mass in adults.


Types
There are various kinds of blood vessels:

They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing away from (arterial) or toward (venous) the . The term "arterial blood" is nevertheless used to indicate blood high in , although the carries "venous blood" and blood flowing in the is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.


Function
Blood vessels function to transport to an animal's body tissues. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and its organs, and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout the circulatory system. (bound to in red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the , hemoglobin is highly saturated (95–100%) with oxygen. In all veins, apart from the , the saturation of is about 75%. (The values are reversed in the pulmonary circulation.) In addition to carrying oxygen, blood also carries , and to the cells of a body and removes .

Blood vessels do not actively engage in the transport of blood (they have no appreciable ). Blood is propelled through arteries and arterioles through pressure generated by the . Blood vessels also transport red blood cells. 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 to and long-term blood loss.

Permeability of the is pivotal in the release of nutrients to the tissue. It is also increased in in response to , and , which leads to most of the symptoms of inflammation (swelling, redness, warmth and pain).


Constriction
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 .

The size of blood vessels is different for each of them. It ranges from a diameter of about 30–25 millimeters for the to only about 5 micrometers (0,005mm) for the capillaries. Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the vascular smooth muscle in the vessel walls. It is regulated by (agents that cause vasoconstriction). These can include factors (e.g., ), a number of (e.g., vasopressin and ) and (e.g., ) from the nervous system.

is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is (termed endothelium-derived relaxing factor for this reason).


Flow
The circulatory system uses the channel of blood vessels to deliver blood to all parts of the body. This is a result of the 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.

The blood pressure in blood vessels is traditionally expressed in (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 (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg.

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.

(2025). 9780071316385, McGraw-Hill.
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.).

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. 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.


Disease
Blood vessels play a huge role in virtually every medical condition. , for example, cannot progress unless the tumor causes (formation of new blood vessels) to supply the malignant cells' metabolic demand. represents around 85% of all deaths from cardiovascular diseases due to the buildup of . Coronary artery disease that often follows after atherosclerosis can cause heart attacks or , resulting in 370,000 worldwide deaths in 2022. 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.

Blood vessel permeability is increased in . Damage, due to or spontaneously, may lead to due to mechanical damage to the vessel . In contrast, occlusion of the blood vessel by , an or a leads to downstream (insufficient blood supply) and possibly ( due to ). Vessel occlusion tends to be a positive feedback system; an occluded vessel creates eddies in the normally or blood currents. These eddies create abnormal fluid velocity gradients which push blood elements, such as cholesterol or bodies, to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage.

(1994). 9780122824708, Academic Press.

The most common disease of the blood vessels is 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. is inflammation of the vessel wall due to autoimmune disease or .

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