Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal 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 cartilage, epithelium, and the lens and cornea of the eye are not supplied with blood vessels and are termed avascular.
There are five types of blood vessels: the arteries, which carry the blood away from the heart; the ; the capillaries, 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.
Capillaries 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 pulmonary artery and the aorta.
Early estimates by Danish physiologist August Krogh suggested that the total length of capillaries in human muscles could reach approximately (assuming a high muscle mass human body, like that of a bodybuilder). However, later studies suggest a more conservative figure of taking into account updated capillary density and average muscle mass in adults.
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 artery carries "venous blood" and blood flowing in the pulmonary vein is 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). Blood is propelled through arteries and arterioles through pressure generated by the Cardiac cycle. Blood vessels also transport red blood cells. Hematocrit 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 anemia and long-term blood loss.
Permeability of the endothelium is pivotal in the release of nutrients to the tissue. It is also increased in inflammation in response to histamine, and , which leads to most of the symptoms of inflammation (swelling, redness, warmth and pain).
The size of blood vessels is different for each of them. It ranges from a diameter of about 30–25 millimeters for the aorta 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 paracrine factors (e.g., ), a number of (e.g., vasopressin and angiotensin) and (e.g., epinephrine) from the nervous system.
Vasodilation is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is nitric oxide (termed endothelium-derived relaxing factor for this reason).
The blood pressure in blood vessels is traditionally expressed in torr (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.
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.
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.
Blood vessel permeability is increased in inflammation. Damage, due to Physical trauma or spontaneously, may lead to hemorrhage due to mechanical damage to the vessel endothelium. In contrast, occlusion of the blood vessel by Atheroma, an embolism blood clot or a foreign body leads to downstream ischemia (insufficient blood supply) and possibly infarction (necrosis due to Ischemia). Vessel occlusion tends to be a positive feedback system; an occluded vessel creates eddies in the normally laminar flow or plug flow blood currents. These eddies create abnormal fluid velocity gradients which push blood elements, such as cholesterol or chylomicron bodies, to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage.
The most common disease of the blood vessels is hypertension 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. Vasculitis is inflammation of the vessel wall due to autoimmune disease or infection.
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