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Tubular channels for blood transport, of which there are three principal types: arteries, capillaries, and veins. Only the larger arteries and veins in the body bear distinct names. Arteries carry blood away from the heart through a system of successively smaller vessels. Capillaries are the smallest but most extensive blood vessels, forming a network everywhere in the body tissues. Veins carry blood from the capillary beds back to the heart through increasingly larger vessels. In certain locations blood vessels are modified for particular functions, as the sinusoids of the liver and the spleen and the choroid plexuses of the brain ventricles. See also Lymphatic system.

Blood vessels are the system of branching and converging tubes which convey blood from the heart to all the various parts of the body and back again, and from the heart to the lungs and back (see blood circulation). The size of blood vessels varies enormously, from a diameter of about 25 mm (1 inch) in the aorta to only 8 μm in the capillaries. This is a 3000-fold range.

The thickness of blood vessel walls also varies enormously, being largest in the large arteries, much less in veins of comparable diameter, and only a single cell thick in the capillaries. Despite the range of sizes the components of the blood vessel walls have a common pattern. All vessels are lined with a single layer of flattened cells called the endothelium. Except for capillaries, all vessels also contain elastic fibres, stiff collagen fibres (similar structure to muscle tendons), and smooth muscle fibres which can constrict or dilate in response to chemical and nervous stimuli. The relative proportions of these components vary in different blood vessels in accordance with their functions.

Recently, the endothelium has been recognized to be of importance in the regulation of the state of constriction or dilatation of the vessels themselves. Of particular note in this respect is ‘endothelial derived relaxation factor’, later shown to be nitric oxide: when this is released, notably in response to the shearing force of the blood on the vessel, it causes dilatation of the vessel.

Large arteries

The aorta and its main branches are called elas-tic arteries. Although they also possess fibrous collagen tissue and smooth muscle, about half of their structure is composed of elastic fibres. These give large arteries a characteristic pale yellow colour. Their wide bore means that they offer little resistance to blood flow, so there is little pressure drop throughout the system of major arteries. The physiological significance of the elastic fibres is that they allow the vessels to expand when blood is ejected intermittently into them from the heart and to constrict again as blood flows out of them into the smaller vessels. The combination of a distensible large vessel and a downstream resistance (arterioles) transforms an intermittent cardiac ejection into a continuous capillary flow.

Small arteries and arterioles

These are the resistance vessels of the circulation and are responsible for determining blood pressure. Arterioles are the vessels at the end of the arterial tree and have a diameter of 20 to 30 μm. Their particular significance is that they have very thick walls in relation to their diameters. Furthermore, the main constituent in their walls is smooth muscle, and the degree of contraction of this muscle regulates the diameter of the vessels and consequently the amount of blood flowing through them. Arterioles are responsible for the largest pressure drop in the circulation. Blood pressure in arteries typically varies from 120 to 80 mm Hg, depending on the phase of the cardiac cycle. In capillaries, the pulsatility is lost and pressure is only about 30 mm Hg.

The muscle in the walls of arterioles possesses an inherent tone. That means that they are normally partly contracted, reducing the size of the lumen to less than the widest possible. The degree of contraction is modified by factors external to the vessels. In particular, the chemical products that are formed as tissues use up energy — the ‘metabolites’ — reach the muscle fibres in the walls of the arterioles and cause them to relax and dilate. This local vasodilatation has the effect of matching local blood flow to tissue energy requirement.

Arterioles can also be regulated by nerves and hormones. These effects tend to be widespread and are concerned mainly with the regulation of arterial blood pressure. Sympathetic nerves have an important role in the control of arterioles. As the frequency of sympathetic nerve impulses increases, more of the transmitter, noradrenaline, is released at the nerve endings, and this causes arterioles to constrict. The adrenal glands also release noradrenaline into the blood but their secretion is mainly of adrenaline. Adrenaline also constricts blood vessels — except those in skeletal muscle, where it dilates them. This diverts blood to the muscle and prepares the body for emergencies as part of the ‘fight or flight’ response.

A 3D image of a brain arteriole constructed from a series of confocal microscopy images. An area of the data was 'digitally' removed — effectively slicing open half of the specimen — to reveal inner structures. Although the blood had been washed out, in this case a single red blood cell remains in the lumen, showing the internal diameter to be only 20-25 micrometres. (Courtesy of C. J. Daly, J. C. McGrath, and S. M. Arribas University of Glasgow.)

Capillaries

These are the ‘exchange vessels’, allowing passage of substances between blood and the fluids outside them which surround the body cells. They consist of a single layer of endothelial cells, with microscopic spaces between adjacent cells which allow the solutes of the blood, including salts, glucose, and dissolved oxygen, to pass into the tissues, and products of tissue metabolism, including carbon dioxide, to pass into the blood. The number of capillaries is so vast that even though they are microscopic their overall resistance to blood flow is low and blood passes through them slowly. The high density of capillaries means the distance for diffusion by the nutrients and gases is small. The more active tissues tend to have a denser supply of capillaries.

Capillaries are formed as a complex system of branching blood vessels between arterioles and venules (microscopic veins). Those near the arteries are at a higher pressure than those near veins. The gaps between endothelial cells are small enough to be almost impermeable to the protein molecules present in the blood, causing the capillary bed to function as a semipermeable membrane. These molecules exert an osmotic force which tends to draw fluid from the tissue spaces into the capillary. This is opposed by the hydrostatic pressure forcing fluid out. A dynamic equilibrium is established, such that at the higher pressure capillaries fluid leaves the circulation, and at the lower pressure ones it is drawn back in. An additional system of vessels, the lymphatics, are fine tubes which provide an alternative route for tissue fluid, via the lymph nodes and back to the circulation.

Disturbance of the balance of the fluid exchange at capillaries can lead to oedema, which is swelling caused by excess tissue fluid. Major causes of this are: a generalized increase in tissue fluid as in heart failure; obstruction to flow through veins or lymphatic vessels such as by cancer growths; and deficiency of blood protein, as in liver or kidney disease or malnutrition, which reduces the osmotic reabsorption force.

Veins

Blood returns from the tissues to the heart along veins. Larger veins possess valves which ensure that blood travels in the correct direction and prevents the development of undue back pressure. Sometimes the valves may cease to function, causing veins to distend abnormally and permanently. This is the cause of varicose veins.

Veins have another important role in addition to being conduits. Approximately 70% of the entire blood volume is contained within the veins, and these are very distensible. This means that they can readily accommodate quite large changes in their volume, either as a result of a change in the total quantity of blood in the circulation (haemorrhage or transfusion), or because of changes in blood distribution (leg veins distend on standing up, for example). The reason that veins can change their volume with little change in pressure is partly because they collapse when empty, which applies to veins above heart level. When filled, the elastic tissue in their walls is readily distensible, although expansion is eventually limited by the relatively indistensible fibrous tissue (collagen).

There is another, active, way in which the volume of blood in veins can be controlled: some veins have the ability to constrict in response to nerve stimulation. Sympathetic nerves supply smooth muscle in the vein walls, and an increase in sympathetic activity, resulting for example from a decreased stimulus to baroreceptors (falling blood pressure), causes venous volume to decrease. The effect of this is to increase filling of the heart and to enhance its output.

Pulmonary vessels

Although the total flow of blood per minute through the lungs is the same as that through the systemic circulation, the pressures are very much lower. Pressure in the pulmonary artery is typically 25/12 mm Hg (systolic/diastolic) compared with 120/80 mm Hg in the aorta and its main branches. The pressure in the lung vessels is lower because they are shorter, wider, have less muscle in their walls, and are very numerous. In particular, there are no muscular resistance vessels like those in the systemic circulation. The pulmonary vessels form a vast low-resistance capillary network which encircles the microscopic air-sacs (alveoli). Gas exchange — of oxygen for carbon dioxide — takes place between blood in the pulmonary capillaries and air in the alveoli.

— Roger Hainsworth

See cardiovascular systemSee also blood circulation; blood pressure; body fluids; lymphatic system; microscopy.

The flexible tubular canals through which blood circulates in the body. Arteries, veins, and capillaries are all kinds of blood vessels. (See circulatory system.)

Any of the vessels conveying the blood; an artery, arteriole, vein, venule, sinusoid or capillary.

• b. v. calcification — in animals usually part of a generalized calcification syndrome; see calcification.
• b. v. congenital defect — see arteriovenous aneurysm, portacaval shunt, patent ductus arteriosus, aortic coarctation.
• coronary b. v. — see coronary arteries.
• b. v. disease — includes arteritis, phlebitis, lymphangitis.
• pulmonary b. v's — (arteries).(veins).
• shunt b. v. — include naturally-occurring arteriovenous anastomoses and those caused by accidental injury or by congenital defect.
• b. v. stenosis — narrowing of the lumen caused by fibrous tissue contraction in the walls, or compression by other adjoining tissues, or congenital defect.

Any one of the network of muscular tubes that carry blood. The kinds of blood vessels are arteries, arterioles, capillaries, venules, and veins.

• Heart and Circulatory and Lymphatic Systems - blood vessel: artery, vein, or capillary

For the singer, see Buster Bloodvessel.

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (August 2008)

Blood vessel
Simple diagram of the human circulatory system
Latin	vas sanguineum

The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart.

Contents 1 Anatomy 2 Types 3 Physiology 4 Role in disease 5 References

Anatomy

The arteries and veins have different structures, veins having two layers and arteries having three:

• Tunica intima (the thinnest layer): a single layer of simple squamous endothelial cells glued by a polysaccharide 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.
• 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 vessel 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.

Types

Blood vessel with an erythrocyte (red blood cell, E) within its lumen, endothelial cells forming its tunica intima (inner layer), and pericytes forming its tunica adventitia (outer layer)

There are various kinds of blood vessels:

• Arteries
  • 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 artery and the iliac artery.
• Arterioles
• Capillaries (the smallest blood vessels)
• Venules
• Veins
  • Large collecting vessels, such as the subclavian vein, the jugular vein, the renal vein and 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 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.

Physiology

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 hemoglobin in 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 hemoglobin is desaturated at about 75%. (The values are reversed in the pulmonary circulation.)

The blood pressure in 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.

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 vasoconstrictors (agents that cause vasoconstriction). These include paracrine factors (e.g. prostaglandins), a number of hormones (e.g. vasopressin and angiotensin) and neurotransmitters (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).

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

Role in disease

Main article: Vascular disease

Blood vessels play a huge 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 lipid lumps (atheromas) in the blood vessel wall, is the most common 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 haemorrhage due to mechanical damage to the vessel endothelium. In contrast, occlusion of the blood vessel by atherosclerotic plaque, by an embolised blood clot or 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 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.^[1]

Vasculitis is inflammation of the vessel wall, due to autoimmune disease or infection.

References

Wikimedia Commons has media related to: Blood vessels

1. ^ Multiphase Flow and Fluidization, Gidaspow et al., Academic Press, 1992

v d e Circulatory system: Arteries and veins (TA A12.0, TH H3.09.02, GA 6.543/GA 7.641) Systemic circulation (Left heart) → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → (Right heart) Pulmonary circulation (Right heart) → Pulmonary arteries → (Lungs) → Pulmonary vein → (Left heart) Blood vessels Endothelium · Tunica intima · Tunica media · Tunica externa Vasa vasorum · Vasa nervorum Rete mirabile · Circulatory anastomosis Arteries Nutrient artery · Arteriole Veins Vena comitans · Superficial vein · Deep vein · Emissary veins · Venous plexus · Venule Lymphatic Lymphatic vessel · Lymph · Lymph capillary M: VAS anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot noco/syva/cong/lyvd/tumr, sysi/epon, injr proc, drug(C2s+n/3/4/5/7/8/9)

v d e List of arteries of head and neck (TA A12.2.05–08, GA 6.549) CC EC sup. thyroid superior laryngeal · sternocleidomastoid branch · infrahyoid branch · cricothyroid branch · glandular branches asc. pharyngeal posterior meningeal · pharyngeal branches · inferior tympanic lingual suprahyoid · dorsal lingual · deep lingual · sublingual facial cervical branches (ascending palatine, tonsillar, submental, glandular) · facial branches (inferior labial, superior labial / nasal septum, lateral nasal, angular) occipital sternocleidomastoid · meningeal · occipital · auricular · descending post. auricular stylomastoid · stapedial · auricular · occipital sup. temporal transverse facial · middle temporal (zygomatico-orbital) · anterior auricular · frontal · parietal maxillary 1st part / mandibular anterior tympanic · deep auricular · middle meningeal (superior tympanic, petrosal) · accessory meningeal · inferior alveolar (mental, mylohyoid) 2nd part / pterygoid to muscles of mastication (deep temporal, pterygoid, masseteric) · buccal 3rd part / pterygopalatine posterior superior alveolar · infraorbital (anterior superior alveolar) · descending palatine (greater palatine, lesser palatine) · artery of the pterygoid canal · sphenopalatine (posterior septal branches, posterior lateral nasal) · pharyngeal IC cervical carotid sinus petrous Vidian · caroticotympanic cavernous/ ophthalmic orbital group:anterior ethmoidal (anterior septal, anterior lateral nasal, anterior meningeal) · posterior ethmoidal · lacrimal (lateral palpebral) · medial palpebral · terminal (supraorbital, supratrochlear, dorsal nasal) ocular group: central retinal · ciliary (short posterior, long posterior, anterior) · hypophysial (superior, inferior) Willis/Cerebral ACA (anterior communicating, medial striate) · MCA (anterolateral central, Orbitofrontal artery, Prefrontal artery, Superior terminal branch, Inferior terminal branch, Anterior temporal branch) · posterior communicating · anterior choroidal SC vertebral artery meningeal · spinal (posterior, anterior) · basilar: pontine · labyrinthine · cerebellar (AICA, SCA, PICA)  · cerebral (PCA) thyrocervical trunk inferior thyroid inferior laryngeal · tracheal · esophageal · ascending cervical · pharyngeal · glandular branches transverse cervical superficial branch · deep branch / dorsal scapular suprascapular acromial branch costocervical trunk deep cervical · Supreme Intercostal artery M: VAS anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot noco/syva/cong/lyvd/tumr, sysi/epon, injr proc, drug(C2s+n/3/4/5/7/8/9)

v d e List of arteries of torso · chest (TA A12.2.01–04,11, GA 6.598) Pulmonary Right pulmonary artery · Left pulmonary artery (Ligamentum arteriosum) Coronary Right coronary: SA nodal · AV nodal · Atrial · Right marginal · Posterior interventricular Left coronary: Anterior interventricular · Left circumflex (Left marginal) Ascending aorta/ aortic arch Brachiocephalic Thyreoidea ima · Right subclavian · Right common carotid Left common carotid External carotid · Internal carotid Carotid body · Carotid sinus · Carotid bifurcation Left subclavian Internal thoracic: Anterior intercostal · Thymic · Pericardiacophrenic · Perforating branches · terminal (Musculophrenic, superior epigastric) Costocervical trunk: Highest intercostal (Posterior intercostal 1–2) · Deep cervical Other Aortic body Descending/ thoracic aorta visceral: Bronchial · Esophageal · Mediastinal parietal: Posterior intercostal 3·11 · Subcostal · Superior phrenic M: VAS anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot noco/syva/cong/lyvd/tumr, sysi/epon, injr proc, drug(C2s+n/3/4/5/7/8/9)

v d e List of arteries of torso – abdomen (TA A12.2.12–15, GA 6.598) AA Parietal inferior phrenic (superior suprarenal) · lumbar · median sacral · Coccygeal glomus Anterior celiac left gastric esophageal branches common hepatic proper hepatic (cystic) · right gastric · gastroduodenal (right gastro-omental, superior pancreaticoduodenal, supraduodenal) splenic pancreatic branches (greater, dorsal) · short gastric · left gastro-omental SMA inferior pancreaticoduodenal · intestinal (jejunal, ileal, arcades, vasa recta)  · ileocolic (colic, anterior cecal, posterior cecal, ileal branch, appendicular)  · right colic · middle colic IMA left colic · sigmoid · superior rectal · marginal Posterior visceral middle suprarenal · renal (inferior suprarenal, ureteral) · gonadal (testicular ♂ / ovarian ♀) terminal/ common iliac IIA Anterior umbilical superior vesical · to ductus deferens ♂ · medial umbilical ligament obturator anterior branch · posterior branch middle rectal vaginal branch ♀ uterine ♀ arcuate · vaginal of uterine · ovarian of uterine · tubal of uterine · spiral V/IV vaginal ♀ · inferior vesical ♂ inferior gluteal accompanying of ischiadic nerve · crucial anastomosis internal pudendal inferior rectal · perineal(posterior scrotal ♂/labial ♀) · bulb of penis ♂/vestibule ♀ · urethral · deep artery of the penis ♂ (helicine)/clitoris ♀ · dorsal of the penis ♂/clitoris ♀ Posterior iliolumbar (lumbar, iliac) · lateral sacral · superior gluteal EIA see arteries of lower limbs M: VAS anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot noco/syva/cong/lyvd/tumr, sysi/epon, injr proc, drug(C2s+n/3/4/5/7/8/9)

v d e List of arteries of upper limbs (TA A12.2.09, GA 6.575) Axillary Shoulder scapular anastomosis · 1st part superior thoracic 2nd part thoracoacromial (deltoid branch) · lateral thoracic 3rd part subscapular (circumflex scapular, thoracodorsal) · anterior humeral circumflex · posterior humeral circumflex Brachial Before split profunda brachii (radial collateral, medial collateral) · ulnar collateral (superior, inferior) Radial forearm: radial recurrent wrist/carpus: dorsal carpal branch · palmar carpal branch hand: superficial palmar branch · princeps pollicis  · Radialis indices artery (radial of index finger) Ulnar forearm: ulnar recurrent (anterior, posterior) · common interosseous (anterior, posterior, recurrent) wrist/carpus: dorsal carpal branch · palmar carpal branch hand: deep palmar branch Arches dorsal carpal arch: dorsal metacarpal (dorsal digital) palmar carpal arch superficial palmar arch: common palmar digital (proper palmar digital) deep palmar arch: palmar metacarpal M: VAS anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot noco/syva/cong/lyvd/tumr, sysi/epon, injr proc, drug(C2s+n/3/4/5/7/8/9)

v d e List of arteries of lower limbs / EIA (TA A12.2.16, GA 6.623) Inferior epigastric cremasteric ♂ / round ligament ♀ Deep circumflex iliac ascending branch of deep circumflex iliac Femoral (EIA after IL) In femoral canal superficial epigastric · superficial circumflex iliac · superficial external pudendal · deep external pudendal (anterior scrotal ♂) Descending genicular saphenous branch · articular branches Profunda femoris medial circumflex femoral (ascending, superficial, deep, acetabular) · lateral circumflex femoral (descending, transverse, ascending) · perforating Popliteal (FA after AH) Genicular superior genicular (medial, lateral) · middle genicular · inferior genicular (medial, lateral) Sural no major branches Anterior tibial tibial recurrent (posterior, anterior) anterior malleolar (medial, lateral) dorsalis pedis: tarsal (medial, lateral) Posterior tibial circumflex fibular · fibular medial plantar · lateral plantar Arches arcuate: dorsal metatarsal / first dorsal metatarsal · deep plantar · dorsal digital arteries plantar arch: plantar metatarsal · common plantar digital · proper plantar digital M: VAS anat(a:h/u/t/a/l,v:h/u/t/a/l)/phys/devp/cell/prot noco/syva/cong/lyvd/tumr, sysi/epon, injr proc, drug(C2s+n/3/4/5/7/8/9)

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