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The blood vessels are part of the cardiovascular system and function to transport blood throughout 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.

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

All blood vessels have the same basic structure. There are three layers, from inside to outside:

• Tunica intima (the thinnest layer): a single layer of simple squamous endothelial cells glued by a polysaccharide intercellular matix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circulary arrange elastic band 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 a 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, the blood vessels in an average human body will stretch approximately 62,000 miles--2.5 times around the earth.^{[citation needed]}

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.

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 70%. (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).

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 lipid lumps (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 haemorrhage. In contrast, occlusion of the blood vessel (e.g. by a ruptured atherosclerotic plaque, by an embolised blood clot or a foreign body) leads to downstream ischemia (insufficient blood supply) and necrosis (tissue breakdown).

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

• vascular surgery
• vascular resistance
• anastomosis
• angioplasty

v • d • e Cardiovascular system
Blood  |  Heart  → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → Heart → Pulmonary arteries → Lungs → Pulmonary vein

v • d • e List of arteries of head and neck
Anterior: CC/EC	superior thyroid (superior laryngeal) - lingual (sublingual) facial: cervical branches (ascending palatine, tonsillar, submental, glandular) - facial branches (inferior labial, superior labial, lateral nasal, angular)
Posterior and ascending: CC/EC	occipital - posterior auricular (stylomastoid) - ascending pharyngeal (meningeal branches)
Terminal, superficial temporal: CC/EC	transverse facial - middle temporal (zygomaticoörbital) - anterior auricular - frontal - parietal
Terminal, maxillary: CC/EC	1st part: anterior tympanic - deep auricular - middle meningeal (superior tympanic) - accessory meningeal - inferior alveolar (mylohyoid) 2nd part: deep temporal - pterygoid branches - masseteric - buccal 3rd part: posterior superior alveolar - infraorbital (anterior superior alveolar) - descending palatine - artery of the pterygoid canal - sphenopalatine
portions #1 and 2: CC/IC	cervical portion (carotid sinus) - petrous portion (Vidian, caroticotympanic)
cavernous portion/ophthalmic: CC/IC	orbital group: lacrimal (lateral palpebral) - supraorbital - posterior ethmoidal - anterior ethmoidal - medial palpebral - supratrochlear - dorsal nasal ocular group: central retinal - ciliary (short posterior, long posterior, anterior) - superior hypophysial - inferior hypophysial
cerebral portion: CC/IC	ACA - anterior communicating - MCA (anterolateral central) - posterior communicating - anterior choroidal - circle of Willis
vertebral artery: SC	meningeal branches - posterior spinal - anterior spinal - PICA - basilar (pontine, labyrinthine, AICA, SCA, PCA)
thyrocervical trunk: SC	inferior thyroid (inferior laryngeal, tracheal, esophageal, ascending cervical) - suprascapular - transverse cervical - dorsal scapular

v • d • e List of arteries of torso - abdomen
AA: Anterior	celiac left gastric: esophageal branches splenic: pancreatic branches (greater, dorsal) – short gastric – left gastro-omental common hepatic: proper hepatic (cystic), right gastric, gastroduodenal (right gastro-omental, superior pancreaticoduodenal, supraduodenal) 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
AA: Posterior	visceral: middle suprarenal – renal (inferior suprarenal, ureteral) – gonadal (testicular ♂/ovarian ♀) parietal: inferior phrenic (superior suprarenal) – lumbar – median sacral terminal: common iliac (IIA, EIA)
IIA: Anterior	umbilical (superior vesical, to ductus deferens) – middle rectal – obturator (anterior branch, posterior branch) - inferior gluteal (accompanying of ischiadic nerve, crucial anastomosis) uterine ♀ (helicine, vaginal of uterine, ovarian of uterine, tubal of uterine) - vaginal ♀/inferior vesical ♂ internal pudendal: inferior rectal - perineal (urethral) - posterior scrotal ♂/labial ♀ - bulb of penis ♂/vestibule ♀ - deep artery of the penis ♂ (helicine)/clitoris ♀ - dorsal of the penis ♂/clitoris ♀
IIA: Posterior	iliolumbar (lumbar, iliac)  – lateral sacral – superior gluteal
EIA	inferior epigastric (cremasteric, round ligament) – deep circumflex iliac – femoral

v • d • e List of arteries of upper limbs
Axillary	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	profunda brachii (radial collateral, medial collateral) - superior ulnar collateral - inferior ulnar collateral
Radial	forearm: radial recurrent - palmar carpal branch - superficial palmar branch wrist: dorsal carpal branch (dorsal carpal arch, dorsal metacarpal, dorsal digital) hand: princeps pollicis - radial of index finger - deep palmar arch (palmar metacarpal)
Ulnar	forearm: anterior ulnar recurrent - posterior ulnar recurrent - common interosseous (anterior, posterior, recurrent) wrist: palmar carpal branch - dorsal carpal branch hand: superficial palmar arch (common palmar digital, proper palmar digital) - deep palmar branch - palmar carpal arch

v • d • e List of arteries of lower limbs
EI: Femoral	superficial epigastric - superficial iliac circumflex - external pudendal (superficial, deep/anterior scrotal arteries) - profunda femoris (lateral circumflex femoral, medial circumflex femoral, perforating) - descending genicular
Popliteal	anterior tibial - sural genicular: superior genicular (medial, lateral) - middle genicular - inferior genicular (medial, lateral)
Anterior tibial	dorsalis pedis: tarsal (lateral - medial) - arcuate - dorsal metatarsal/first dorsal metatarsal - deep plantar tibial recurrent (posterior, anterior) anterior malleolar (medial, lateral)
Posterior tibial	circumflex fibular - fibular medial plantar - lateral plantar (plantar arch, plantar metatarsal)