Vein - wikipedia Vein This article is about the blood vessels. For the vascular system in plant leaves, see Leaf veins. For other uses, see Vein ( disambiguation ). Vein The main veins in the human body Structure of a vein, which consists of three main layers. The outer layer is connective tissue, called tunica adventitia or tunica externa ; a middle layer of smooth muscle called the tunica media, and the inner layer lined with endothelial cells called the tunica intima. Details System Circulatory system Identifiers Latin vena TA A12. 0.00. 030 A12. 3.00. 001 FMA 50723 Anatomical terminology ( edit on Wikidata ) Veins are blood vessels that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart ; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart. In contrast to veins, arteries carry blood away from the heart. Veins are less muscular than arteries and are often closer to the skin. There are valves in most veins to prevent backflow. Contents 1 Structure 1.1 Venous system 1.2 Microanatomy 2 Function 3 Clinical significance 3.1 Diseases 3.1. 1 Venous insufficiency 3.1. 2 Deep vein thrombosis 3.1. 3 Portal hypertension 3.1. 4 Other 3.2 Imaging 3.3 Veins of clinical significance 3.4 Phlebology 4 History 5 See also 6 References 7 External links 7.1 Scientific publications Structure Veins are present throughout the body as tubes that carry blood back to the heart. Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small. Superficial veins are those closer to the surface of the body, and have no corresponding arteries. Deep veins are deeper in the body and have corresponding arteries. Perforator veins drain from the superficial to the deep veins. These are usually referred to in the lower limbs and feet. Communicating veins are veins that directly connect superficial veins to deep veins. Pulmonary veins are a set of veins that deliver oxygenated blood from the lungs to the heart. Systemic veins drain the tissues of the body and deliver deoxygenated blood to the heart. Most veins are equipped with valves to prevent blood flowing in the reverse direction. Veins are translucent, so the color a vein appears from an organism 's exterior is determined in large part by the color of venous blood, which is usually dark red as a result of its low oxygen content. Veins appear blue because the subcutaneous fat absorbs low - frequency light, permitting only the highly energetic blue wavelengths to penetrate through to the dark vein and reflect back to the viewer. The colour of a vein can be affected by the characteristics of a person 's skin, how much oxygen is being carried in the blood, and how big and deep the vessels are. When a vein is drained of blood and removed from an organism, it appears grey - white. Venous system See also : List of veins of the human body Branches of Inferior Vena Cava The largest veins in the human body are the venae cavae. These are two large veins which enter the right atrium of the heart from above and below. The superior vena cava carries blood from the arms and head to the right atrium of the heart, while the inferior vena cava carries blood from the legs and abdomen to the heart. The inferior vena cava is retroperitoneal and runs to the right and roughly parallel to the abdominal aorta along the spine. Large veins feed into these two veins, and smaller veins into these. Together this forms the venous system. Whilst the main veins hold a relatively constant position, the position of veins person to person can display quite a lot of variation. The pulmonary veins carry relatively oxygenated blood from the lungs to the heart. The superior and inferior venae cavae carry relatively deoxygenated blood from the upper and lower systemic circulations, respectively. The portal venous system is a series of veins or venules that directly connect two capillary beds. Examples of such systems include the hepatic portal vein and hypophyseal portal system. The peripheral veins carry blood from the limbs and hands and feet. Microanatomy Microscopically, veins have a thick outer layer made of connective tissue, called the tunica externa or tunica adventitia. There is a middle layer of bands of smooth muscle called tunica media, which are, in general, much thinner than those of arteries, as veins do not function primarily in a contractile manner and are not subject to the high pressures of systole, as arteries are. The interior is lined with endothelial cells called tunica intima. The precise location of veins varies much more from person to person than that of arteries. Function Veins serve to return blood from organs to the heart. Veins are also called `` capacitance vessels '' because most of the blood volume ( 60 % ) is contained within veins. In systemic circulation oxygenated blood is pumped by the left ventricle through the arteries to the muscles and organs of the body, where its nutrients and gases are exchanged at capillaries. After taking up cellular waste and carbon dioxide in capillaries, blood is channeled through vessels that converge with one another to form venules, which continue to converge and form the larger veins. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped through the pulmonary arteries to the lungs. In pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the cycle of blood circulation. The return of blood to the heart is assisted by the action of the muscle pump, and by the thoracic pump action of breathing during respiration. Standing or sitting for a prolonged period of time can cause low venous return from venous pooling ( vascular ) shock. Fainting can occur but usually baroreceptors within the aortic sinuses initiate a baroreflex such that angiotensin II and norepinephrine stimulate vasoconstriction and heart rate increases to return blood flow. Neurogenic and hypovolaemic shock can also cause fainting. In these cases, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness. Jet pilots wear pressurized suits to help maintain their venous return and blood pressure. The arteries are perceived as carrying oxygenated blood to the tissues, while veins carry deoxygenated blood back to the heart. This is true of the systemic circulation, by far the larger of the two circuits of blood in the body, which transports oxygen from the heart to the tissues of the body. However, in pulmonary circulation, the arteries carry deoxygenated blood from the heart to the lungs, and veins return blood from the lungs to the heart. The difference between veins and arteries is their direction of flow ( out of the heart by arteries, returning to the heart for veins ), not their oxygen content. In addition, deoxygenated blood that is carried from the tissues back to the heart for reoxygenation in the systemic circulation still carries some oxygen, though it is considerably less than that carried by the systemic arteries or pulmonary veins. Although most veins take blood back to the heart, there is an exception. Portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins, and blood is taken back into the heart. Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver. Clinical significance Diseases Venous insufficiency Main article : Chronic venous insufficiency Venous insufficiency is the most common disorder of the venous system, and is usually manifested as spider veins or varicose veins. Several varieties of treatments are used, depending on the patient 's particular type and pattern of veins and on the physician 's preferences. Treatment can include Endovenous Thermal Ablation using radiofrequency or laser energy, vein stripping, ambulatory phlebectomy, foam sclerotherapy, lasers, or compression. Postphlebitic syndrome is venous insufficiency that develops following deep vein thrombosis. Deep vein thrombosis Main article : Deep vein thrombosis Deep vein thrombosis is a condition in which a blood clot forms in a deep vein. This is usually the veins of the legs, although it can also occur in the veins of the arms. Immobility, active cancer, obesity, traumatic damage and congenital disorders that make clots more likely are all risk factors for deep vein thrombosis. It can cause the affected limb to swell, and cause pain and an overlying skin rash. In the worst case, a deep vein thrombosis can extend, or a part of a clot can break off and land in the lungs, called pulmonary embolism. The decision to treat deep vein thrombosis depends on its size, a person 's symptoms, and their risk factors. It generally involves anticoagulation to prevents clots or to reduce the size of the clot. Portal hypertension The portal veins are found within the abdomen and carry blood through to the liver. Portal hypertension is associated with cirrhosis or disease of the liver, or other conditions such as an obstructing clot ( Budd Chiari syndrome ) or compression from tumours or tuberculosis lesions. When the pressure increases in the portal veins, a collateral circulation develops, causing visible veins such as oesophageal varices. Other Thrombophlebitis is an inflammatory condition of the veins related to blood clots. Imaging Video of venous valve in action Ultrasound, particularly duplex ultrasound, is a common way that veins can be seen. Veins of clinical significance The Batson Venous plexus, or simply Batson 's Plexus, runs through the inner vertebral column connecting the thoracic and pelvic veins. These veins get their notoriety from the fact that they are valveless, which is believed to be the reason for metastasis of certain cancers. The great saphenous vein is the most important superficial vein of the lower limb. First described by the Persian physician Avicenna, this vein derives its name from the word safina, meaning `` hidden ''. This vein is `` hidden '' in its own fascial compartment in the thigh and exits the fascia only near the knee. Incompetence of this vein is an important cause of varicose veins of lower limbs. The Thebesian veins within the myocardium of the heart are valveless veins that drain directly into the chambers of the heart. The coronary veins all empty into the coronary sinus which empties into the right atrium. The dural venous sinuses within the dura mater surrounding the brain receive blood from the brain and also are a point of entry of cerebrospinal fluid from arachnoid villi absorption. Blood eventually enters the internal jugular vein. Phlebology Venous valves prevent reverse blood flow. Phlebology is the medical specialty devoted to the diagnosis and treatment of venous disorders. A medical specialist in phlebology is termed a phlebologist. A related image is called a phlebograph. The American Medical Association added phlebology to their list of self - designated practice specialties in 2005. In 2007, the American Board of Phlebology ( ABPh ), now known as the American Board of Venous & Lymphatic Medicine ( ABVLM ), was established to improve the standards of phlebologists and the quality of their patient care by establishing a certification examination, as well as requiring maintenance of certification. Although not currently a Member Board of the American Board of Medical Specialties ( ABMS ) ; the American Board of Venous & Lymphatic Medicine certification exam is based on ABMS standards. The American College of Phlebology ( ACP ) is one of the largest medical societies in the world for physicians and allied health professionals working in the field of phlebology. The ACP has 2000 members. The ACP encourages education and training to improve the standards of medical practitioners and the quality of patient care. The American Venous Forum ( AVF ) is a medical society for physicians and allied health professionals dedicated to improving the care of patients with venous and lymphatic disease. The majority of its members manage the entire spectrum of venous and lymphatic diseases -- from varicose veins, to congenital abnormalities to deep vein thrombosis to chronic venous diseases. Founded in 1987, the AVF encourages research, clinical innovation, hands - on education, data collection and patient outreach. History Image of veins from William Harvey 's De Motu Cordis et Sanguinis, 1628 This section needs expansion. You can help by adding to it. ( March 2016 ) The valves in veins were described by Jacques Dubois, but their true function was later discovered by William Harvey. See also May -- Thurner syndrome Nutcracker syndrome Thoracic outlet syndrome References Jump up ^ Albert, consultants Daniel ( 2012 ). Dorland 's illustrated medical dictionary. ( 32nd ed. ). Philadelphia, PA : Saunders / Elsevier. p. 2042. ISBN 978 - 1 - 4160 - 6257 - 8. Jump up ^ Kienle, Alwin ; Lilge, Lothar ; Vitkin, I. Alex ; Patterson, Michael S. ; Wilson, Brian C. ; Hibst, Raimund ; Steiner, Rudolf ( 1 March 1996 ). `` Why do veins appear blue ? A new look at an old question ''. Applied Optics. 35 ( 7 ) : 1151. PMID 21085227. doi : 10.1364 / AO. 35.001151. Jump up ^ Maton, Anthea ; Jean Hopkins ; Charles William McLaughlin ; Alexandra Senckowski ; Susan Johnson ; Maryanna Quon Warner ; David LaHart ; Jill D. Wright ( 1993 ). Human Biology and Health. Englewood Cliffs, New Jersey : Prentice Hall. ISBN 0 - 13 - 981176 - 1. Jump up ^ Kahn SR ( August 2006 ). `` The post-thrombotic syndrome : progress and pitfalls ''. British Journal of Haematology. 134 ( 4 ) : 357 -- 65. PMID 16822286. doi : 10.1111 / j. 1365 - 2141.2006. 06200. x. Jump up ^ Power, D'Arcy, William Harvey, in Masters of Medicine ' series, pub. T. Fisher Unwin, London, 1897 External links Look up vein in Wiktionary, the free dictionary. Merck Manual article on veins American Venous Forum American College of Phlebology American Board of Phlebology American College of Phlebology Foundation Australasian College of Phlebology Information from the Australasian College of Phlebology Website In economics : Arterial and venous industries Animated Venous Access tutorials Scientific publications Phlebology : The Journal of Venous Disease Phlebology : 1986 -- 1999 Human systems and organs Musculoskeletal Skeletal system Bone Carpus Collar bone ( clavicle ) Thigh bone ( femur ) Fibula Humerus Mandible Metacarpus Metatarsus Ossicles Patella Phalanges Radius Skull ( cranium ) Tarsus Tibia Ulna Rib Vertebra Pelvis Sternum Cartilage Joints Fibrous joint Cartilaginous joint Synovial joint Muscular system Muscle Tendon Diaphragm Circulatory system Cardiovascular system peripheral Artery Vein Lymphatic vessel Heart Lymphatic system primary Bone marrow Thymus secondary Spleen Lymph node CNS equivalent Glymphatic system Nervous system Brain Spinal cord Nerve Sensory system Ear Eye Integumentary system Skin Subcutaneous tissue Breast Mammary gland Immune system Myeloid Myeloid immune system Lymphoid Lymphoid immune system Respiratory system Upper Nose Nasopharynx Larynx Lower Trachea Bronchus Lung Digestive system Mouth Salivary gland Tongue upper GI Oropharynx Laryngopharynx Esophagus Stomach lower GI Small intestine Appendix Colon Rectum Anus accessory Liver Biliary tract Pancreas Urinary system Genitourinary system Kidney Ureter Bladder Urethra Reproductive system Female Uterus Vagina Vulva Ovary Placenta Male Scrotum Penis Prostate Testicle Seminal vesicle Endocrine system Pituitary Pineal Thyroid Parathyroid Adrenal Islets of Langerhans General anatomy : systems and organs, regional anatomy, planes and lines, superficial axial anatomy, superficial anatomy of limbs Arteries and veins Vessels Arteries Nutrient artery Arteriole Metarteriole Capillaries Types Continuous Fenestrated Sinusoidal Precapillary sphincter Precapillary resistance Veins Vena comitans Superficial vein Deep vein Emissary veins Venous plexus Venule Lymph Lymphatic vessel Lymph Lymph capillary Circulatory system Systemic Left heart → Aorta → Arteries → Arterioles → Capillaries → Venules → Veins → Vena cava → ( Right heart ) Pulmonary Right heart → Pulmonary arteries → Lungs → Pulmonary vein → Left heart Microanatomy Tunica intima Endothelium Internal elastic lamina Tunica media Tunica externa Vasa vasorum Vasa nervorum Vascular nerves Rete mirabile Circulatory anastomosis Veins in the human body Veins of the head and neck External jugular Retromandibular maxillary pterygoid plexus superficial temporal anterior auricular Direct posterior auricular transverse cervical suprascapular anterior jugular jugular venous arch Internal jugular Diploic / brain Cerebral Superficial : superior superficial middle inferior inferior anastomotic ( Labbé ) superior anastomotic ( Trolard ) Deep : great internal basal deep middle superior thalamostriate Cerebellar superior inferior Sinuses To COS superior sagittal straight inferior sagittal occipital To CS sphenoparietal intercavernous superior ophthalmic ethmoidal central retinal nasofrontal vorticose veins inferior ophthalmic To IJV sigmoid : transverse petrosquamous superior petrosal inferior petrosal basilar plexus internal auditory veins condylar Facial / common facial frontal supraorbital angular superior labial inferior labial deep facial Direct lingual dorsal lingual deep lingual sublingual pharyngeal superior thyroid superior laryngeal middle thyroid Brachiocephalic Vertebral occipital occipital emissary suboccipital venous plexus deep cervical Direct inferior thyroid inferior laryngeal thymic Veins of the thorax and vertebral column Thorax Coronary Coronary sinus Great cardiac ( Left marginal ) Posterior of the left ventricle Oblique of the left atrium ( Ligament of the left vena cava ) Middle cardiac Small cardiac vein Anterior cardiac ( Right marginal ) Smallest cardiac veins Right atrial veins Right ventricular veins Pulmonary right inferior pulmonary vein right superior pulmonary vein left inferior pulmonary vein left superior pulmonary vein superior vena cava Brachiocephalic internal thoracic anterior intercostal superior epigastric left superior intercostal supreme vertebral subclavian axillary : lateral thoracic thoracoepigastric dorsal scapular external jugular pericardiacophrenic internal jugular Azygos right superior intercostal bronchial intercostal / posterior intercostal 5 - 11 accessory hemiazygos / hemiazygos superior phrenic Vertebral column vertebral venous plexuses external internal spinal posterior anterior basivertebral intervertebral Veins of the abdomen and pelvis To azygos system ascending lumbar subcostal IVC ( Systemic ) To IVC or left renal vein inferior phrenic hepatic central veins of liver liver sinusoid suprarenal renal gonadal ovarian ♀ / testicular ♂ pampiniform plexus ♂ lumbar common iliac Common iliac Unpaired median sacral vein Internal iliac posterior : iliolumbar superior gluteal lateral sacral anterior : inferior gluteal obturator uterine ♀ uterine plexus ♀ vesical vesical plexus prostatic plexus ♂ deep of penis ♂ / clitoris ♀ posterior scrotal ♂ / labial ♀ vaginal plexus / vein ♀ middle rectal internal pudendal inferior rectal bulb of penis ♂ / vestibule ♀ rectal plexus External iliac inferior epigastric deep circumflex iliac vein Portal vein ( Portal ) Splenic short gastric left gastroepiploic pancreatic inferior mesenteric superior rectal left colic Superior mesenteric right gastroepiploic pancreaticoduodenal jejunal ileal middle colic right colic ileocolic appendicular Direct cystic left gastric / esophageal right gastric paraumbilical Veins of the human arm General Subscapular Axillary Circumflex scapular Superficial arm : Cephalic Accessory cephalic Median cubital Basilic Median antebrachial hand / forearm : Dorsal network Intercapitular Dorsal metacarpal Superficial palmar arch Palmar digital Deep arm : Radial Ulnar Brachial hand / forearm : Deep palmar arch Palmar metacarpal Veins of the human leg thigh femoral deep vein of the thigh popliteal deep leg fibular anterior tibial posterior tibial superficial leg small saphenous great saphenous external pudendal superficial of penis / clitoris foot dorsal arch dorsal metatarsal dorsal digital plantar arch plantar metatarsal common digital plantar digital GND : 4187475 - 4 NDL : 00575032 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Vein&oldid=800916889 '' Categories : Veins Hidden categories : All articles with unsourced statements Articles with unsourced statements from March 2016 Articles to be expanded from March 2016 All articles to be expanded Articles using small message boxes Wikipedia articles with GND identifiers Use dmy dates from April 2017 Talk Contents About Wikipedia Afrikaans Aragonés ܐܪܡܝܐ Авар Azərbaycanca বাংলা Bân - lâm - gú Беларуская Български Boarisch Bosanski Català Čeština Cymraeg Dansk Deutsch Eesti Ελληνικά Español Esperanto Euskara فارسی Français Gaeilge Galego 한국어 Հայերեն हिन्दी Hrvatski Ido Bahasa Indonesia Interlingua Íslenska Italiano עברית Basa Jawa ಕನ್ನಡ Қазақша Kiswahili Kurdî Лакку Latina Latviešu Lietuvių Magyar Македонски മലയാളം Bahasa Melayu Монгол မြန်မာဘာသာ Nederlands नेपाली 日本 語 Norsk Norsk nynorsk Occitan Oʻzbekcha / ўзбекча ਪੰਜਾਬੀ Pangasinan Plattdüütsch Polski Português Română Runa Simi Русский Shqip Sicilianu Simple English Slovenčina Slovenščina کوردی Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Tagalog தமிழ் తెలుగు ไทย Тоҷикӣ Türkçe Українська اردو ئۇيغۇرچە / Uyghurche Tiếng Việt Võro Walon West - 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Question: why is it important for veins to have valves
Answer: to prevent backflow