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== '''Embryology''' == * Processes of blood vessel development ** Vasculogenesis in which vessels arise by coalescence of angioblasts *** Formation of major vessels ** Angiogenesis where vessels sprout from existing vessels *** Formation of the remainder of the system === '''Arterial system''' === ** '''Early vascular development''' *** Truncus arteriosus forms from the outflow tracts of both ventricles, and divides into the aorta and pulmonary trunk in about week 5 *** Aortic sac - most distal part of truncus arteriosus ==== '''Aortic arches''' ==== ** An artery to each pharyngeal arch, arising from the aortic sac ** The five arches are named I, II, III, IV and VI (the fifth arch either never forms or forms then involutes) ** The first arch is obliterated before the fifth arch is formed, but they are numbered as they appear ** The aortic sac forms a right and left horn, which become the brachiocephalic artery and proximal aortic arch respectively ==== Vitelline arteries ==== ** Initially a number of paired vessels supplying the yolk sac ** Gradually fuse and form the arteries in the dorsal mesentery of the gut (coeliac and SMA) ==== Umbilical arteries ==== ** Initially paired ventral branches of the dorsal aorta, coursing to the placenta in close association with the allantois ** They change origin early on to the common iliac artery and persist as the proximal part of that vessel ** Remain paired until birth ** Distal parts are obliterated at birth to form the medial umbilical ligaments ** IMA derived from umbilical arteries === '''Venous system''' === ==== '''Early development''' ==== *** Sinus venosus develops in 4th week, receiving venous blood from the right and left sinus horns initially *** The horns in turn each receive blood from: (all forming week 5) **** Vitelline vein - from yolk sac to sinus venosus **** Umbilical vein - oxygenated blood from chorionic villi to embryo **** Common cardinal vein - draining the blood of the embryo proper *** The left sinus horn rapidly loses importance due to left-to-right blood shunts, but the right side enlarges rapidly ==== '''Vitelline veins''' ==== *** Also called omphalomesenteric veins *** Carry blood from the yolk sac to the sinus venosus *** Left vitelline vein obliterated in week 5 *** A: Originally both veins form a plexus around the duodenum and pass through the septum transversum (future diaphragm) *** B: Hepatic sinusoids (vascular network) forms in the midst of the plexus to support the growing liver cords in the septum *** C: Atrophy of right umbilical vein and left vitelline vein *** D: **** SMV derives from distal right vitelline vein. The point proximal to where it is joined by the splenic vein becomes the PV. **** The more proximal right vitelline vein (proximal to hepatic sinusoids) forms the retro-hepatic IVC ==== '''Umbilical veins''' ==== ** Originate in chorionic villi and carry oxygenated blood to the embryo ** Initially pass on each side of the liver, but some connect to the hepatic sinusoids, and a channel also continues to the hepatocardiac channel (known as ductus venosus, which allows umbilical vein blood to bypass the sinusoidal plexus of the liver). ** Right umbilical vein obliterated in week 5, and then the left umbilical vein is the only one to carry blood from the placenta to the liver. ** After birth, the left umbilical vein and ductus venosus are both obliterated and form the ligamentum teres hepatis and ligamentum venosum respectively ==== '''Cardinal veins''' ==== ** Initially the main venous drainage system of the embryo - anterior cardinal veins for cephalic part and posterior veins for the rest of the embryo. The two veins join into the common cardinal veins prior to entering the sinus horns. ** Additional veins form as part of this system in weeks 5-7: *** Subcardinal veins - draining kidneys *** Sacrocardinal veins - drain the lower extremities *** Supracardinal veins - drain the body wall by way of intercostal veins, taking over the function of the posterior cardinal veins ** SVC formed by the right common cardinal vein and the proximal right anterior cardinal vein ** The anastomosis between the two sacrocardinal veins forms the left common iliac vein (see diagram below) ==== '''SVC embryology''' ==== ** The persisting right anterior cardinal vein ** Below the entrance of azygos vein, it is the persisting right common cardinal vein ==== '''Embryology of IVC''' ==== ** Complex - three embryological veins contributing to different parts ** Starts off as paired right and left system, but the left side shunts vein to the right, and the left veins gradually become smaller/disappear ** Right vitelline vein forms hepatic and supra-hepatic IVC ** Right subcardinal vein - forms renal segment of IVC ** Right sacrocardinal vein becomes infra-renal/'sacrocardinal' IVC ==== '''Azygos embryology''' ==== ** Essentially, originate from the left and right supracardinal veins ** 4th to 11th right intercostal veins empty into the right supracardinal vein, which together with the proximal-most part of the original posterior cardinal vein forms the azygos vein. The left supracardinal vein forms the hemiazygos vein. === '''Circulation before and after birth''' === ** Before birth, blood returns to the fetus via the umbilical vein. Most diverts past the liver through the ductus venosus directly into IVC. ** Mixing of oxygenated and deoxygenated blood can occur at the following places: *** Liver (mixes with some blood from portal system) *** IVC (after ductus venosus) *** Right atrium (mixes with blood from head/upper limbs) *** Left atrium (mixes with blood from lungs) *** Entrance of ductus arteriosus into the descending aorta * === '''Circulatory changes at birth''' === ** Caused by cessation of placental blood flow, and beginning of respiration *** Ductus arteriosus closes, so the amount of blood flowing through lungs increases rapidly **** Mediated by bradykinin, released by the lungs during initial inflation *** Closure of umbilical arteries *** Closure of umbilical vein and ductus venosus *** Closure of oval foramen, pushing septum primum against septum secundum * == Arteries of the neck == * See 'neck' == Subclavian artery == * See 'neck' == '''Thoracic aorta''' == * Surface markings ** Arch begins at second right costal cartilage and ends at second left costal cartilage, and reaches up as far as mid-manubrium ** Descending aorta starts at second left costal cartilage, and ends in midline at about xiphisternum * Ascending aorta ** Emerges pericardium, ends at the level of manubriosternal joint where it becomes the arch * Aortic arch ** Upper convexity reaches as high as the midpoint of the manubrium ** Branches *** Brachiocephalic trunk **** Arises from arch about 2cm below sternal notch **** Divides behind right SCJ into right subclavian and right common carotid **** Can sometimes have the thyroid ima as a branch *** Left common carotid *** Subclavian * Descending thoracic aorta ** Commences at the lower border of T4. At first just to left of midline, but slants medially to so that the diaphragmatic hiatus is in the midline. ** Branches: *** 9 posterior intercostal arteries *** Bronchial arteries *** Some small oesophageal vessels *** Be aware of arteria radicularis magna - a larger intercostal or lumbar artery supplying cord segments, which can lead to paraplegia if clamped excessively * Variants ** Brachiocephalic artery and left CCA can arise as a single trunk ** Double aortic arch - early obstructive symptoms ** Aberrant right subclavian artery - arteria lusoria - 1% *** Passes posterior to oesophagus and can cause compressive symptoms == '''Abdominal aorta''' == * Course ** Becomes the abdominal aorta as it passes behind the median arcuate ligament, at the level of T12 ** Passes downwards behind the peritoneum on the bodies of the lumbar vertebrae, inclining slightly to the left ** Bifurcates at L4 ** Surface markings - just below xiphisternum to just below umbilicus * Posterior relations ** Anterior spinal ligament ** Cisterna chyli above ** Left lumbar veins and left sympathetic trunk below * Branches ** Single ventral arteries to gut *** Coeliac *** SMA *** IMA ** Paired branches to other viscera *** Suprarenal *** Renal *** Gonadal ** Paired branches to abdominal wall *** Subcostal *** Inferior phrenic *** Lumbar - four pairs arising directly posteriorly ** Unpaired parietal branch *** Median sacral == '''Coeliac artery''' == * Artery to the foregut, supplying from the distal oesophagus to D2, liver, GB, spleen, pancreas * Branches at T12, almost perpendicular to the aorta, then courses anterolaterally in the lesser sac for 12mm * Most often forms a trifurcation ** Hepatic artery - see 'liver' *** Passes to the right beneath the posterior peritoneum of the omental bursa, enters the hepatoduodenal ligament and ascends to the hilum of the liver on the left side of the CBD ** Splenic artery - see 'spleen' *** Descends beneath the peritoneum to undulate along the cephalad border of the pancreas, where it gives off a dorsal pancreatic branch and others ** Left gastric artery - see 'stomach' * Left gastric ('coronary vein') crosses over the coeliac trunk in its course from the lesser curve of the stomach to the portal vein * Variations: ** Branching patterns *** Bifurcation into hepatosplenic and left gastric 50-75% *** Trifurcation 10-20% *** Quadrifurcation (GDA, right/left hepatic, dorsal pancreatic) ** Any of the three main branches may arise independently from the aorta/SMA (most commonly right hepatic from SMA) ** Common origin of coeliac and SMA == '''SMA''' == * Supplies the midgut - from the entrance of the CBD to a level just short of the splenic flexure * Arises from the front of the aorta, 1cm below the coeliac trunk, at L1 level, behind neck of pancreas. Forms a sharp caudal angle, which contains the left renal vein (between SMA and aorta). * Directed steeply downwards behind the splenic vein and the body of the pancreas, with SMV on its right, closely associated as far cephalad as behind the pancreas. * Anterior to left renal vein, uncinate process of the pancreas, and D3, from top down in that order * Then enters the upper end of the mesentery of the small intestine, along with the vein, and passes down to the right along the root of the mesentery * For description of intestinal arcades, see 'small intestine' topic * Branches ** '''Inferior pancreaticoduodenal''' (arise from posterior, at the level of the uncinate process). This artery joins the pancreaticoduodenal arcade to connect with coeliac circulation. ** '''Right colic''' (arises from right, within the fused mesentery of the right colon) ** '''Ileocolic''' (arises from right), which then divides into superior and inferior. This can arise in common with the right colic artery. Proceeds down root of mesentery. ** '''Middle colic''' (arises from right, at the level of the lower border of the neck of pancreas). Then divides into right and left branches at the intestinal border of the transverse mesocolon, which both run along the margin of the transverse colon. *** Right branch anastomoses with the ascending branch of right colic *** Left branch supplies the transverse colon almost to the splenic flexure, where it anastomoses with a branch of the left colic *** The middle colic therefore leaves a large avascular window to its left in the transverse mesocolon, where the lesser sac/posterior wall of stomach can be accessed. ** '''Jejunal and ileal branches''' (arise from left) * Variations: ** Replaced/accessory right hepatic 10-15% ** Replaced common hepatic/splenic/left gastric (rare) == '''Inferior mesenteric artery''' == * Emerges near the lower border of D3, as the duodenum crosses the aorta * Remains closely applied to the aorta as it passes to the left into the fused mesentery of the left colon * Branches ** Left colic (within 5cm of origin). The 'meandering mesenteric artery' of Riolan can form arcades with the middle colic artery. ** Several sigmoidal branches into sigmoid mesentery ** Terminates as the superior rectal branch. This crosses over the left iliac vessels to reach the posterior wall of the upper rectum. * Variant ** Double/absent IMA ** Arc of RIolan == '''Common iliac artery''' == * Formed by bifurcation of the aorta at L4, to the left of midline * Pass to front of SIJ where they bifurcate into EIA and IIA == '''External iliac artery''' == * Continues in the line of the CIA, along the pelvic brim on psoas * Branches - both just before inguinal ligament ** '''Inferior epigastric''' - comes off just before inguinal ligament. Runs towards the umbilicus, penetrating the transversalis fascia below the arcuate line of the posterior rectus sheath, to reach the lower third of the rectus abdominis muscle. Immediately after its origin, sends a small branch as part of spermatic cord. ** '''Deep circumflex iliac''' - runs behind the lateral portion of the inguinal ligament. * Variation ** 20% of patients have corona mortis - an obturator artery arising from inferior epigastric (see picture) then across the pectinate line of the pubis. Can be injured in femoral hernia repairs. == '''Internal iliac arteries''' == * Course ** Passes downwards and soon divides into small posterior and larger anterior divisions ** '''Mnemonic''' - I Love Going Places In My Very Own Underwear (ILGPIMVOU) *** Iliolumbar *** Lateral sacral *** Gluteal (superior/inferior) *** Pudendal internal *** Inferior vesical, superior vesical *** Middle rectal *** Vaginal artery *** Obturator *** Umbilical and uterine ** Posterior *** Iliolumbar *** Lateral sacral *** Superior gluteal ** Anterior *** Three associated with the bladder **** Superior vesical (which continues as the obliterated umbilical) - usually the first and highest from this division **** Inferior vesical **** Obliterated umbilical - medial umbilical ligament *** Three other visceral branches **** Middle rectal **** Uterine **** Vaginal *** Three parietal branches **** Obturator (in 80%) **** Internal pudendal **** Inferior gluteal = '''Venous system''' = == '''Jugular veins''' == * See 'neck' == '''Brachiocephalic veins''' == * Begin behind the sternal ends of the clavicles * Right: 2-3cm long, descends to finish behind the right first costal cartilage * Left: crosses obliquely behind upper manubrium, to end behind the right first costal cartilage where it joins the right vein to form SVC == '''Superior vena cava (SVC)''' == * Path ** Commences at the lower border of the first costal cartilage by confluence of the two brachiocephalic veins ** Vertically downwards behind the right border of the sternum ** Enters upper border of right atrium * Tributaries ** Both brachiocephalic veins (at its beginning) ** Azygos vein == '''Inferior vena cava''' == * Course ** Formed by the confluence of right and left common iliac veins, in front of L5 ** Ascends retroperitoneally to the right of the aorta, and passes in a groove on the liver *** There is a tributary-free plane anterior to the retrohepatic cava - can use finger to bluntly dissect here, below the major veins ** Perforates the tendinous part of the diaphragm ** Enters the inferoposterior part of the right atrium * Features ** No valves in the IVC distal to a somewhat vestigial valve at its entry into RA * Relations ** Crossed at its origin by right CIA ** IVC crossed by root of mesentery and right gonadal artery ** Lies behind duodenal C/head of pancreas ** Posterior to epiploic foramen - covered only by peritoneum *** Therefore posterior to PV - this is where a portocaval shunt would be performed - not always straightforward due to limited mobility of PV ** Two lowermost lumbar arteries cross behind it lower down ** Right renal artery crosses behind ** Inferior phrenic and adrenal arteries cross behind it ** Right adrenal gland lies to its right ** Right sympathetic chain is overlain by abdominal IVC * Tributaries ** Lumbar veins *** Usually four pairs in total, but only the caudal 3rd and 4th pairs drain into IVC *** 1st and 2nd veins drain into azygos system *** Major significance is that the left lumbar veins can get damaged as they pass behind the aorta during mobilisation *** Right veins can be damaged when the IVC is retracted anteriorly during lumbar sympathectomy ** Right gonadal vein (L2) *** Just below right renal vein ** Renal veins *** Left vein usually higher than right by 1-2cm *** Both veins usually enter at 45 degree angle *** Right vein 2cm, left vein longer and draped across front of aorta ** Supra-renal veins *** Right - very short and enters IVC just below liver *** Left - joins left renal veins ** Inferior phrenic vein *** Drains the under-surface of the diaphragm *** Right vein drains into IVC usually above the entrance of the right hepatic vein *** Left vein often double, with a posterior branch draining into the left adrenal or left renal vein, but the anterior branch passes in front of the oesophageal hiatus to drain into IVC or left hepatic vein ** Hepatic veins *** See 'liver' * Exposure ** See 'vascular exposures' * Variants ** IVC duplication (up to renal veins) ** Left sided IVC ** Left renal vein behind aorta, or paired sandwich ** Azygos continuation of the IVC ** Circumcaval ureter == '''Portal venous system''' == * See 'liver' == '''Azygos system''' == * '''Azygos system''' ** The thoracic wall and upper lumbar region are drained by the posterior intercostal and lumbar veins into the azygos veins. ** '''Azygos''' *** Embryology **** See above **** Essentially from the left and right supracardinal veins of the embryo *** Course **** In adults, the functional commencement is at the union of the ascending lumbar vein with the subcostal vein of the right side (T12) **** Passes through the aortic opening of the diaphragm under the right crus **** Lies posterolateral to oesophagus, on the vertebral bodies, with thoracic duct medial to it **** Arches over the right main bronchus at about T4 and passes forward to enter the SVC *** Tributaries **** Lower eight right posterior intercostal veins **** Superior intercostal vein at its convexity **** Bronchial veins from the right lung **** Some veins from the middle third of the oesophagus **** Hemiazygos - two connections at levels T8 and T9 *** Highly variable ** '''Hemiazygos''' *** Lies longitudinally on the left side of the bodies of the thoracic vertebrae *** Highly variable combination of hemiazygos and accessory hemiazygos *** Tributaries **** Accessory hemiazygos - left posterior intercostal veins 5-8 **** Hemiazygos - left posterior intercostal veins 9-12 *** Ends with drainage into azygos at T8 (accessory hemiazygos) and T9 (hemiazygos) [[Category:Anatomy]]
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