Editing Liver (section)

=== '''Portal vein''' ===

** Provides about 75% of      hepatic inflow, and 50-70% of oxygen requirement. No valves, so      accommodates high flow at low pressure
** Embryology
*** Forms from right and left       vitelline veins
*** Left vitelline vein becomes       lower part of PV
*** Upper part of right       vitelline vein becomes upper part of PV
** Course
*** Forms behind the neck of       the pancreas from SMV and splenic vein. Also gets blood from left       gastric vein, with variable point of inflow.
*** Length 5.5-8cm, diameter       1cm
*** Runs behind D1 (with GDA       just anterior to it) and into the hepatoduodenal ligament, where it runs       along the right border of the lesser omentum (posterior to CBD and PHA)
*** Tributaries below duodenum       enter from the right, posteriorly and the left, but not anteriorly;       gentle blunt dissection anterior to PV can open up a plane behind the       neck of the pancreas
*** Does not tend to have tribs       above the duodenum, but some tributaries may be found lower down
**** Pancreaticoduodenal vein -        enters on the right side, behind or immediately above the duodenum
**** Right and sometimes left        gastric vein - if entering PV, do so on the left, usually behind the        duodenum
*** Divides into main right and       left branches at the hilum of the liver
** Branches
*** Right portal triad - short       extrahepatic course (1.5cm), which appears to be the continuation of the       main PV, before entering the liver and branching into anterior and       posterior sectoral branches. Division to right lateral sector describes       a gentle curve towards right posterior liver; right medial sector can       arise at an angle. These branches can sometimes be seen       extra-hepatically and can come off the main PV. Usually a small caudate       process branch off the main right PV.
*** Left portal triad - longer       extrahepatic course (4cm) and runs transversely along the base of       segment IV in a peritoneal sheath, which is the upper end of the lesser       omentum. Then runs anterior and caudally in the umbilical fissure,       giving branches to segments II and III on the left, branches to the left       side of the caudate lobe, and recurrent branches to segment IV on the       right. Ligamentum teres is attached to the vein in the umbilical       fissure.
** Tributaries
*** SMV and splenic vein
*** Right and left gastric       veins
*** Superior       pancreaticoduodenal veins
*** Cystic vein/s into the       right branch of the portal vein
*** Periumbilical veins running       with ligamentum teres
** Portosystemic shunts
*** Submucosal veins in upper       stomach and oesophagus - drain cephalad into azygos system - varices
*** Varices of the inferior       rectal (haemorrhoidal) veins in the wall of the rectum. Note that these       are different to haemorrhoids - these are true veins, occurring cephalad       to the haemorrhoidal venous plexuses.
*** Ligamentum teres -       occasionally the umbilical vein itself opens up, but more often small       para-umbilical veins which run with the ligamentum teres, enlarge and       allow dilatation of contiguous veins in the umbilicus and abdominal wall
*** Retroperitoneum - multiple,       usually small, veins from the splenic venous system can enlarge and       communicate with diaphragmatic, adrenal, or renal veins
** Variations
*** Standard anatomy seen in       70%
*** Most common variation is       the 'portal vein trifurcation': main PV divides into left, right       posterior and right anterior branches.
*** Other common variation is       the right posterior PV originating as the first branch of the PV, or the       right anterior PV arising from the left PV
*** Other very rare variations:
**** PV found anteriorly to        neck of pancreas and duodenum
**** Direct entry of PV to IVC
**** Pulmonary vein entering PV
**** Congenital absence of left        branch of PV (with right branch coursing peripherally through the liver        to supply the left liver)
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