Editing Vascular system (section)

== '''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
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=== '''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
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