Intermediate - Vascular Overview

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Intermediate Heart Development Timeline.jpg

Some understanding of embryonic vascular development is helpful in a study of cardiac embryology and the embryonic circulation. Early in the third week of embryonic development vasculogenesis begins whereby endothelial cell precursors form aggregations as angioblastic cords. This process underlies the initial formation of the endocardial heart tubes as well as the primitive blood vessels. The cords coalesce to form blood vessels while continued angiogenesis, driven by metabolic requirements and specifically hypoxia, allows for the creation of a vascular network.

Development of Arteries

The aortic arches

Upon folding of the embryo the paired dorsal aortae connecting to the cranial end of the heart tube are brought ventrally to form the first aortic arches. Additional aortic arches develop over the next few weeks which are later remodelled to form the arteries of the upper body. Caudal to the arches, the paired dorsal aortae fuse to form a single median dorsal aorta which develops the following branches:

  • Ventral (gut) branches - derived from the vitelline arteries
  • Lateral branches - supply retroperitoneal structures
  • Dorsolateral branches (intersegmental arteries) - supply the head, neck, body wall, limbs and vertebral column

Development of Veins

Three paired veins drain into the primordial heart tube:

  • Vitelline veins - return poorly oxygenated blood from the yolk sac
  • Umbilical veins - carry well-oxygenated blood from the primordial placenta
  • Common cardinal veins - return poorly oxygenated blood from the body of the embryo

The vitelline venous system gives rise to the liver sinusoids and portal system and forms the ductus venosus which acts as a shunt from the umbilical vein to the IVC. The IVC is formed during a left-to-right shift in the embryonic veins and is composed of:

  • A hepatic segment - from the hepatic vein and sinusoids
  • A prerenal segment - from the right subcardinal vein
  • A renal segment - from subcardinal and supracardinal anastomosis
  • A postrenal segment - from right supracardinal vein

The following two diagrams give an overview to the embryonic vasculature:

The three embryonic circulations
The embryonic cardiovascular system

Fetal Circulation

Fetal circulation

Fetal circulation consequently differs from the adult one predominantly due to the presence of 3 major vascular shunts:

  • Ductus venosus - between the umbilical vein and IVC
  • Foramen ovale - between the right and left atrium
  • Ductus arteriosus - between the pulmonary artery and descending aorta

The main function of these shunts is to redirect oxygenated blood away from the lungs, liver and kidney (whose functions are performed by the placenta).

Oxygenated blood is carried from the placenta to the foetus in the umbilical vein, most of which then passes through the ductus venosus to the IVC while some blood supplies the liver via the portal vein. Blood from the liver drains into the IVC through the hepatic veins. The blood in the IVC is a mixture of oxygenated blood from the umbilical vein and desaturated blood from the lower limbs and abdominal organs (e.g. the liver). This blood enters the right atrium where most of it is directed to the left atrium through the foramen ovale and from here to the left ventricle and aorta. The remainder of the blood in the right atrium passes with blood from the SVC (from the head and upper limbs) to the right ventricle and pulmonary artery where most of it passes to the aorta via the ductus arteriosus. The blood passes from the aorta to the hypogastric arteries, umbilical arteries and then back to the placenta.

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Angioblastic cords: Groups or ‘columns’ of embryonic precursor cells which will form the walls of both arteries and veins.

Angiogenesis: Growth of new blood vessels.

Aortic arch arteries: (Or pharyngeal arch arteries.) Each early developing pharyngeal arch contains a lateral pair of arteries arising from the aortic sac, above the heart, and running into the dorsal aorta. Later in development these arch arteries are extensively remodelled to form specific components of the vascular system.

Dorsal aortae: Two largest arteries either side of the midline which later fuse to form the descending portion of the aorta.

Hypoxia: Pathological condition in which part or all of the body suffers from inadequate oxygen supply.

Retroperitoneal: Refers to abdominal organs located external to the peritoneal cavity.

Vasculogenesis: Growth of blood vessels from endothelial precursor cells which migrate and differentiate to form blood vessels. Differs from angiogenesis which refers to the growth of blood vessels from pre-existing ones.