Basic - Vascular Heart Connections
|Begin Basic||Primitive Heart Tube||Embryonic Heart Divisions||Vascular Heart Connections|
|Cardiac Embryology||Begin Basic||Begin Intermediate||Begin Advanced|
Some understanding of the vascular system of an embryo is useful in completely understanding cardiac development. In the primitive heart tube section it was stated that the blood islands dispersed throughout the embryo form the early blood vessels. The islands coalesce to form vessels and these expand and continue to develop, forming vascular networks. The vascular system can be thought of in terms of arteries and veins and the major embryonic vessels can be seen in the diagram on the right.
Development of Veins
We already learnt that blood travels through the embryonic heart from the sinus venosus. There are three paired veins which form to drain into the sinus venosus:
- 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 sinus venosus soon shifts to the right to be incorporated into the right atrium as there is a shift in the venous system from the left to the right side of the embryo. The inferior vena cava and superior vena cava form and drain into the sinus venosus. In the left atrium the four pulmonary veins form, which will return oxygenated blood from the lungs.
Development of Arteries
We saw in the fusion animation that the dorsal aortae develop at the same time as the early heart tubes. These connect to the heart tubes prior to fusion via the first aortic arch arteries. Other arches develop, which go on to form the arteries of the head and neck. We also previously saw the way in which the aorta and pulmonary trunk form. The dorsal aorta gives off branches which supply blood to the rest of the embryo:
- Gut (ventral/front) branches
- Lateral (side) branches
- Intersegmental arteries
As the embryo progresses to a fetus the vasculature is still remarkably different to that of the adult, including the presence of three vascular shunts:
- foramen ovale (seen previously) - blood travels from the right atrium to the left atrium
- ductus venosus - blood from the umbilical vein bypasses the liver to enter the inferior vena cava
- ductus arteriosus - blood passes from the pulmonary trunk into the aorta
These shunts allow blood to bypass the lungs, liver and kidneys, whose functions are performed by the placenta while in utero.
The following diagram shows the movement of blood throughout the fetal circulation. The main flow of blood is as follows:
- Placenta → umbilical vein → ductus venosus → inferior vena cava → right atrium → foramen ovale → left atrium → left ventricle → aorta → hypogastric arteries → umbilical arteries → placenta.
Blood that passes from the right atrium to the right ventricle travels:
- Right ventricle → pulmonary trunk → ductus arteriosus → aorta
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Aorta: The largest artery in the human body originating in the left ventricle. The aorta ascends, arches over the heart and then descends through the abdomen.
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.
Artery: Blood vessels that carry blood away from the heart.
Blood vessel: An elastic tubular channel that transports blood around the body.
Dorsal aortae: Two largest arteries either side of the midline which later fuse to form the descending portion of the aorta.
Fetus: (Also foetus). In mammals, term describes the period of development following the embryonic period. In humans, the development week 9 to 36 is the fetal stage (second and third trimester). This term is also used non-scientifically to describe the human conceptus at both embryonic and fetal stages of development.
Inferior vena cava (IVC): Large vein which carries deoxygenated blood from the lower half of the body to the right atrium.
Placenta: The developmental organ formed from maternal and fetal contributions in animals with placental development. In human, the placenta at term is a discoid shape "flat cake" shape; 20 cm diameter, 3 cm thick and weighs 500-600 gm. The placenta has many different functions including metabolism, transport and endocrine.
Pulmonary trunk: A vessel that arises from the right ventricle of the heart, extends upward, and divides into the right and left pulmonary arteries that transport deoxygenated blood to the lungs.
Superior vena cava (SVC): Short vein which carries deoxygenated blood from the upper half of the body to the right atrium.
Vascular networks: Assemblies of blood vessels throughout the body.
Vascular shunts: Blood vessels that redirect the flow of blood. In the embryo there are 3 major developmental shunts.
Vein: Blood vessel which carries blood towards the heart.