Notes
Fetal Circulation
Sections
Notes
Prenatal circulation
- Oxygenated blood arrives via the placenta; thus, the fetus is dependent upon maternal oxygen stores.
- High vasculature pressure in the lungs prevents significant pulmonary blood flow; thus, blood is shunted from the lungs, and the liver, as well, via special temporary structures.
- Arteries carry blood Away from the heart.
- Veins Return blood to the heart.
Superior vena cava and right heart flow:
- Superior vena cava drains head, neck, an upper extremities.
Pathway:
- Blood from the superior vena cava drains into the right atrium, then to the right ventricle.
- Right ventricle pumps the blood into the pulmonary trunk (aka, artery).
- The majority of low oxygenated blood is shunted directly to the aorta via the ductus arteriosus.
- Within the pulmonary trunk, a small amount of blood is sent through the high pressure pulmonary arteries to the right and left lungs.
Inferior vena cava and left heart flow:
- Inferior vena cava delivers oxygenated blood from placenta to left heart.
- Inferior vena cava drains oxygen-poor blood from the trunk and lower extremities.
Pathway:
- Oxygen-rich blood leaves the placenta via the left umbilical vein.
- Most of this blood is shunted directly to the inferior vena cava, where it mixes with relatively oxygen-poor blood returning from the trunk and lower extremities (oxygen-rich blood bypasses liver vasculature).
- De-oxygenated blood from trunk/lower extremities ascends through the IVC and mixes with the oxygenated blood from the left umbilical vein.
- Referred to as "mixed blood" in diagram.
- Blood from the inferior vena cava passes from right atrium, through the foramen ovale, to the left atrium (bypasses right ventricle/pulmonary system).
- This blood mixes with the small amount of blood returning from the pulmonary veins.
- After the left ventricle pumps the blood into the aorta, it is first sent to the Head, Neck, and Upper Extremities. Thus, the developing brain receives mixed blood with a relatively high oxygen concentration.
- After the aortic branches, remaining blood intermixes with blood from the ductus arteriosus (from the SVC, which, again, is deoxygenated).
- Blood passing through the descending aorta is sent to the trunk and lower extremities.
- Via the umbilical arteries, blood returns to the placenta (where it is oxygenated).
Post-natal circulation:
- Structures that shunt blood from the liver and lungs regress after birth.
- The first post-natal breath alters pulmonary pressures and opens the pulmonary vasculature, allowing blood flow.
- The foramen ovale closes due to changes in atrial pressures.
- The temporary structures regress.
Summary of Post-natal changes:
- The ductus arteriosus, which carried blood from the pulmonary trunk to the aorta, becomes the ligamentum arteriosus.
- The ductus venosus, which shunted blood from the hepatic vasculature directly to the inferior vena cava, becomes the ligamentum venosum.
- The umbilical arteries, which delivered low-oxygen blood from the fetus to the placenta, become the medial umbilical ligaments.
- The left umbilical vein, which delivered high-oxygen blood from the placenta to the fetus, becomes the ligamentum teres.
- The foramen ovale, which allowed direct passage from the right to left atrium, closes; it is represented by the fossa ovalis, which can be felt in the dorsal aspect of the interatrial septum.
- Review internal features of the heart
- Review adult circulation
- Review Systemic vs. pulmonary circulation
Full-Length Text
Here we will learn about fetal circulation and key post-natal anatomical and physiological transformations.
To begin, start a table, and, regarding prenatal circulation, denote that:
Oxygenated blood arrives via the placenta; thus, the fetus is dependent upon maternal oxygen stores.
High vasculature pressure in the lungs prevents significant pulmonary blood flow;
Thus, blood is shunted from the lungs, and the liver, as well, via special temporary structures.
Although, in adults we simply remember that arteries are vessels that carry blood away from the heart and carry oxygenated blood whereas veins are vessels that return blood TO the heart and carry de-oxygenated blood, the blood oxygen concentration in fetal circulation is much more complicated. So, we must erase the connection between the direction of blood flow and oxygen content.
[As an aside, remember that in adults: the pulmonary veins carry oxygenated blood whereas the pulmonary arteries carry deoxygenated blood.]
Also, although the superior and inferior vena cava both deliver blood to the right side of the heart, we distinguish the path that each vessel's blood flows through the heart, itself – the difference in flow is largely attributed to the shape of the inferior vena cava's valves.
So to illustrate fetal circulation, split the page into: Superior Vena Cava & Right Heart and Inferior Vena Cava and Left Heart.
We start with the superior vena cava and right heart because it's simpler.
Draw the heart.
Label the atria and ventricles.
Indicate the right heart atrioventricular valve (aka, tricuspid valve).
Then, include the superior vena cava, pulmonary trunk, and a segment of lung tissue. Also, indicate the head, neck, and upper extremities, which drain into the superior vena cava.
Show that blood from the superior vena cava drains into the right atrium, passes through the right atrioventricular valve (aka, tricuspid valve) to the right ventricle,
Which pumps the blood into the pulmonary trunk (aka, artery).
Specify that within the pulmonary trunk, a small amount of blood is sent through the high pressure pulmonary arteries to the right and left lungs;
Then, show that the rest is shunted directly to the aorta via the ductus arteriosus; we'll soon see that the ductus arteriosus is what joins blood from the SVC with blood that arrived from the IVC.
Now for the IVC and the left heart.
Draw the inferior vena cava.
Redraw the heart, a snippet of the pulmonary arteries and lungs, and the ductus arteriosus.
We need a more elaborate diagram, here, because the IVC receives oxygenated blood from the placenta.
Draw the placenta, and liver; In the heart, include the foramen ovale (which is how blood crosses from the right atrium to the left-side of the heart).
Now, show that oxygen-rich blood leaves the placenta via the left umbilical vein (notice that this is a vein because it is carrying blood towards the heart). At the liver, we see that much of this blood bypasses the hepatic vasculature and is, instead, shunted directly to the inferior vena cava, where it mixes with relatively oxygen-poor blood returning from the trunk and lower extremities.
Next, show that de-oxygenated blood ascends through the IVC and mixes with the oxygenated blood from the left umbilical vein – indicate that the blood now has a mixed-oxygen concentration.
Then, show that once it enters the right atrium, blood from the inferior vena cava passes through the foramen ovale to the left atrium.
Then, draw the ascending aorta, which gives rise to three major arteries at its arch to feed the: Head, Neck, and Upper Extremities. Thus, the developing brain receives mixed blood with a relatively high oxygen concentration.
Also, show that it intermixes with blood from the ductus arteriosus (from the SVC, which, again, is deoxygenated.
So, draw the descending aorta, which bifurcates distally.
And show that blood descends through it to the: Trunk & Lower Extremities.
Now, also show that via the umbilical arteries, blood returns to the placenta (where it is oxygenated).
As a final point, draw the pulmonary veins and show that a small amount of blood from lungs mixes with the blood from the IVC.
As mentioned earlier, the structures that shunt blood from the liver and lungs regress after birth.
Denote in our table that the first post-natal breath alters pulmonary pressures and opens the pulmonary vasculature, allowing blood flow:
The foramen ovale closes due to changes in atrial pressures, and, tyhe temporary structures regress.
To summarize these anatomical pre- to post-natal changes, create a small table.
The ductus arteriosus, which carried blood from the pulmonary trunk to the aorta, becomes the ligamentum arteriosus.
The ductus venosus, which shunted blood from the hepatic vasculature directly to the inferior vena cava, becomes the ligamentum venosus.
The umbilical arteries, which delivered low-oxygen blood from the fetus to the placenta, become the medial umbilical ligaments.
The left umbilical vein, which delivered high-oxygen blood from the placenta to the fetus, becomes the ligamentum teres.
Finally, the foramen ovale, which allowed direct passage from the right to left atrium, closes; it is represented by the fossa ovalis, which can be felt in the dorsal aspect of the interatrial septum.