Since we’ve been on a roll, let’s do one more on the development of the cardiovascular system. There is a lot that goes on, so there is plenty to talk about. We already have the heart formed so let’s give it some places to pump to!

 

The Venous System

So the timeline here is week 4. Three pairs of veins have been formed that are draining into the tubular heart. These are the vitelline veins, the umbilical veins and the common cardinal veins.

The Vitelline veins’ job is to carry deoxygenated blood from the umbilical vesicle. They pass through the septum transversum and enter the sinus venosus. Remember where these structures are? If not, flip back to our earlier article that deals with the formation of the heart. The left vitelline vein regresses and doesn’t really seem to do much. The right vitelline vein, though, forms most of the hepatic portal system and part of the inferior vena cava. Those are some big deal structures!

The umbilical veins are responsible for carrying oxygenated blood from the chorionic sac. They pass both sides of the liver and also enter the sinus venosus. As the liver develops, though, they lose their connection to the heart and instead dump their blood into the liver. This time it’s the right vein that regresses and the left that continues to carry oxygen from the placenta. We also need to mention the ductus venosus here. It provides a bypass through the liver so that blood can pass from the placenta and up to the heart without having to go through the hepatic capillary network.

Last in this section is the common cardinal veins, who carry deoxygenated blood from the body of the embryo. This is the main drainage system for the embryo, too. The cranial portion is served by the anterior cardinal and the caudal is drained by the posterior cardinal. It’s those two veins who merge to form the common cardinal veins.

The posterior cardinal veins develop as part of the interim kidney, but mostly disappear when they do. Part of the posterior cardinal vein sticks around to become the roots of the azygos vein and the common iliac vein, though. Since so much of the posterior cardinal vein disappears, two veins are developed to replace and supplement them. The subcardinal veins and the supracardinal veins, and the right supracardinal vein is going to stick around to become the inferior portion of the inferior vena cava. The subcardinal isn’t without it’s benefits, though. They form the left renal, suprarenal and gonadal veins, a segment of the inferior vena cava, and both the azygos and the hemiazygos veins.

Okay, we’ve mentioned a few different things that form the inferior vena cava, so let’s break that structure down a bit. These are the segments and where they are derived from: Hepatic segment (hepatic vein aka proximal right vitelline vein); prerenal segment (right subcardinal vein); renal segment (the anastomosis of the subcardinal and the supracardinal); and the postrenal segment (right supracardinal vein).

 

The Arterial System

Once again, we have four different parts! Dorsal aorta and intersegmental arteries, pharyngeal arch arteries, umbilical arteries and the vitelline arteries. We’re going to skip the pharyngeal arch arteries so we can come back to them.

Initially, the dorsal aorta is actually paired arteries that run the entire length of the embryo. The caudal end fuses to make the lower thoracic and the abdominal aorta, which is sort of the bulk of it. In the cranial area, the right artery will regress so the left can become the primordial aorta. The dorsal aorta also has branches that carry blood to the somites. These are called the intersegmental arteries. In the neck, they grow up to be the vertebral artery. In the thorax, they become the intercostal arteries. The abdomen leads to the lumbar arteries. The 5th pair of intersegmental arteries become the common iliac arteries. Finally, the sacral region leads to the lateral sacral arteries.

Next, we have the vitelline artery. Its unpaired and leads from the umbilical vesicle into the primordial gut. Eventually, we’ll make the celiac trunk, and both the superior and inferior mesenteric arteries out of it.

Then last, we have the umbilical arteries. This one is kind of cool because it is directly connected to vessels of the chorion. The proximal parts become the internal iliac and superior vesical arteries, while the distal part will eventually disintegrate and become the medial umbilical ligaments.

 

Pharyngeal Arch Arteries

These are arteries on either side of the aortic sac that end in the dorsal aortae. There are 6 pairs altogether, but it’s important to note that they don’t develop together. In fact, the first 2 pairs will have disappeared when the 6th one appears.

Arch I will give rise to the maxillary arteries and part of the external carotid arteries.

Arch II leaves a dorsal remnant that becomes the stapedial arteries.

Arch III does two things. The proximal part becomes the common carotid and the distal becomes the internal carotid.

Arch IV kicks in on the aortic arch, but so does the proximal part of the aortic sac and the left dorsal aorta. The right IV will make the proximal portion of the right subclavian artery with the distal side coming from the right dorsal aorta and right 7th intersegmental artery. By the way, left subclavian artery comes from the left 7th intersegmental artery. Kinda makes sense.

The left arch artery VI also splits up proximally and distally. Proximal leads to the left pulmonary artery and distal will make the ductus arteriosus. The right VI does something similar but the proximal leads to the right pulmonary artery and actually, the distal part just goes away.

 

Lymphatics

Of course, when we talk about fluid going around in the cardiovascular system, we probably should at least touch on the lymphatic system, too. We pretty much figure that the cardiovascular system is going to be done round week 6 and the lymphatic system will develop around two weeks later. By the end of week 8, we have 6 primary lymph sacs: the paired jugular, paired iliac, the singular retroperitoneal and the singular cisterna chyli. The lymphatic vessels also develop pretty similar to blood vessels.

After awhile, they connect to the lymph sac and pass along to main veins. Head, neck and upper limbs drain into the jugular sac, primordial gut goes to both the retroperitoneal sac and the cisterna chyli, and final the lower trunk and limbs goes into the iliac.

The probably big thing to note is that the left and thoracic ducts connect the jugular sacs to the cisterna chyli. They make an anastomosis between the left and right ducts. What is important is that the caudal right duct and the cranial left duct end up taking off, and that leads to the creation of the thoracic duct!
Maybe we’ll tackle lymphatics more sometime, but this feels like quite a bit to keep track of for now. Hopefully, this gives you a pretty decent idea of what is going on so that you can fill in the rest of the details later! Best of luck!