Foetal (Fetal) Circulation | Before and At Birth | Cardiac Physiology | Embryology

Summary

This video from Bite Size Med explores fetal circulation, highlighting the unique differences from adult circulation due to the non-functional lungs and reliance on the placenta for gas exchange. It details the three key shunts in the fetal circulatory system: the ductus venosus, foramen ovale, and ductus arteriosus, which facilitate blood flow bypassing the lungs. The video also explains the physiological changes at birth that lead to the closure of these shunts, transitioning the baby's circulation to an adult pattern.

Takeaways

• 🌟 Fetal circulation is distinct from adult circulation due to the non-functional lungs and reliance on the placenta for gas exchange.
• 🔄 The umbilical cord has two arteries and one vein, with the arteries carrying deoxygenated blood and the vein carrying oxygenated blood.
• 🚦 Three shunts are crucial for fetal circulation: the ductus venosus, foramen ovale, and ductus arteriosus, allowing blood to bypass the liver, lungs, and directing it towards vital organs.
• 📍 The ductus venosus shunts blood from the umbilical vein to the inferior vena cava, bypassing the liver.
• 🫀 The foramen ovale allows oxygenated blood to pass from the right atrium to the left atrium, bypassing the lungs.
• 🩸 The ductus arteriosus channels blood from the pulmonary artery to the aorta, ensuring only a small portion goes to the lungs.
• 🌡️ High pulmonary vascular resistance in the fetus due to collapsed lungs and high blood flow to the placenta due to low systemic vascular resistance.
• 👶 At birth, the first breath decreases pulmonary vascular resistance, leading to closure of the shunts as the circulatory system adapts to lung breathing.
• ⏳ After birth, the functional closure of the ductus arteriosus occurs within hours to days, followed by anatomical closure over months.
• 🔗 The remnants of fetal circulatory structures persist into adulthood as ligaments and other anatomical markers.

Q & A

What is the primary difference between fetal and adult circulation?

The primary difference is that in fetal circulation, the lungs are non-functional, and the placenta performs the gas exchange, whereas in adults, the lungs are fully functional.

Why does the umbilical vein carry oxygenated blood and the umbilical arteries carry deoxygenated blood?

The umbilical vein carries oxygenated blood because it transports blood from the oxygenating placenta to the fetus. The umbilical arteries carry deoxygenated blood because they transport blood from the fetus to the placenta for oxygenation.

What are the three shunts in fetal circulation and their functions?

The three shunts are the ductus venosus, the foramen ovale, and the ductus arteriosus. The ductus venosus allows blood from the placenta to bypass the liver. The foramen ovale allows blood to pass from the right atrium to the left atrium, bypassing the lungs. The ductus arteriosus allows blood to bypass the lungs by shunting from the pulmonary artery to the aorta.

How does the blood from the umbilical vein enter the fetal heart?

The blood from the umbilical vein enters the inferior vena cava (IVC) and then bypasses the liver through the ductus venosus, entering the IVC and right atrium.

What directs the oxygenated blood towards the foramen ovale in the right atrium?

The Eustachian valve at the junction of the IVC and the right atrium directs the more oxygenated blood towards the foramen ovale.

Why does the majority of oxygenated blood from the left ventricle go to the brain and coronary circulation?

The majority of oxygenated blood from the left ventricle goes to the brain and coronary circulation because these are vital structures that require a good supply of oxygen.

What causes the right-to-left shunt in the fetal circulation?

The right-to-left shunt in fetal circulation is caused by the high pulmonary vascular resistance due to the collapsed lungs and the lower systemic vascular resistance in the placenta, leading to higher pressures on the right side of the heart compared to the left.

What changes occur in the fetal circulation at birth?

At birth, the baby takes a breath, the lungs expand, and the pulmonary vascular resistance falls. This causes the pressures in the left side of the heart to become higher than the right, reversing the shunts and closing them.

How do the shunts close after birth?

The foramen ovale closes due to the left atrial pressure becoming higher than the right, causing a flap to close. The ductus arteriosus closes due to high oxygen tension and loss of prostaglandin E2, leading to constriction and eventual fibrous tissue growth. The ductus venosus closes as the umbilical vein flow stops and portal venous pressure rises.

What are the remnants of the fetal circulation structures in adults?

In adults, the remnants are the ligamentum teres (from the umbilical veins), the ligamentum venosum (from the ductus venosus), the ligamentum arteriosum (from the ductus arteriosus), and the fossa ovalis (from the foramen ovale).

Outlines

00:00

🔬 Fetal Circulation and its Unique Characteristics

This paragraph introduces the concept of fetal circulation, highlighting the key differences from adult circulation due to the non-functional lungs and the role of the placenta in gas exchange. It explains the unique blood flow through the umbilical arteries and vein, with deoxygenated blood in the arteries and oxygenated blood in the vein. The circulatory pathway is detailed, emphasizing the role of three shunts: the ductus venosus, foramen ovale, and ductus arteriosus. These shunts allow blood to bypass the liver and lungs, directing oxygenated blood to vital organs and tissues. The paragraph concludes with an explanation of why these shunts occur, due to the high pulmonary vascular resistance in the fetus and the lower systemic vascular resistance in the placenta.

05:01

🌱 Changes in Circulation at Birth and Beyond

The second paragraph discusses the physiological changes that occur at birth, which alter the fetal circulatory system. It explains how the first breath of the baby reduces pulmonary vascular resistance and increases systemic vascular resistance, leading to the closure of the foramen ovale and ductus arteriosus. The paragraph also describes the functional and anatomical closure of these shunts and the reasons behind them, such as increased oxygen tension and the loss of prostaglandin E2. It concludes with the remnants of these fetal structures in adults, such as the ligamentum venosum, ligamentum arteriosum, and fossa ovalis, and the transformation of the umbilical arteries into the medial umbilical ligament.

Keywords

💡Fetal circulation

Fetal circulation refers to the blood flow in a fetus, which is distinct from adult circulation. In the fetus, the lungs are non-functional, and the placenta handles gas exchange. This circulatory system is reliant on specific shunts that bypass the lungs and liver, ensuring oxygenated blood reaches vital organs. The video focuses on explaining these unique features and the changes that occur at birth.

💡Placenta

The placenta is an organ that facilitates the exchange of oxygen, nutrients, and waste between the mother and the fetus. In fetal circulation, the placenta performs the role of the lungs by oxygenating blood. The video emphasizes how the umbilical vein carries oxygenated blood from the placenta to the fetus’s heart, highlighting the importance of this organ in fetal development.

💡Umbilical cord

The umbilical cord connects the fetus to the placenta and contains two umbilical arteries and one umbilical vein. Unlike typical circulation, the umbilical vein carries oxygenated blood from the placenta to the fetus, while the umbilical arteries carry deoxygenated blood back to the placenta. This is a key concept in understanding the flow of blood in fetal circulation, as described in the video.

💡Shunts

Shunts are natural bypasses in fetal circulation that allow blood to flow around non-functional organs like the lungs and liver. The video discusses three major shunts: the ductus venosus, the foramen ovale, and the ductus arteriosus. These structures are crucial in directing oxygenated blood to vital areas like the brain while bypassing the lungs and liver until birth.

💡Ductus venosus

The ductus venosus is the first shunt in fetal circulation that bypasses the liver. It allows oxygenated blood from the umbilical vein to flow directly into the inferior vena cava. The video highlights its role in ensuring that most blood reaches the heart and brain while avoiding the liver, which is not fully functional during fetal life.

💡Foramen ovale

The foramen ovale is the second shunt in fetal circulation, an opening between the right and left atria of the heart. It allows oxygenated blood to flow from the right atrium to the left atrium, bypassing the lungs. In the video, it is explained as a critical mechanism that directs oxygenated blood to the left side of the heart, ensuring vital organs receive oxygen during fetal development.

💡Ductus arteriosus

The ductus arteriosus is the third shunt in fetal circulation, a vessel that connects the pulmonary artery to the aorta. This shunt diverts blood away from the non-functional lungs and into the systemic circulation. The video explains its importance in bypassing the lungs and describes how it closes after birth, becoming the ligamentum arteriosum.

💡Pulmonary vascular resistance

Pulmonary vascular resistance refers to the resistance encountered by blood flow in the lungs. In fetal life, the resistance is high due to the collapsed state of the fetal lungs. The video explains how this high resistance maintains the right-to-left shunt through the ductus arteriosus and foramen ovale, ensuring blood bypasses the lungs until birth.

💡Patent foramen ovale

A patent foramen ovale occurs when the foramen ovale does not close after birth, leaving an opening between the atria. In the video, it is noted that while the shunt usually closes after birth due to increased pressure in the left atrium, a failure to close results in a patent foramen ovale. This condition is often asymptomatic and unnoticed.

💡Patent ductus arteriosus

A patent ductus arteriosus is a condition where the ductus arteriosus fails to close after birth, allowing blood to flow from the aorta to the pulmonary artery. The video explains how the ductus arteriosus typically closes after birth as pulmonary vascular resistance falls and systemic pressure rises. If it remains open, it causes a left-to-right shunt, which can lead to complications.

Highlights

Fetal circulation differs from adult circulation due to non-functional lungs and reliance on the placenta for gas exchange.

The liver is partially functional, receiving less blood compared to the placenta, which receives a lot.

The umbilical cord has two arteries and one vein, with the artery carrying deoxygenated blood and the vein carrying oxygenated blood.

The placenta oxygenates blood, leading to oxygenated blood in the umbilical vein and deoxygenated blood in the arteries.

Fetal circulation is dependent on three shunts: one at the liver, one at the heart, and one at the great vessels.

The ductus venosus shunts oxygenated blood from the umbilical vein to the inferior vena cava, bypassing the liver.

The Eustachian valve directs oxygenated blood towards the foramen ovale, the second shunt.

The foramen ovale allows blood to bypass the lungs by directing it from the right atrium to the left atrium.

The ductus arteriosus, the third shunt, connects the pulmonary artery to the aorta, bypassing the lungs.

Only about 12% of blood from the right ventricle reaches the lungs, with the rest going through the ductus arteriosus.

The high pulmonary vascular resistance in the fetus causes right-to-left shunting at the foramen ovale and ductus arteriosus.

At birth, the baby's first breath reduces pulmonary vascular resistance, leading to closure of the shunts.

The foramen ovale closes due to increased left atrial pressure, preventing right-to-left shunting.

The ductus arteriosus closes due to increased oxygen tension and loss of prostaglandin E2, which relaxes vessels.

If the ductus remains open, it's called a patent ductus arteriosus, leading to a left-to-right shunt.

The ductus venosus closes as portal blood is redirected into the hepatic sinusoids after birth.

In adults, remnants of the fetal circulation structures remain as ligaments and the fossa ovalis.

Transcripts

hi everyone welcome to bite size med

where we talk about quick

bite size concepts in medicine for study

and rapid review

this video is on fetal circulation

the circulation in the fetus differs

from that of an adult for one main

reason

the lungs are non-functional and the

placenta does the job of gas exchange

the liver is partially functional so

neither the lung nor the liver need much

blood

versus the placenta which gets a lot

the umbilical cord from the placenta has

two umbilical arteries and one umbilical

vein

compared to the usual arteries and veins

the umbilical artery has deoxygenated

blood

and the umbilical vein oxygenated blood

why does that happen because the

placenta is the one that's oxygenating

blood

so the vein which carries blood from the

placenta to the heart

will be oxygenated and the arteries

which come from the heart to the

placenta

will bring deoxygenated blood for

oxygenation

now let's look at the circulatory

pathway so fetal circulation

is dependent on shunts there are three

bypass

shunts one at the liver one at the heart

and one at the great vessels

the umbilical vein carries oxygenated

blood

from the placenta to the inferior vena

cava

this is 70 to 80 percent saturated

the blood bypasses hepatic circulation

and enters the ivc

via the first shunt that's the ductus

venosus

so very little blood enters the hepatic

sinusoids

in the ivc the oxygenated blood from the

ductus venosus

tends to stay relatively separate from

the deoxygenated blood

coming from the rest of the lower half

of the body and portal circulation

the blood enters the right atrium from

the ivc

there is a valve of the ivc called the

eustachian valve at the junction with

the right atrium

and that directs the more oxygenated

blood towards the second shunt

the foramen ovale

this is an opening in the atrial septum

so that blood can reach the left atrium

the rest of the deoxygenated blood

coming from the head end of the fetus

through the superior vena cava

and from the lower half of the body and

portal circulation through the ivc

they go through the tricuspid valve into

the right ventricle

from the right ventricle blood enters

the pulmonary artery

the better oxygenated blood from the

left atrium whose saturation now would

be around 65 percent

enters the left ventricle from the left

ventricle the blood enters the aorta

majority of the oxygenated left

ventricular blood

goes to the brain and coronary

circulation so the vital structures are

getting good oxygen

to the origin of the left subclavian

artery is shunt

iii the ductus arteriosus it's a

connection between the pulmonary artery

and the aorta so blood from the

pulmonary artery

goes through the ductus arteriosus to

the aorta

thanks to this shunt only around 12

percent of blood that comes out of the

right ventricle

reaches the lungs the rest goes through

the ductus arteriosus to the aorta

from the aorta blood gets supplied to

the rest of the systems

it passes through the umbilical arteries

back to the placenta for oxygenation

so in both these shunts blood moves from

the right side

to the left side why does that happen so

the fetal lungs are collapsed

the alveoli are closed and the pulmonary

vessels are collapsed

if the vessels are closed the resistance

to blood flow

is high that's the pulmonary vascular

resistance

so that's high and thus the pulmonary

arterial pressure is high

as is the right atrial and the right

ventricular pressure

the placenta on the other hand has large

vessels

so the vascular resistance in systemic

circulation is

low because of the low systemic vascular

resistance

there's a low aortic pressure a low left

atrial and left ventricular pressure

so since the right atrial pressure is

higher than the left atrium

blood flows through the foramino valley

from the right atrium to the left

similarly the pulmonary arterial

pressure is higher than the aortic

so blood flows from the pulmonary artery

to the aorta

so right to left shunt and thus they

bypass the lungs

so blood gets immediately recirculated

through systemic arteries without the

lungs

so what happens at birth the baby takes

a breath and the lungs expand

the vessels are no longer compressed so

the resistance to flow is lesser

so the pulmonary vascular resistance

will fall

during fetal life there's some amount of

hypoxia that keeps the pulmonary vessels

constricted

but once there is aeration there is

vasodilation

so that also reduces the pulmonary

vascular resistance

that in turn causes a reduction of the

pulmonary artery pressure

the right atrial and the right

ventricular pressure loss of blood flow

to the placenta makes the systemic

vascular resistance rise

so high aortic pressure left atrial

pressure

and left ventricular pressure so now the

pressure of the left circulation is

higher than the right

with the left atrial pressure being

higher than the right atrium

blood will flow backwards through the

foramen ovale

there's a flap towards the left side of

the atrial septum

that will close backwards so the shunt

closes

and there's no more flow in most people

it'll close permanently

if it doesn't it's called a patent

foramen ovale

but because in general the left atrial

pressure is gonna stay higher than the

right

the flap stays closed and most of the

times it goes unnoticed

high aortic pressure and low pulmonary

artery pressure means blood flows

backwards through the ductus arteriosus

as well

in a few hours the muscles in the wall

will constrict

more so over the next one to eight days

the flow stops and this is a functional

closure over the next one to four months

once fibrous tissue grows into the lumen

that's when it anatomically closes so

what makes the ductus close

there are two possible reasons one is

high oxygen tension

the partial pressure of oxygen increases

from 15 to 20 millimeters of mercury

during fetal life

to 100 millimeters of mercury once

breathing starts

also there's a loss of prostaglandin e2

and that has vessel relaxing effects

so pg e2 keeps the ductus open

if the ductus remains open it's called a

patent ductus arteriosus

however contrary to fetal life the shunt

here is a left to right shunt

from the aorta to the pulmonary artery

because of the pressure changes

the last shunt the ductus venosus flow

through the

umbilical vein stops the portal blood

goes into the ductus phenosis

a small amount goes through the hepatic

channels in around one to two hours the

muscles in the wall of the ductus

venosus contract and it closes

so a rise in the portal venous pressure

pushes the portal venous blood into the

hepatic sinusoids

a patent ductus venosus is quite rare

in adults these structures they remain

as vestiges

the umbilical arteries become the medial

umbilical ligament

not to be confused with the median

umbilical ligament which is a remnant of

the arrakis

the umbilical veins become ligamentum

tears of the liver

the others are pretty easy the ductus

venosus becomes the ligamentum venosum

the ductus arteriosus becomes the

ligamentum arteriosum

and the foramen ovale it remains as the

fossa ovalis

so that's the fetal circulation and the

changes that happen at birth

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