Fetal Circulation

1. Where are pharyngeal arches developed from?

   • How many pairs are they?

2. Describe the linings of these pharyngeal arches

3. What migrates to these pharyngeal arches and what is the result of this?

Development of Pharyngeal Arches:

• @ 4th-5th week, series of bulges @ future face and anterior neck appear → Pharyngeal Arches (6 pairs)

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Characteristics of Pharyngeal Arches:

• lined by ectoderm and endoderm

• Neural crest cells migrate into pharyngeal arches to surround cores of paraxial mesoderm

  • together: forms most structures in the pharyngeal arches.

1. Define Pharyngeal Arches

   • How many are them?

   • Where are they and why are they important?

2. What do they contain/have? What do they recruit and what does this develops?

Pharyngeal Arches:

• Def: (5) paired transient swellings

• Location: surrounds foregut; btw developing brain and heart;

• Importance: critical for dev. of head and neck structures.

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Contents:

• Contain all 3 germ layers & has own artery, nerve, cartilage, and muscles

• recruit neural crest cells → cartilages, bones, muscles, nerves, arteries of the head and neck

1. What is the first structure to develop in each pharyngeal arch?

   • Describe this structure

   • Where does this structure terminate? What does this form?

   • Describe the temporal property of this developmental step

Aortic (Pharyngeal) Arch Artery:

• first structure to develop in each pharyngeal arch

• Communicating blood vessel btw dorsal aorta and ventral aorta (aortic sac in humans)

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Termination:

• Each arch terminates in R/L dorsal aorta

• R/L dorsal aorta → fuse @ caudal → descending aorta

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Temporal Property:

• Transient; appear in a cranial-to-caudal sequence and are not all present simultaneously

1. Where does the Ventral Aorta (aortic sac) developed from? Describe its initial structure

2. What does the aortic sac develops into?

Development of the Aortic Sac (ventral aorta)

• develops from the truncus arteriosus

• initially exists as paired structures → fusion → aortic sac

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What the Aortic Sac Forms:

• forms right and left horns

  • R: brachiocephalic artery

  • L: proximal ascending aorta

Describe what the Six aortic arches develops into

Development of the 6 Aortic Arches:

1. First arch → maxillary A.

2. Second Arch → hyoid and stapedial A. (temporary embryonic structures ) → orbital, dural or maxillary branches

3. Third Arch → Common/External/Internal (first part) Carotid A.

   • rest of the internal carotid is formed by the dorsal aorta

4. Fourth Arch

   • L: aortic arch

   • R: proximal right subclavian artery

     • Distal segment from 7th intersegmental artery

5. Fifth Arch → nothing develops here

6. Sixth Arch

   • R: proximal right pulmonary artery

   • L: proximal left pulmonary artery and ductus arteriosus

Where does the recurrent laryngeal nerve hook around? What is the function of this nerve?

Recurrent Laryngeal N. hooks/function:

• Left Side: hooks around the ductus arteriosus

• Right Side: subclavian artery

  • due to absence of the distal part of the right sixth arch.

• Function: main nerve supply to the larynx

1. What are Vitelline Arteries?

2. What happens when these Arteries Fuses and the function of these structures

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1. What does the vitilline Veins form before entering the sinus venosus?

2. How do Vitilline veins relate to liver development?

3. What are the Vitilline veins a precursor of?

Vitelline Arteries:

• paired branches of the dorsal aorta that supply the yolk sac

Fusion of Vitilline Arteries:

• Fusion → three unpaired arteries in the dorsal mesentery of the gut

  • celiac (foregut)

  • superior mesenteric (midgut)

  • inferior mesenteric (hindgut).

• supply the parts of the gastrointestinal tract derived from each part of the primitive gut.

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Vitelline Veins:

• Before entering sinus venosus → forms plexus around duodenum and passes through septum transversum

• Form hepatic sinusoids when liver cords grow into the septum transversum

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Vitelline Veins Development:

• Right Side:

  • hepatocardiac portion of IVC

  • superior mesenteric vein

• Left Side:

  • Nothing

• Plexus around duodenum → portal vein

1. What are Umbilical Arteries?

2. What does it become?

3. Describe what happens to it after birth

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1. Initially, where are the Umbilical Veins? What happen to these veins ?

2. What is the Ductus Venosus?

3. What happens to these veins after Birth?

Umbilical Arteries:

• paired ventral branches of the dorsal aortae

• shift to become branches of the common iliac arteries

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Umbilical Arteries After Birth:

• persist as internal iliac and umbilical arteries

  • distal umbilical arteries → obliterated → medial umbilical ligaments

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Umbilical Veins:

• Initially pass on each side of the liver, connecting with hepatic sinusoids

  • Right umbilical V. and Prox. Left Umbilical V. disappears

• left umbilical vein will carry blood from placenta to liver

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Ductus Venosus:

• forms btw L Umbilical V. and right hepatocardiac channel (IVC)

• shunts oxygenated blood from placenta past the liver into the heart

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Umbilical V. After Birth

• left umbilical vein → ligamentum teres hepatis

• ductus venosus → ligamentum venosum

What three major pairs of veins forms the venous systems

• Vitelline (omphalo-mesenteric) veins: carry blood from the yolk sac

• Umbilical veins: carry oxygenated blood to the embryo from chorionic villi

• Cardinal veins: drain the body of the embryo proper

1. What are the Cardinal Veins consists of?

2. What happens when these veins fuse

Cardinal Veins:

• Consists of Anterior/Posterior Cardinal Veins

  • A: Drains cephalic part

  • P: Drains rest of the embryo

• Anterior and Posterior Cardinal Veins will fuse → common cardinal vein and will drain into the sinus horn

1. What forms the SVC?

2. How does the left brachiocephalic vein form?

3. How does the left superior intercostal vein form?

• SVC: Right anterior cardinal v. + common cardinal v.

• Left Brachiocephalic v.: anastomosis between the anterior cardinal veins

• Left Superior Intercostal v.: terminal portion of the left posterior cardinal veins
  

Describe Fetal Circulation

Placenta → fetus via umbilical vein → bypasses liver via ductus venosus → IVC

• @ IVC: placental blood mixes with deoxygenated blood

IVC → RA → LA via foramen Ovale 

• Small portion of blood @  enters RV → Pulmonary trunk → Aorta via ductus arteriosus

LA → LV  → Systemic circulation  → Placenta via 2 umbilical veins

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NOTE: at all stages, deoxygenated and oxygentated blood mixes

Describe the circulatory changes @ birth

• Describe what happen to some vasculatures

Circulatory Changes at Birth:

Baby’s First Breath → Lung caps are filled w/ blood → O2 blood from Lungs → LA  

• this causes LA > RA in pressure →  holds the valves of foramen ovale shut

  • Foramen Ovale Valve + septum secundum → fossa ovalis

  • Probe patency: in 20% of peeps; fusion does not occur

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Changes @ vasculature:

• ductus arteriosus → ligamentum arteriosum

• distal umbilical arteries → medial umbilical ligaments

• umbilical vein → ligamentum teres hepatis (round ligament of the liver)

• ductus venosus → Ligamentum venosum

1. After birth, what normally closes the Ductus Arteriosus?

2. What is the consequences of patent ductus arteriosus (PDA)?

3. How can you detect PDA in a patient

Ductus Arteriosus Closure due to:

• Increased O2 Content

• decreased prostaglandin levels (constriction of DA)

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Symptoms/consequences of PDA:

• L → R shunt due to higher pressure in aorta

• increased pulmonary artery (PA) pressure → right
  ventricular hypertrophy (RVH) → R →L shunt →late cyanosis

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Detection:

• murmur at first intercostal space left parasternal border

1. Define Coarctation of the Aorta

2. What are the two types and describe the differences between the two of them

3. Symptoms/Consequences?

Coarctation of the Aorta:

• Def: congenital narrowing of the aorta

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Two Types:

• Preductal coarctation

  • usually associated w/ patent ductus arteriosus (PDA)

  • Surgery in infancy is required for survival

• Postductal coarctation

  • not usually associated w/ PDA

  • collateral circulation develops and patients typically survive into adulthood (but not beyond 50 w/o surgery)

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Symptoms/consequences:

• Higher blood pressure in upper vs. lower extremities

• Weak pulses in lower extremities

• Compensatory development of collateral circulation

Describe the collateral circulation created during Coarticulation of the Aorta

Anterior and posterior intercostal arteries:

• Supplies intercostal muscles or the thoracic aorta

• Increased size of collaterals present as “notching of the ribs” in CXRs

• ***These collateral is also used to maintain blood supply to anterior intercostal spaces in surgery when internal thoracic
  artery is used for cardiac bypass surgery.***

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Superior/Inferior epigastric artery:

• Superior from internal thoracic artery and Inferior from external iliac

• Provides collateral circulation from Upper limb (subclavian A.) to the Lower Limb (Femoral A.)

• ***These collaterals could be used to bypass obstruction of abdominal aortic aneurysm or atherosclerotic plaques in the common iliac, external iliac arteries***