TBL13 - Development of the Vascular System

Question 1

What are two early intraembryonic veins and what do they unite to form?

Answer 1

Early intraembryonic veins include the bilateral anterior and posterior cardial veins that unite to form the short common cardinal veins

Question 2

When shunting of venous blood is directed towards the right sinus horn, what is formed by an anastomosis between the left and right anterior cardinal veins? Blood from which locations is channeled mostly to the right side? What forms the SVC?

Answer 2

1) During shunting of venous blood toward the right sinus horn, the left brachiocephalic vein is formed by an anastomosis between the left and right anterior cardinal veins
2) Thus, most venous blood from the left side of the head and left upper limb is channeled to the right side
3) Union of the right anterior and common cardinal veins forms the SVC

Question 3

What is the fate of the posterior cardinal veins? What veins form the IVC?

Answer 3

1) The posterior cardinal veins are obliterated but after formation of the left-to-right venous shunt, the azygos vein is derived from the proximal portion of the right posterior cardinal vein
2) The coalescence of other veins that drain the kidneys, lower limbs, and body wall, forms the IVC

Question 4

What does the aortic sac generate and where does this occur? What is the aortic sac continuous with?

Answer 4

1) At the cranial end of the heart tube, the aortic sac generates the left and right dorsal aortae
2) The aortic sac is continuous with the truncus arteriosus

Question 5

What forms between the aortic sac and the dorsal aortae? What do these structures course through on their way to the dorsal aortae?

Answer 5

1) Pairs of aortic arches arise from the aortic sac and terminate in the dorsal aortae
2) From the aortic sac, the aortic arches course through the mesenchyme of paired pharyngeal arches to the dorsal aortae

Question 6

How many pairs of aortic arches does the aortic sac form? How are they numbered and why?

Answer 6

1) The aortic sac originally generates six pairs of aortic arches
2) The arches are numbered I, II, III, IV, and VI because the fifth pair form incompletely and regress

Question 7

What is the fate of aortic arches I and II? Fate of aortic arch III? Fate of the right aortic arch IV? Fate of the left aortic arch IV?

Answer 7

1) Aortic arches I and II disappear
2) Aortic arch III forms the right and left common carotid arteries
3) The right 4th aortic arch forms the right subclavian artery
4) The left 4th aortic arch completes the arch of the aorta and forms the left subclavian artery

Question 8

What is the arch of the aorta continuous with? What is this structure formed by? Where does the right dorsal aorta travel?

Answer 8

1) The arch of the aorta is continuous with the descending aorta, which is formed by the left dorsal aorta and continues as the single midline aorta
2) The right dorsal aorta disappears beyond the right subclavian artery

Question 9

When the right dorsal aorta fails to disappear, what forms? What is required if dyspnea or dysphagia occur?

Answer 9

1) When the right dorsal aorta fails to disappear, a double arch of the aorta wraps around the trachea and esophagus
2) Surgical division of the vascular ring is required if dyspnea or dysphagia occur

Question 10

What does coarctation of the aorta result in?

Answer 10

Coarctation of the aorta is an abnormal narrowing (stenosis) that reduces the size of the descending aortic lumen thereby obstructing blood flow to the inferior part of the body

Question 11

Compare the locations of preductal and postductal coarctations. Compare their clinical consequences.

Answer 11

1) Preductal coarctations occur above the entrance of the ductus arteriosus, while postductal coarctations occur below the entrance of the ductus arteriosus
2) In coarctation of the aorta, the arch of the aorta or thoracic aorta has an abnormal narrowing (stenosis) that diminishes the caliber of the aortic lumen, producing an obstruction to blood flow to the inferior part of the body

Question 12

Why is neonatal surgery required to correct preductal coarctation of the aorta, and why can postductal coarctation of the aorta be asymptomatic in newborns?

Answer 12

1) In preductal coarctation of the aorta, an embryo can shunt blood from the ductus arteriosus to the descending aorta. Once the baby is born, the ductus arteriosus is converted to the ligamentum arteriosus and can no longer shunt blood. There is therefore no more blood traveling through the aorta and surgery is required for survival
2) In postductal coarctation of the aorta, blood flows through the subclavian artery after passing the ligamentum arteriosus. This allows blood to go through the internal thoracic arteries and into the anterior intercostal arteries
3) Also in postductal coarctation of the aorta, blood can flow from the aorta directly to the posterior intercostal arteries
4) These branches create a collateral circulation that will return blood to the abdominal aorta

Question 13

With postductal coarctation, which arteries typically create a compensatory collateral circulation and why can notable pulsations occur in the intercostal spaces?

Answer 13

1) When the coarctation is inferior to this site (postductal coarctation), a good collateral circulation usually develops between the proximal and distal parts of the aorta through the intercostal and internal thoracic arteries
2) The collateral vessels may become so large that they cause notable pulsation in the intercostal spaces and erode the adjacent surfaces of the ribs, which is visible in radiographs of the thorax

Question 14

What does aortic arch VI form? What does it form specifically on the left side? What occurs to the ductus arteriosus after birth?

Answer 14

1) Aortic arch VI (aka pulmonary arch) forms the right and left pulmonary arteries that extend from the pulmonary trunk
2) On the left side, aortic arch VI forms the ductus arteriosus that joins the left pulmonary artery with the arch of the aorta
3) After birth, obliteration of the ductus arteriosus forms the ligamentum arteriosum

Question 15

Why can a patent ductus arteriosus in premature infants result in the cascade of pulmonary hypertension, left ventricular hypertrophy, and left atrial distension? Why is the condition fatal?

Answer 15

1) A patent ductus arteriosus causes a left to right shunt of blunt from the descending aorta to the pulmonary arteries
2) This extra blood now empties into the lung and right ventricle, causing an increase in pulmonary hypertension. The lungs are now returning an extra amount of oxygenated blood to the left atrium. This will cause left atrial distention and result to left atrial hypertrophy
3) This can result in death because blood is not circulating properly

Question 16

Why does PDA cause a continuous murmur heard parasternally at the left 2nd ICS?

Answer 16

Because a continuous flow of blood is traveling in the wrong direction from the aorta, through the patent ductus arteriosus, into the pulmonary arteries and into the lungs, a murmur is heard continuously

Question 17

What does the umbilical vein do? What does the ductus venosus do? Where does blood from the IVC travel to?

Answer 17

1) The umbilical vein delivers oxygenated blood from the placenta to the fetus
2) Most of the oxygenated blood enters the ductus venosus to bypass the liver and empty into the IVC
3) Blood from the IVC traverses the foramen ovale into the left atrium and passes into the left ventricle for pumping into the aorta

Question 18

Where does blood from the SVC travel to? Why is blood shunted through the ductus arteriosus?

Answer 18

1) Blood from the SVC passes through the right atrium into the right ventricle for pumping into the pulmonary trunk
2) Swallowed (aspirated) amniotic fluid collapses the pulmonary vessels in the fetal lungs (due to increase fluid pressure on the alveolar septa); thus, high pulmonary vascular resistance diverts blood from the pulmonary trunk into the aorta via the ductus arteriosus

Question 19

What do the common iliac arteries do in the fetus?

Answer 19

The common iliac arteries deliver deoxygenated blood from the fetus to the umbilical arteries for return to the placenta

Question 20

What occurs to the umbilical arteries and veins a few minutes after birth? What does obliteration of the ductus venosus forms?

Answer 20

1) A few minutes after birth, vasoconstriction closes the umbilical arteries
2) Shortly after the arterial constriction, closure of the umbilical vein and ductus venosus occurs
3) Obliteration of the ductus venosus forms the ligamentum teres

Question 21

What occurs to the septum primum-derived valve after birth? How is the fossa ovale formed?

Answer 21

1) After birth, increased left atrial pressure presses the septum primum-derived valve of the foramen ovale against the septum secundum
2) The fossa ovale is formed by fusion of the valve of the foramen ovale with the septum secundum, which typically occurs between the 8th and 12th month postnatally

Question 22

Why does left atrial pressure increase and right atrial pressure decrease when a newborn begins to breathe?

Answer 22

Instead of blood being shunted from the right atrium directly to the left atrium as in the embryo, blood can now circulate through the chambers of the heart in addition to the pulmonary circulation

Question 23

Why would crying create cyanotic periods in neonates but not normal 8-12 month old infants?

Answer 23

Crying would allow the foramen ovale to be open and shunt deoxygenated blood to the left atrium to be pumped to the body. This would result in cyanotic periods. Once the foramen ovale closes, this can no longer happen (8-12 months)