Fetal Circulation

Question 1

what are the four fetal shunts and what does this mean?

Answer 1

• the fetal shunts are blood flow channels in fetal circulation that close at/near birth and are thus not present in the adult
  
  • placental circulation
  • ductus arteriosis
  • ductus venosus
  • foramen ovale

Question 2

• placental circulation
  
  • connects what structures?
  • serves what purpose?

Answer 2

• blood flows from the placenta –> umbilical veins –> fetus –> umbilical arteries –> placenta
  
  • placental circulation effectively serves as the lungs, GI tract, and kidneys of developing fetus

Question 3

• the ductus arteriosus
  
  • connects what structures?
  • serves what purpose?
  • becomes what adult remnant?

Answer 3

• connects the pulmonary artery to the descending aorta
• bypasses the lungs (blood goes directly from right heart to systemic circulation)
  
  • this is b/c the lungs in the fetus are collapsed. with the introduction of O₂ to the lungs, the ductus arteriosus closes.
• becomes the ligamentum arteriosum

Question 4

• the ductus venosus
  
  • connects what structures?
  • serves what purpose?
  • becomes what adult remannt?

Answer 4

• connects the umbilical vein to the inferior vena cava
• bypasses the the liver (blood from fetus bypasses portal circulation)
• becomes the ligamentum venosum

Question 5

• the foramen ovale
  
  • connects what structures?
  • serves what purpose?
  • becomes what adult remant?

Answer 5

• connects the right atrium to the left atrium
• bypasses the right ventricle and lung
• becomes the fossa ovalis/valve of the foramen ovale in the adult

Question 6

• the placenta
  
  • comes from what progenitor cells?
  • interfaces with what blood supplies and via what structures?

Answer 6

• comes from trophoblast cells.
  
  • these trophoblast cells grow into the endometrium, where they form villi. the villi interact with maternal sinuses, which are supplied by branches of the uterine artery (spiral arteries) and drained by the branches of the uterine vein. vasculature within the villi delivers materal sinus blood to/from the fetus.
    
    • umbilical veins: carry O2 rich blood from maternal finus –> fetus
    • umbilical arteries: carry O2 poor blood from fetus —> maternal sinus
  • these two circulations are separated by a barrier of capillary endothelium + villous epithelium

Question 7

across what barries does gas diffuse from the mother to the fetus? what challenges does the anatomy pose to gas diffusion?

Answer 7

• gas exchange occurs across the placental villi.
• diffusion distance posses the biggest challenge to oxygenation of fetal blood.
  
  • by fick’s law, flux (diffusion) is inversely proportional to diffusion distance/barrier thickness.
  • recall that O₂ rich blood goes from uterine arteries –> maternal sinuses –> umbilical veins –> fetus
    
    • as a result of this travel distance, O2 content drops significantly from maternal blood (85-95 mmHg) to the umbilical vein (30 mmHg)

Question 8

what mechanisms influence “O2 carrying capacity” in fetal circulation and why is this important?

Answer 8

• fetal blood has an increased O2 carrying capacity, meaning the amount of O2 bound to hemoglobin (Hb) at a given O2 partial pressure is higher than in the mother. this compensates low O2 partial pressure in fetal blood
  
  • hb concentration is 50% greater in the fetus than adult
  • hb affinity for O₂ is greater in fetus than adult

Question 9

what is the O2 dissociation curve and its relevance to fetal circulation

Answer 9

• the O2 dissociation curve models hemoglobin affinity for O2 by plotting O2 content against O2 partial pressure
  
  • the fetal curve is left shifted relative to the adult curve
    
    • this means that, at a given O2 pressure, fetal blood carries more O2 per gram of Hb than adult blood
      
      • this is a key component of the increased O2 capacity seen in fetal circulation

Question 10

define the bohr effect and its role in fetal circulation

Answer 10

• the Bohr effect is the right shift in the maternal O₂ dissociation curve when there is increased H+ and CO₂ detected in placental blood.
  
  • this means that at a given PO₂, maternal Hb has a lesser affinity for O2, and more O2 is “free” for the Hb that reaches the fetus.
    
    • this is another fetal compensation for the large maternal-fetal diffusion distance

Question 11

how does the placental barrier change throughtout pregnancy and why is this relavent to fetal circulation?

Answer 11

During pregnancy there is a progressive…

• increase in placental cross‐sectional area (A)
• thinning of the villar membranes (T)
  
  • by Fick’s law, both changes increase flux of O2 from maternal to fetal blood.
    
    • another fetal compensatory mechanism for maternal-fetal diffusion distance

Question 12

summarize the fetal adaptations that compensate for large maternal-fetal diffusion distance

Answer 12

• high Hb concentration in fetal blood (50% > maternal)
• greater affinity of fetal Hb for O2 - i.e. fetal left shit
• Bohr effect (H+/CO₂ lowers materal Hb affinity for O2) - i.e. maternal right shift
• barrier changes - larger placental surface area + villi thinning

Question 13

• discuss which nutrients diffuse across the placenta via
  
  • passive diffusion
  • facilitated diffusion
  • and active transport

Answer 13

Nutrient exchange in placenta

• Passive diffusion
  
  • fatty acids
  • small ions such (Na+, K+, Cl)
  • waste products
• Facilitated diffusion
  
  • glucose (voa GLUT 1 & GLUT 3; NOT insulin dependent)
• Active transport
  
  • amino acids
  • absorbic acid
  • Ca++
  • PO₄

Question 14

discuss the series vs parallel arrangement of blood flow at different types in the development

Answer 14

• in the fetus: blood flow is arranged in “parallel”
  
  • because of the foramen ovale, both the RA and LA receive blood from systemic circulation
  • because of the ductus arteriosus, the aorta receives blood from both the LV and the PA
• in the adult: blood flow is in series
  
  • body –> RA–> RV –> lung –> LA –> LV –> aorta –> body

Question 15

• defined “combined ventricular ouput”
  
  • what are the sources of this ouput?
  • where does this blood go?

Answer 15

• fetal cardiac output, which consists of blood from both ventricles (right ventricular output + left ventricular ouput)
  
  • LVO =constititutes 1/3 of the CVO. supplies the
    
    • carotids (perfuse head)
    • subclavian arteries (perfuses upper limb)
    • & coronary arteries
  • RVO = constitutes 2/3 of the CVO
    
    • most of the RVO crosses the ductus arteriosus to supply the descending aorta, which perfuses the
      
      • abdominal organs
      • lower limbs
    • remaining goes to pulmonary circulation to perfuse the lungs

Question 16

where in fetal circulation is blood most oxygenated?

Answer 16

the umbilical vein

Question 17

• what percentage of the CVO goes to the placenta & where does it travel after this?

Answer 17

• 40-50% of the CVO goes directly to the placenta via the umbilical arteries
• the blood is oxygenated in the maternal sinusoids, which the feed into teh umbilical veins
  
  • umbilical vein drains go to the fetus, where it then mixes with portal blood
    
    • half of this blood passes through the liver –> hepatic veins –> IVC
    • half of this blood bypasses portal circulation via the ductus venosus and feeds directly in the IVC

Question 18

discuss the concept of “streamlining”

Answer 18

• the right atrium receives blood form both the umbilical vein (high oxygen saturation) and maternal tissues (low oxygen saturation).

Question 19

what are the sources of left ventricular blood?

Answer 19

• LA (carrying blood from the lungsP
• RA
  
  • IVC (the portion of IVC blood carrying the more oxygenated blood - i.e., blood that came from the umbilical arteries)

Question 20

what are the sources of right ventricular blood in the fetus?

Answer 20

• basically all the feeders of the RA because that’s what’s feeding the RV
  
  • IVC (the portion of IVC blood that came from fetal tissue and is thus deoxygenated)
  • SVC
  • coronary sinus

Question 21

contrast the oxygen saturation of the IVC compared to the SVC

Answer 21

• IVC more oxygenated the SVC
  
  • this is because the SVC only is only draining fetal tissues (head, upper torso) while the IVC contains blood from fetal tissues (lower limbs) as well as blood from the umbilical vein (fresh O2 rich blood from maternal sinusoids)

Question 22

what fetal heart chamber has the most O₂ rich blood?

Answer 22

the left ventricle.

• receives the O₂ rich portion of blood from the IVC (via the right atrium)
• receives O2 rich blood from the lungs (via the LA)

Question 23

contrast the oxygen saturation of the blood in the head vs the body in fetal circulation

Answer 23

head = received more more saturated blood

• the head receives blood from coronary arteries, which branch off the aortic arch proximal to the ductus arteriosis. this means they are transmitting blood only from the LV, the most oxygenated chamber.
• the lower body receives blood from the ascending aorta distal to the ductus arteriosus, which is mixed blood containing both LV blood + pulmonary artery blood, and is thus less oxygenated

Question 24

what keeps the foramen ovale open in fetal circulation? what would cause this to change?

Answer 24

• the fact that the right atrial pressure > right ventricular pressure
  
  • if pressures reverse and Prv > Pra, foramen ovale would close

Question 25

what keeps the ductus arteriosus open in fetal circulation? what might cause this to change?

Answer 25

• the mean pressure in the pulmonary artery > mean pressure in the aorta
  
  • this is due to the fact that diastolic Ppa > diastolic Pao
    
    • ​this is due to the high vascular resistance in the PA
  • their systolic pressures are near equal
• this ductus arterious may close if the pressure gradient reverses such that Pao > Ppa

Question 26

contrast cardiac cells in fetal heart and the adult heart. what is the significance of these differences?

Answer 26

• 1. fewer contractile elements
  
  • increased pre-load –> little increase in stroke volume since frank-starling mechanisms not intact
    
    • since CO = HR X SV, cardiac output is largely dependent on HR.
      
      • feta HR is high (140-160 min)
  • increased afterload –> big decrease in stroke volume
    
    • cardiac contractility can’t overcome afterload due to sparse cardiac muscle
• 2. SNS innervation to cardiac cells not fully developed
  
  • ​futher limits contractility

Question 27

what key changes in pulmonary circulation occur after birth & what is their significance?

Answer 27

• 1. a decrease in pulmonary vascular resistance
     * birth –> baby breaths –> O2 enters lungs and causes vasodilation of the pulmonary vessels –> there is a massive fall in pulmonary vascular resistance –> which increases pulmonary venous flow
     
     • increased flow from pulmonary veins into aorta increases LA pressure
       
       • ​increased P_LA contributes to closure of foramen ovale
         
         • pulmonary vascular resistnace continues to fall in the following weeks
• 2. placental transfusion: a decrease in intrathoraic pressure “sucks blood from placenta” into systemic circulation. idk why