Foetal Circulation Flashcards
Explain how foetal circulation is modified in comparison to adult circulation
- 3 by-passes to avoid liver/lungs
- by-pass of hepatic circulation
- by-pass of pulmonary circulation by foramen ovale and ductus arteriosus
What surrounds and is contained in the umbilical cord?
- surrounded by fetal membrane and amnion
- contains Wharton’s jelly which contains 2 umbilical arteries and 1 umbilical vein
What is the function of the umbilical vein?
carries oxygen and nutrient rich blood from the placenta
What are the divisions of the umbilical vein?
- smaller branch to the liver (liver then has branch to IVC)
- larger branch: the ductus venosus which by-passes the liver to drain to IVC
What order can blood be passed through the vessels from drainage to the IVC?
- right atrium to left atrium through foramen ovale
- to left ventricle
- to aorta
What order can blood be passed through the vessels from drainage to the SVC?
- right atrium
- right ventricle
- pulmonary trunk
- ductus arteriosus (90%)
- aorta
- pulmonary trunk
- pulmonary arteries (10%)
- pulmonary veins
- left atrium, left ventricle
- aorta
(blood is NOT oxygenated)
Where does the blood in the aorta go?
- one third gets distributed around the abdomen, pelvis and lower limbs
- 2 thirds goes to placenta by internal iliac artery then to umbilical artery then placenta
Explain the effect tying and cutting the umbilical cord has on postnatal circulation
- loss of blood flow through placenta
- increased systemic vascular resistance as blood must now be distributed in systemic circulation
- therefore increased aortic, left ventricular and left atrial pressure
Explain the effect of the first breath on post natal circulation
- expansion of the lungs expands the before collapsed alveoli
- decreased pulmonary vascular resistance
- therefore reduced pulmonary arterial, right ventricular and right atrial pressures
Explains what happens once pulmonary circulation is established upon the first breath
- blood from pulmonary circulation is returned to left atrium
- increases pressure in left atrium
- valve of foramen ovale is pushed rightward and closes
- becomes structurally closed in 4 months to become fossa ovalis
What is patent foramen ovale?
- when foramen ovale doesn’t close
- usually asymptomatic
- can cause paradoxical emboli
Explain how the obliteration of ductus arteriosus occurs
- occurs due to the systemic pressure changes to cause blood that normall flows through ductus arteriosus from pulmonary trunk to aorta to be reversed
- functionally closed after 1hr due to increased oxygen/decreased prostaglandin causing constriction of the vessel
- structural closure between 1-4 months by thickening of tunica intima to become ligamentum arteriosum
Patent ductus arteriosus
- when ductus arteriosus isn’t obliterated
- due to pressure gradient blood flows back into pulmonary circulation
- results in pulmonary hypertension ad congestive cardiac failure
Explain how obliteration of ductus venosus occurs
- blood flow through umbilical vein stops
- muscle wall of vessel contracts
- portal venous blood flows through hepatic sinuses
- occurs 1-3hrs after
- becomes ligamentum venosum
Where can the ligamentum venosum be identified in adults?
Between caudate and left lobe of liver
What does the umbilical vein become after birth?
- round ligament of the liver: ligamentum teres hepatis
What does the umbilical arteries become after birth?
medial umbilical ligaments within medial umbilical folds
Explain the development from a zygote to a morula
- 2 cell stage: 30hr
- 4 cell stage: 40hr
- morula (16 cell stage): 72hrs
surrounded by zona pellucida which disappears after 4 days
Explain the development from a morula to a blastocyst
- morula travels down fallopian tube, enters uterine cavity and implants on endometrium
- fluid accumulates in intracellular spaces
- spaces merge to form single cavity (blastocyst cavity)
- inner cell mass at one pole where cells will differentiate to form the primary germ layers in gastrulation
Explain the development of a blastocyst
- day 8 inner cell mass differentiates into
- epiblast adjacent to amniotic cavity
- hypoblast adjacent to blastocyst cavity/primitive yolk sac
- layers form bilaminar disc
- cavity forms within epiblast to become amniotic cavity
What side and region is the primitive streak found on the bilaminar disc?
- dorsal side
- caudal region
Explain the formation of the trilaminar disc
- cells of epiblast begin to migrate through primitive streak and creates a third layer between epiblast and hypoblast
- epiblast differentiates to ectoderm
- hypoblast differentiates to endoderm
- middle layer becomes mesoderm
Explain the events that occur in the trilaminar disc of a 17 day old embryo
- prenotochordal cells migrate through primitive streak to form notochordal plate in endoderm
- plate detaches from endoderm to form notochord
- activates signalling pathway for tissue differentiation and development
- events occur in cranial to caudal direction
What does the mesoderm become?
divided into:
- paraxial mesoderm: somites and head mesenchyme
- intermediate mesoderm: urogenital system
- lateral plate mesoderm: walls of body cavities, serous membranes and circulatory system
