Fetal circulation
INTRODUCTION:
- The liver and heart of the fetus receive blood with very high oxygen saturation
Placental circulation consists of independent circulation of blood in two systems:
- Uteroplacental circulation
- Fetoplacental circulation
UTEROPLACENTAL CIRCULATION (maternal circulation):
- Circulation of maternal blood through the intervillous space
- The primitive uteroplacental circulation is functionally established during end of first month
- Volume:500 mL →350 mL in villi system and 150 mL in intervillous space
- Intervillous blood flow at term:500–600 mL per minute, completely replaced about 3–4 times per minute
- Pressure within intervillous space:10–15 mm Hg(uterine relaxation);30–50 mm Hg (uterine contraction)
- Fetal capillary pressure:20–40 mm Hg
Arterial circulation:
- Initailly at 12 weeks cytotrophoblastic invasion into the spiral arteries up to the intradecidual portion
- Secondary invasion of trophoblast between 12 weeks and 16 weeks up to radial arteries
- Spiral arteries are converted to large bore uteroplacental arteries
- Net effect is funneling of the arteries which reduces pressure of blood to 70–80 mm Hg & increases blood flow before it reaches the intervillous space.
- Extravillous trophoblast:Trophoblast cells that do not take part in villous structure
- Endovascular: Invades lumen of the spiral arteries & replaces the endothelium
- Interstitial:Invades inner third of myometrium.
- NK cells prevent Further invasion
Venous drainage:
- Intervillous space → Uterine veins
Circulation in the intervillous space:
- Arterial blood → Chorionic plate → Lateral dispersion → Migration toward the basal plate→ Uterine veins.
FETOPLACENTAL CIRCULATION:
- The two umbilical arteries carry the impure blood from the fetus
- Umbilical arteries → Underneath amnion → Chorionic plate→ Break up → Branches enter stems of the chorionic villi → Primary, secondary and tertiary vessels(villi) → Terminal capillary/shunts → venous channels
- Fetal blood is returned directly to the placenta through the two hypogastric arteries.
- The distal portions of the hypogastric arteries atrophy and obliterate within 3 to 4 days after birth; remnants are called umbilical ligaments.
- Maternal and fetal bloodstreams shows countercurrent flow facilitating material exchange
- Villous capillary pressure:20–40 mm Hg
FETAL BLOOD CHARACHTERISTICS:
- Maximum level of alpha feto protein is seen in Fetal serum
- Heart receives blood with high oxygen saturation
- Blood in IVC has more saturation than blood in SVC as IVC carries the oxygenated blood of umblical vein.
- The left ventricular blood has more oxygen saturation that right ventricular blood because it carries the blood of IVC,while blood in right ventricle is a mixture of blood from IVC and SVC
- Fetal Hb shows high pO2 saturation compared to adult Hb because Affinity to binding to DPG is different in fetal Hb
- The left ventricular output is approximately half of right ventricular output because volume of blood reaching in left atrium is considerably lower than volume of blood reaching in right atrium.
- Aorta and pulmonary trunk are connected by ductus arteriosis, and pulmonary trunk has pressure slightly higher or equal to that of arota –> So, blood flows from pulmonary trunk to aorta
- The pressure in right and left ventricles are equal
- Fetal blood flow:400 mL/min
PLACENTAL BARRIER (placental membrane):
In early pregnancy:
- Syncytiotrophoblast
- Cytotrophoblast,
- Basement membrane
- Stromal tissue
- Endothelium of the fetal capillary wall with its basement membrane.
- 0.025 mm thick
Near term:
- Attenuation of syncytial layer
- Sparse cytotrophoblast and distended fetal capillaries almost fill the villus
- Vasculosyncytial membrane: Zone of villi with thin syncytiotrophoblast,(alpha zones for gas exchange)
- Thick “beta zones” of the terminal villi are for hormone synthesis
- IgG passes this barrier
Exam Important
- The primitive uteroplacental circulation is functionally established during end of first month.
- Fetal blood is returned to the umbilical arteries and the placenta through the Hypogastric arteries
- The immunoglobulin which passes the placental barrier in humans is IgG
- Trophoblast, Fetal capillary endothelium & Mesoderm are components of placental barrier
- The liver and heart of the fetus receive blood with very high oxygen saturation
- Maximum level of alpha feto protein is seen in Fetal serum
- Heart receives blood with high oxygen saturation
- Fetal Hb shows high pO2 saturation compared to adult Hb because Affinity to binding to DPG is different in fetal Hb
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