HUBS3511 · Human Reproduction And Pregnancy
Fetal and Neonatal Physiology
In the fetus the placenta is the lung and the real lungs are fluid-filled and useless, so the fetal heart is plumbed with three shunts that divert blood around the liver and lungs, and the umbilical vein carries the most oxygenated blood in the whole body — the reverse of the usual “veins are deoxygenated” rule. Fetal haemoglobin (HbF) has a higher O₂ affinity than adult HbA (a left-shifted curve), so at the low O₂ of the placenta the fetus pulls oxygen off the mother. At birth two events — the first breath (PVR falls) and cord clamping (SVR rises) — reverse the pressures and the three shunts close. HUBS3511 also examines DOHaD (the developmental origins of health and disease, a story about mismatch between a thrifty fetal phenotype and postnatal plenty) and the neonatal transition (lung fluid clearance, surfactant, functional residual capacity). Learn the fetal circulation as one loop: placenta → back to placenta.
What this chapter covers
- 01Fetal circulation: the three shunts and the umbilical vessels
- 02Why bypass the lungs: high fetal pulmonary vascular resistance
- 03Fetal vs adult haemoglobin: HbF, the left shift and the double Bohr effect
- 04The switch at birth: ↓PVR + ↑SVR → shunts close (and adult remnants)
- 05Functional vs anatomical shunt closure; PFO and PDA
- 06DOHaD: thrifty phenotype, epigenetic programming and mismatch
- 07Neonatal transition: lung fluid clearance, surfactant, FRC
- 08Delayed cord clamping
Worked example: the three fetal shunts and what each bypasses
- +1Ductus venosus: umbilical vein → inferior vena cava, bypassing the liver.
- +1Foramen ovale: right atrium → left atrium, bypassing the lungs.
- +1Ductus arteriosus: pulmonary artery → aorta, again bypassing the lungs.
- +1Why bypass the lungs: fetal lungs are fluid-filled and collapsed, so pulmonary vascular resistance is very high; the shunts are pressure-driven short-cuts around them.
- +1Umbilical vein: it carries oxygenated blood FROM the placenta TO the fetus, because the reference organ is the placenta (not the lungs); the umbilical arteries carry deoxygenated blood back.
Key terms
- Ductus venosus
- The fetal shunt that carries oxygenated umbilical-vein blood past the liver into the inferior vena cava. It closes within minutes of birth and becomes the ligamentum venosum.
- Foramen ovale
- The fetal opening between the right and left atria that bypasses the lungs while right-sided pressure exceeds left. After birth, rising left atrial pressure presses it shut; it becomes the fossa ovalis (a persistent opening is a patent foramen ovale).
- Fetal haemoglobin (HbF)
- Haemoglobin with two α and two γ chains. Because the γ chain binds 2,3-BPG poorly, HbF has a higher O₂ affinity than adult HbA (a left-shifted dissociation curve), letting the fetus pull O₂ off maternal blood at the low placental PO₂.
- DOHaD
- The Developmental Origins of Health and Disease (Barker) hypothesis: an adverse in-utero environment programmes long-term disease risk via epigenetic changes (e.g. DNA methylation). The risk arises from MISMATCH between a thrifty fetal phenotype and a nutrient-rich postnatal life.
- Surfactant
- A substance that lowers alveolar surface tension to prevent collapse and raise lung compliance at birth. Deficiency in preterm infants causes respiratory distress.
Fetal and Neonatal Physiology FAQ
Why does the umbilical vein carry oxygenated blood?
Because the reference organ is the placenta, not the lungs. The umbilical vein brings oxygenated blood from the placenta to the fetus; the two umbilical arteries return deoxygenated blood to the placenta. This reverses the usual “veins are deoxygenated” convention.
Which way is the fetal Hb curve shifted, and why?
Left. HbF has a higher O₂ affinity than maternal HbA (because its γ chains bind 2,3-BPG poorly), so its curve is left-shifted. A left shift means it holds O₂ more tightly — exactly what is needed to pull O₂ off the mother at the low placental PO₂. Saying HbF releases O₂ more easily is backwards and would starve the fetus.
What closes the shunts at birth?
Two events: the first breath inflates the lungs (PVR drops, pulmonary flow surges) and cord clamping removes the low-resistance placenta (SVR rises). This reverses the pressure gradient — left now higher than right — so the foramen ovale presses shut, and rising O₂ with falling prostaglandins closes the ductus arteriosus and ductus venosus.
What is DOHaD really about?
Mismatch — not simply “bad womb = bad outcome”. A nutrient-poor fetus develops a thrifty phenotype that expects scarcity; if postnatal life is nutrient-rich, those settings misfire and raise the risk of obesity, type 2 diabetes, hypertension and cardiovascular disease. Low birth weight is the classic marker and the mediator is epigenetic, not a DNA-sequence change.
Exam move
Learn the fetal circulation as a single traced loop (placenta → umbilical vein → ductus venosus → RA → foramen ovale → LA → brain; pulmonary artery → ductus arteriosus → aorta; umbilical arteries → placenta), keeping each shunt paired with what it bypasses (ductus venosus → liver; foramen ovale and ductus arteriosus → lungs). For haemoglobin, do not memorise numbers — reason from the shift: left = higher affinity = holds O₂, which is what the fetus needs to pull O₂ off the mother. Rehearse the birth switch as the reverse of the fetal loop (↓PVR + ↑SVR → pressures reverse → shunts close), and name the failure lesions (PFO, PDA). For DOHaD, the one-word answer is MISMATCH, with low birth weight as the marker and epigenetics as the mediator.
