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Fetal Circulation and Its Physiology

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1. Introduction

1. The placenta serves as the combined lung, gut, and kidney of the fetus.
2. The heart pumps in parallel (not in series as in adults) — thanks to two right-to-left shunts.
3. Three specialized shunts divert blood away from the non-functional lungs and reduce hepatic first-pass.

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2. The Placenta as the Gas Exchange Organ

• Umbilical vein carries oxygenated blood from placenta to fetus (O₂ saturation ~80%)
• Umbilical arteries (×2) carry deoxygenated blood from fetus to placenta (O₂ saturation ~58–60%)

Memory tip: "Umbilical Vein carries O₂-rich blood" — the Vein is the exception to the rule that veins carry deoxygenated blood.

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3. The Three Fetal Shunts

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4. Step-by-Step Blood Flow in the Fetus

Step-by-step route:

• Highly oxygenated blood (~80% saturation) leaves the placenta via the single umbilical vein under high pressure.

• On approaching the liver, the blood splits:
  • ~50% passes through the ductus venosus directly into the IVC (bypasses the liver)
  • ~50% enters the hepatic sinusoids via portal circulation and reaches the IVC through hepatic veins
• Both streams merge in the IVC, mixing with deoxygenated blood from the lower limbs → IVC saturation ~67%

• The blood enters the right atrium and is guided toward the foramen ovale by the valve of the inferior vena cava (Eustachian valve).
• The crista dividens (lower edge of septum secundum) diverts most IVC blood through the foramen ovale into the left atrium.
• A small portion remains in the right atrium and mixes with poorly oxygenated blood from the SVC and coronary sinus.

• Blood that crosses the foramen ovale mixes with a small amount of deoxygenated pulmonary venous return in the left atrium.
• From the left ventricle, it enters the ascending aorta.
• The first branches of the ascending aorta are the coronary and carotid arteries → therefore the heart and brain receive the best-oxygenated blood in the fetus.

• Mixed (medium-saturation) blood from the right atrium (SVC + small IVC component) passes to the right ventricle and is ejected into the pulmonary trunk.
• Pulmonary vascular resistance is high in fetal life (vasoconstriction of unexpanded lung vessels) → only ~10% enters the lungs.

• ~90% of blood in the pulmonary trunk passes through the ductus arteriosus into the descending aorta, mixing with blood from the proximal aorta.
• From the descending aorta:
  • ~65% → umbilical arteries → back to the placenta for reoxygenation
  • ~35% → viscera and inferior body (kidneys, lower limbs, gut)

Sites of Blood Mixing (Langman's Classification)

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5. Why the Heart Pumps in Parallel

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6. Fetal Hemoglobin (Hb F) — Physiological Basis

• Hb F has a higher O₂ affinity than adult Hb A → its O₂–dissociation curve is shifted left
• Mechanism: Hb F binds 2,3-DPG less effectively than Hb A → less right-shifting of the curve → greater O₂ uptake at any given PO₂
• This allows fetal red cells to load O₂ from maternal blood in the placenta even at relatively low PO₂ values

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7. Fetal Pulmonary Circulation

• Pulmonary vasculature is vasoconstricted in utero due to low PO₂ and high CO₂
• Pulmonary vascular resistance is much higher than systemic resistance
• Pulmonary blood flow is minimal (~10% of right ventricular output)
• The walls of pulmonary arteries are thick; thinning begins at birth with lung expansion

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8. Circulatory Changes at Birth (Transitional Neonatal Circulation)

Trigger: Lung Expansion

• Lungs expand → pulmonary vascular resistance falls to <20% of the in-utero value
• PO₂ rises → oxygen acts as a vasodilator via nitric oxide (NO) production
• Pulmonary blood flow increases dramatically

Sequential Events at Birth

Timeline of Ductus Arteriosus Closure:

• Functional closure: within hours of birth
• 24 hours: 20% ducts functionally closed
• 48 hours: ~80% closed
• 96 hours: 100% closed in term neonates
• Anatomic closure (ligamentum arteriosum): by 12th postnatal week

Role of Prostaglandins

• In utero: PGE₂ and PGI₂ keep the ductus arteriosus open (vasodilation of ductal smooth muscle)
• At birth: cyclooxygenase activity falls, PG synthesis drops → ductus constricts
• In prematurity: ductus may stay patent → treated with indomethacin (COX inhibitor)
• TGF-β is involved in the subsequent anatomic closure

Foramen Ovale Permanent Closure:

• Anatomic closure by the 3rd postnatal month — septum primum proliferates and adheres to left margin of septum secundum
• Leaves the oval fossa (fossa ovalis) as a permanent anatomical remnant

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9. Adult Derivatives of Fetal Structures

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10. Clinical Correlates

Patent Ductus Arteriosus (PDA)

• Failure of ductus arteriosus to close after birth
• 2–3× more common in females
• Creates a left-to-right shunt (aorta → pulmonary artery) → pulmonary vascular overload
• Associated with maternal rubella infection and prematurity
• Treatment: Indomethacin (COX inhibitor → ↓ PGE₂ → ductal constriction); surgical ligation if unresponsive
• Continuous "machinery" murmur

Patent Foramen Ovale (PFO)

• ~25–30% of adults have a probe-patent foramen ovale
• Usually asymptomatic but can cause paradoxical embolism (venous thrombus → systemic circulation)

Fetal Asphyxia

• Any disturbance interfering with placental O₂/CO₂ exchange (maternal hypotension, abruption, cord prolapse, severe anemia)
• Causes: low PO₂, high PCO₂, acidosis → depressed myocardial function and reduced cardiac output
• Brain forced to shift to anaerobic metabolism → ATP depletion, endothelial damage, hemorrhage

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11. Summary Table: Key Oxygen Saturations in Fetal Circulation

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12. Quick Review — Key Exam Points

1. Umbilical vein carries oxygenated blood (one vein, two arteries in cord)
2. Ductus venosus bypasses the liver; ~50% of umbilical venous blood uses it
3. Crista dividens directs IVC blood through the foramen ovale
4. Heart and brain get the best-oxygenated blood (via ascending aorta)
5. High pulmonary vascular resistance is the reason the ductus arteriosus is necessary
6. Hb F has higher O₂ affinity than Hb A because it binds 2,3-DPG less
7. At birth: lungs expand → ↑ PO₂ → ↑ pulmonary flow → ↑ LA pressure → foramen ovale closes
8. O₂ constricts the ductus arteriosus; prostaglandins (PGE₂, PGI₂) dilate it
9. Parallel → Serial pump transition is completed within minutes–hours of birth
10. Right ventricular wall is thicker than left in fetus/neonate; reverses by end of first month

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Fetal Physiology — Detailed Exam Notes

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1. Overview

• The fetus lives in an oxygen-poor, fluid-filled intrauterine environment
• All gas exchange, nutrition, and waste removal occur via the placenta — not the lungs, gut, or kidneys independently
• Multiple organ systems are structurally present but functionally immature, with maturation occurring in a carefully programmed sequence
• The fetus must undergo a profound physiological transition at birth to survive extrauterine life

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2. Fetal Cardiovascular Physiology

Unique Features

• Heart pumps in parallel (not series) due to two intracardiac/extracardiac shunts
• Three obligatory shunts: ductus venosus, foramen ovale, ductus arteriosus
• Right ventricle output > left ventricle output in fetal life
• Best-oxygenated blood preferentially directed to brain and myocardium via ascending aorta

Cardiac Output

• Neonatal cardiac output is heart-rate dependent (stroke volume is fixed due to non-compliant myocardium)
• Normal fetal heart rate: 110–160 bpm
• Bradycardia = myocardial ischemia, impending arrest

Persistent Fetal Circulation (PFC) / Persistent Pulmonary Hypertension of the Newborn (PPHN)

• Hypoxia or acidosis in the first days of life can prevent or reverse the normal transition → right-to-left shunting resumes
• Creates a vicious cycle: ↑ pulmonary vascular resistance → right-to-left shunt → hypoxemia → acidosis → more ↑ PVR
• Treatment: mechanical ventilation, supplemental O₂, inhaled nitric oxide (pulmonary vasodilator), milrinone, ECMO in refractory cases

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3. Fetal Respiratory Physiology

Fetal Breathing Movements (FBM)

• Begin in the late first trimester (~10–12 weeks), confirmed by Doppler
• Occupy <50% of any 24-hour period; become more regular near term
• Purpose: development and conditioning of respiratory muscles and lung growth
• Hypoxia and tactile stimulation promote FBM
• FBM decrease just before labor — clinically important (absent FBM may precede labor)

Fetal Lung Development (5 Stages)

Surfactant

• Produced by type II alveolar pneumocytes (alveolar epithelial cells)
• Composed of dipalmitoylphosphatidylcholine (DPPC, lecithin), phosphatidylglycerol, apoproteins
• Function: reduces alveolar surface tension → prevents alveolar collapse at end-expiration
• Production begins ~30 weeks; sufficient for extrauterine life by 34–36 weeks
• L:S ratio (lecithin:sphingomyelin) ≥ 2:1 → lung maturity (clinical test on amniotic fluid)
• Glucocorticoids (e.g., betamethasone given to mother) accelerate surfactant production → given between 24–34 weeks when preterm delivery is anticipated

Lung Fluid

• In utero, alveoli and airways are filled with ~90 mL of plasma ultrafiltrate (lung fluid)
• Volume approximates the functional residual capacity (FRC) of the neonatal lung
• Essential for lung growth — restricting it impairs lung expansion
• At onset of labor: catecholamines and arginine vasopressin (AVP) ↓ fluid production and activate epithelial Na⁺ channels (ENaCs) for active fluid reabsorption
• During vaginal delivery: "vaginal squeeze" expels remaining fluid
• Residual fluid reabsorbed by pulmonary capillaries and lymphatics

Respiratory Distress Syndrome (RDS) / Hyaline Membrane Disease

• Result of surfactant deficiency in premature neonates (<34 weeks)
• Alveolar collapse, progressive hypoxemia
• Treatment: exogenous surfactant therapy, CPAP/mechanical ventilation

Transient Tachypnea of the Newborn (TTN)

• Delayed reabsorption of fetal lung fluid
• Occurs in C-section babies (no vaginal squeeze) and small preterm neonates
• Improves within 2–48 hours; diagnosis of exclusion; supportive treatment

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4. Fetal Hematology

Fetal Hemoglobin (Hb F)

• Composition: α₂γ₂ (two alpha + two gamma chains) — vs. adult Hb A (α₂β₂)
• Higher O₂ affinity than Hb A → O₂ dissociation curve shifted LEFT
• Mechanism: Hb F binds 2,3-DPG less effectively → less right-shifting effect → greater O₂ loading at lower PO₂
• Allows fetal RBCs to extract O₂ from maternal blood even at low PO₂ values in the placenta

O₂-Affinity Transition at Birth

• After birth, O₂ demands increase but Hb F impairs O₂ unloading to tissues
• During 1–2 month transition to Hb A: rising 2,3-DPG levels and mild acidosis reduce O₂ affinity → better O₂ delivery to tissues

Hematopoiesis — Sites (Developmental Sequence)

Mnemonic: "Young Liver Synthesizes Blood" — Yolk sac → Liver → Spleen → Bone marrow

Fetal Blood Counts

• Fetal Hb (Hb F) constitutes ~80% of total Hb at birth, falling to <5% by 6 months
• Nucleated RBCs present in fetal circulation (erythroblasts) — marker of fetal stress if seen postnatally in excess
• Fetal polycythemia: Hct ~50–65% at term (compensates for low O₂ saturation)

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5. Fetal Renal Physiology

Development and Function

• Definitive kidney (metanephros) begins forming at ~5 weeks; nephrons form between 8–36 weeks
• Fetal kidneys begin producing urine by ~10 weeks
• By 20 weeks, fetal urine is the main source of amniotic fluid

Amniotic Fluid

• Produced by: fetal urine (mainly), fetal lung fluid, umbilical cord, fetal skin (early)
• Swallowed by the fetus at ~500 mL/day (recycled)
• Normal volume at term: 800–1000 mL
• Oligohydramnios (↓ amniotic fluid): renal agenesis, obstructive uropathy → "Potter sequence" (pulmonary hypoplasia, limb deformities, facial features)
• Polyhydramnios (↑ amniotic fluid): impaired fetal swallowing (esophageal atresia, anencephaly), excess production

Renal Immaturity

• Fetal kidneys are physiologically immature — low GFR, poor concentrating ability
• Primary waste excretion occurs via placenta (not fetal kidneys)
• Kidneys gain functional significance only after birth

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6. Fetal Gastrointestinal Physiology

Development Timeline

Meconium

• Composed of intestinal secretions, mucus, bile, swallowed amniotic fluid, desquamated cells
• Present from ~16 weeks onwards
• Normally expelled within 24–48 hours after birth
• Meconium-stained amniotic fluid indicates fetal distress → risk of meconium aspiration syndrome

GI Maturation Implications

• Many digestive enzymes not at normal levels until 34 weeks
• Premature feeding must be cautious; delayed feeding causes villous atrophy
• Incomplete gut barrier function + lack of IgA → risk of necrotizing enterocolitis (NEC) in premature neonates

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7. Fetal Neurological Physiology

CNS Development

• Neural tube closes by week 4; failure causes neural tube defects (anencephaly, spina bifida)
• Neuronal proliferation: 10–18 weeks
• Neuronal migration: 12–24 weeks
• Synaptogenesis and myelination: begins ~24 weeks, continues until years after birth

Germinal Matrix

• High concentration of thin-walled blood vessels present between 24–34 weeks
• Poorly tolerant to blood pressure changes → prone to rupture → intraventricular hemorrhage (IVH) in premature neonates
• Premature infants cannot autoregulate cerebral blood flow → any hemodynamic instability raises IVH risk

Fetal Hypoxia and the Brain

• Fetal tissues have remarkable resistance to hypoxia (poorly understood)
• Chronic O₂ deficiency → forced shift to anaerobic metabolism → ATP depletion, ADP/inorganic phosphate accumulation → neuronal/glial injury → endothelial damage → cerebral/intraventricular hemorrhage

Retina Development

• Retinal angiogenesis: 16 weeks → complete by 40 weeks
• Dependent on hypoxic environment stimulating VEGF
• Premature birth + O₂ exposure inhibits VEGF → arrested retinal vasculogenesis → Retinopathy of Prematurity (ROP)
• Risk highest at <28 weeks; incidence ~84% in this group

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8. Fetal Endocrine Physiology

Thyroid

• Fetal thyroid begins concentrating iodine by 10–12 weeks; TSH secretion by 10 weeks
• Fetal T₄ levels rise progressively from mid-gestation
• Maternal T₄ crosses the placenta to provide some hormonal support in early pregnancy
• Neonatal thyroid function is tested at birth (neonatal screening for congenital hypothyroidism)

Adrenal Cortex (Fetal Zone)

• The fetal adrenal gland has a unique "fetal zone" (large inner zone) absent in adults
• Produces DHEA-S → converted by placenta to estrogens (especially estriol)
• Estriol levels are used as a marker of fetoplacental unit function
• At birth: fetal zone involutes → replaced by adult cortex

Insulin and Glucose

• Fetal pancreatic β-cells functional from ~10–12 weeks
• Glucose crosses the placenta freely (facilitated diffusion) → fetal glucose = ~70–80% of maternal
• Fetal glucose stores (glycogen) at birth can last only ~24 hours
• Glucose turnover in neonates: 4–9 mg/kg/min (vs. 2–3 in adults)
• 90% of glucose used by the brain → hypoglycemia (glucose <45 mg/dL) causes neurological injury

Fetal Adrenal Medulla (Stress Response)

• Fetal adrenal medulla produces epinephrine and norepinephrine
• Catecholamine surge at birth: initiates lung fluid clearance, heart rate increase, and mobilization of glycogen

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9. Fetal Immune System

• Passive immunity: IgG crosses the placenta from ~22 weeks onwards (active transport via FcRn receptors); maximal transfer in third trimester
• At birth, neonatal IgG levels ≈ maternal levels
• IgM does NOT cross placenta (large molecular weight) → presence of IgM in neonatal blood indicates intrauterine infection (e.g., TORCH)
• IgA absent in gut lumen until well after birth → provided by breast milk (secretory IgA)
• Cellular immunity: T lymphocytes identifiable in Peyer patches by week 18; functional but immature

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10. Fetal Energy and Metabolic Physiology

• Carbohydrates predominate in perinatal period; fats take over as primary fuel after early neonatal phase
• Infants unable to feed must receive dextrose-containing IV fluids immediately

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11. Physiological Transition at Birth

Respiratory Initiation

• Initiated within 30 seconds of birth, sustained within 90 seconds
• Triggers: mild hypoxia/acidosis, cord clamping, cold, pain, touch, noise, light
• Lung expansion → ↑ alveolar and arterial O₂ tension → potent stimulus for pulmonary arterial vasodilation
• Pulmonary vascular resistance falls to <20% of fetal values with first few breaths

Cardiovascular Transition (Parallel → Serial)

Role of Chemical Mediators at Birth

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12. Fetal Asphyxia — Physiological Consequences

• Maternal hypotension
• Abruptio placentae
• Cord prolapse (compression in birth canal)
• Severe maternal anemia
• Twin-to-twin transfusion syndrome

1. Depressed myocardial function → ↓ cardiac output → worse O₂ delivery
2. Brain shifts to anaerobic metabolism: ATP ↓, ADP and phosphate ↑
3. Neuronal/glial integrity disrupted
4. Endothelial damage → cerebral hemorrhage, IVH
5. Chronic deficiency → increased risk of metabolic derangements, cerebral palsy

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13. Summary: Key Numbers to Remember

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14. Quick Clinical Associations

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