Topic 5

Critical Period For Circulatory System Development

Day 20-50 Post Fertilisation

In what week does the Circulatory System begin development

Week 3

In what week is there a functional heartbeat

Week 4

Describe the maternal blood flow to and from the placenta

Blood flows from uterine arteries → maternal sinuses → uterine vein

How does gas exchange occur between mother and fetus

Across capillary walls in chorionic villi - foetal & maternal blood do not mix

Foetal vein & artery direction of blood flow

Foetal vein: carries oxygenated blood toward the fetus from the placenta (via the umbilical vein).

Foetal artery: carries deoxygenated blood away from the fetus to the placenta (via the umbilical arteries).

What is the function of the ductus venosus

It links the umbilical vein to the inferior vena cava, bypassing the liver - Allows Blood to Bypass Foetal Liver

How is blood flow through ductus venosus regulated

Flow Regulated by Sphincter

50-80% of Blood flow can Avoid Hepatic Sinuses

Why is the ductus venosus bypass important for the heart

It prevents cardiac overload during high venous return (e.g. uterine contractions).

Foramen Ovale function

Links Right Atrium with Left Atrium

Avoids Oxygen Rich Blood Going to Pulmonary Circulation

More Direct Route To Ascending Aorta → Up to the Brain

What does the ductus arteriosus connect

The pulmonary artery to the descending aorta

Ductus arteriosus purpose

To bypass the non-functioning fetal lungs. Only ~10% of fetal blood passes through the lungs (for their growth & development)

Describe pathway of foetal circulation around body

Up/Down arrow for each of these at birth

Causes foramen ovale to close

Most Common Atrial Septal Defect

Patent Foramen Ovale

What is the clinical significance of a patent foramen ovale

None, as pressure in the left heart exceeds the right, keeping it closed

Clinical significance of patent foramen ovale occurring with other defects

Cyanosis of Skin and Mucus Membrane (due to right-to-left shunting)

What triggers ductus arteriosus closure

Increased oxygen tension (PO₂) and decreased prostaglandin E₂ levels (constriction)

PO2 in Foetal Ductus arteriosus vs PO2 in neonatal ductus arteriosus in mmHg

PO2 in Foetal Ductus arteriosus = 15-20mmHg

PO2 in neonatal ductus arteriosus = >100 mmHg

What PO₂ level is critical for closure of ductus arteriosus

50 mmHg

What happens if ductus arteriosus remains patent

Increased Re-circulation & Cardiac Output

Decreased Cardiac and Respiratory Reserves

Patent ductus arteriosus prevalence

1in 5500

How is a Patent ductus arteriosus treated

When does the ductus venosus close

Within 1–3 hours of birth

What causes the closure of ductus venosus

Increased portal pressure (6–10 mmHg) forcing blood through the liver

What happens to fetal respiratory movements during pregnancy

They decrease in the third trimester

Why do fetal respiratory movements decrease in the third trimester

To reduce swallowing of waste products and promote fetal growth

To allow the Foetus to Survive at Low PO2, the Oxygen Dissociation Curve Shifts to what side

LHS

How is Hb-F helpful to allow the Foetus to Survive at Low PO2

At Any PO2 Hb-F binds 20-30% More O2 than Hb-A

How does the mother help to allow the Foetus to Survive at Low PO2

Increased oxygen given up by Mother

How does uterine blood flow change during pregnancy

It increases 20-fold to ~500 mL/min

What hormones regulate uterine blood flow change during pregnancy

Oestrogen (vasodilation), progesterone (venoconstriction), and placental hormones

What is the Bohr effect

The shift of the O₂ dissociation curve due to pH changes — alkalosis → LHS (more oxygen binds), acidosis → RHS (less oxygen binds)

Explain the double Bohr effect in the placenta

Foetal PCO2 = 46mmHg; Maternal PCO2 = 45mmHg

Foetal blood: CO₂ diffuses out → ↓ Foetal PCO₂ → ↑ pH → Hb binds more O₂.

Maternal blood: gains CO₂ → ↓ pH → Hb releases O₂.

This enhances O₂ transfer to the foetus

Foetal O2 Dissociation Curve Shifted to LHS

Maternal O2 Dissociation Curve Shifted to RHS

What triggers the first breath

Mild asphyxia (↑ PCO (hypercapnia)₂, ↓ O₂ (hypoxia)) stimulates central chemoreceptors

Explain

First Breath

The first inspiration is the most difficult — requires high transpulmonary pressure (~60–80 cmH₂O) to overcome:

• Surface tension in fluid-filled alveoli,

• Viscous resistance of the lung fluid,

• Chest wall recoil (very compliant chest).

A few mins → a few days

With each breath:

More alveoli are aerated,

Surfactant spreads over alveolar surfaces, reducing surface tension,

FRC is established.

The P–V curve moves upward and to the left, meaning:

• Higher compliance (steeper slope),

• Less pressure needed to achieve a given volume,

• The lung can now maintain some air at end-expiration (FRC).

What is pulmonary surfactant

A phospholipid-protein mixture that reduces surface tension in alveoli

How does surface tension arise

Surface tension arises from the difference between the attractive forces on molecules at an air-liquid interface

Results in a tension on the surface film that resists expansion

WHy is surfactant important

Prevents alveolar collapse during exhalation and contributes to innate lung defence against pathogens

Where and when (gestation weeks) is it produced

By type II pneumocytes, between 24 - 34 weeks gestation

Surfactant main components

• 70–80% dipalmitoylphosphatidylcholine (phospholipid)

• 10% proteins (SP-A, SP-B, SP-C, SP-D)

• 10% neutral lipids (cholesterol)

What increases surfactant production

Cortisol, estrogen, prolactin, thyroid hormones (T₃/T₄), hypoxia, prostaglandins

What decreases or alters surfactant composition

Ozone (↓ SP-A), NO₂ (alters lipids), TNF-α (↓ production)

Effect of Insulin on Surfactant

Most prevalent cause of respiratory distress & death in premature newborns

Lack of surfactant

Is surfactant amphipathic / hydrophilic / hydrophobic

amphipathic (hydrophilic & hydrophobic)

What can be given to mothers at risk of early delivery to reduce risk of respiratory distress caused by a lack of surfactant

Cortisol increases surfactant