SN6
SHORT NOTE
5M
★ High-yield
Describe the fetal circulation and the changes that occur at birth.
Peculiarities of Fetal Circulation
- The placenta, not the lung, is the organ of gas exchange
- The lungs are collapsed, with very high pulmonary vascular resistance
- Three shunts divert blood away from the lungs and liver
- The two ventricles work in parallel, not in series
- No blood in the fetus is fully oxygenated — the highest saturation is about 80% in the umbilical vein
The Three Shunts
| Shunt | Connects | Purpose | Becomes after birth |
|---|---|---|---|
| Ductus venosus | Umbilical vein → inferior vena cava | Bypasses the liver | Ligamentum venosum |
| Foramen ovale | Right atrium → left atrium | Bypasses the lungs | Fossa ovalis |
| Ductus arteriosus | Pulmonary artery → aorta | Bypasses the lungs | Ligamentum arteriosum |
- Umbilical arteries → medial umbilical ligaments; umbilical vein → ligamentum teres
Course of Fetal Blood
Placenta → Umbilical vein (80% saturated) → Ductus venosus → IVC → Right atrium → Foramen ovale → left atrium → LV → aorta → brain and heart → SVC blood → RV → pulmonary artery → ductus arteriosus → descending aorta → Umbilical arteries → placenta
- The crista dividens streams the well-oxygenated IVC blood across the foramen ovale → so the brain and heart receive the best-oxygenated blood
- Only 10–15% of the cardiac output reaches the lungs
Changes at Birth
| Event | Consequence |
|---|---|
| Lungs expand with the first breath | ↓ Pulmonary vascular resistance → ↑ pulmonary blood flow |
| Cord clamped | Loss of the low-resistance placenta → ↑ systemic resistance |
| ↑ Left atrial pressure > right atrial | Foramen ovale closes functionally (anatomically by 1 year) |
| ↑ Arterial PO2; ↓ prostaglandin E2 | Ductus arteriosus constricts — functionally in 10–15 h, anatomically in 1–3 months |
| Cessation of umbilical flow | Ductus venosus closes in 1–3 hours |
Applied Aspects
- Patent ductus arteriosus — commoner in prematurity and after maternal rubella; continuous "machinery" murmur. Closed with indomethacin (prostaglandin inhibitor); kept open with prostaglandin E1 when a duct-dependent circulation exists
- Patent foramen ovale — present in about 25% of adults; a route for paradoxical embolism
- Fetal haemoglobin with its left-shifted curve is essential for placental oxygen uptake
All questions in Cardiovascular System
- Q1Describe the cardiac cycle. Explain the pressure and volume changes in the left ventricle during one cycle. Add a note on the jugular venous pulse. ★
- Q2Define cardiac output. Describe its determinants and the factors regulating it. Add a note on the methods of its measurement. ★
- Q3Describe the short-term and long-term mechanisms regulating arterial blood pressure. Add a note on the renin–angiotensin system. ★
- Q4Describe the conducting system of the heart. Explain the genesis of the pacemaker potential and the properties of cardiac muscle. ★
- Q5Describe the normal electrocardiogram. Explain the leads, waves and intervals with their normal values, and give the uses of the ECG. ★
- SN1Enumerate the heart sounds. Give their causes, characteristics and areas of auscultation. ★
- SN2State Starling’s law of the heart. Explain its basis and significance. ★
- SN3Describe the baroreceptor reflex and give its significance. ★
- SN4Describe the coronary circulation and the factors regulating coronary blood flow. ★
- SN5Describe the triple response of Lewis. Add a note on the axon reflex.
- SN6Describe the fetal circulation and the changes that occur at birth. ★
- SN7Define shock. Classify it and describe the compensatory mechanisms. ★