Explain the physiological changes during pregnancy, and parturition
The foetal circulation has a number of structural differences:
- Two umbilical arteries
The umbilical artery returns deoxygenated blood to the placenta.
- PO2 of 18mmhg (SpO2 45%)
- One umbilical vein
The umbilical vein supplies oxygenated blood to the foetus.
- Has a PaO2 of 28mmHg (SpO2 70%)
- 60% of blood from the umbilical vein enters the IVC
- 40% of blood enters the liver
- Two ducts:
- Ductus venosus
Shunts blood from the umbilical vein to the IVC.
- Ductus arteriosus
Shunts blood from the pulmonary trunk to the descending aorta.
- Ductus venosus
- A foramen ovale
Shunts blood from the right atrium to the left atrium.
- Immature myocardium
Foetal myocardium does not obey Starlings Law, and does not adjust contractility for any given preload. Therefore:
These structural difference alter the pathway of blood circulation:
- Oxygenated blood returns via the umbilical vein
- 40% flows to the liver
- 60% is returned to the IVC
- Oxygenated blood in the IVC is directed via the Eustachian valve through the foramen ovale
- Blood returning from the SVC is directed into the RV, and then into the descending aorta by the ductus arteriosus
- ~10% of RV output flows through the pulmonary circulation
This arrangement has several features:
- Blood with the most oxygen is delivered to the arch vessels to supply the brain
- Blood with the least oxygen is delivered to the umbilical arteries for gas exchange
- Both the RV and the LV eject into systemic circulations, and are of similar size and wall thickness
Changes at Birth
Several changes happen at birth:
- Placental circulation is lost
There is a transition from circulation in parallel to circulation in series.
- An FRC is established
Reversal of hypoxic pulmonary vasoconstriction results in a rapid drop in PVR.
The cord is clamped
The systemic vascular bed volume falls, and SVR increases.
The fall in PVR lowers RV afterload
- The rise in SVR increases LV afterload
When LAP exceeds RAP, the foramen ovale closes
Increased left sided afterload causes flow reversal in the ductus arteriosus
There is progressive closure of the ductus over hours to days, under the influence of prostaglandins and oxygenated blood flowing through the duct.
The ductus venosus progressively fibroses over a period of days to weeks
- Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.
- Brandis K. The Physiology Viva: Questions & Answers. 2003.