The baby/placenta has a separate blood system from the mother. The placenta does the job of the lungs by exchanging gas (oxygen and carbon dioxide) via the intervillous space between the baby’s and the mother’s blood system. Before birth, a third of the baby/placenta blood volume is in the placenta at any given time to facilitate this gas exchange.
After birth this ‘placental’ blood volume is transferred through the pulsing cord into the baby increasing the baby’s circulating blood volume. This has two major effects:
1. Provides the extra blood volume needed for the heart to direct 50% of it’s output to the lungs (8% before birth). This extra blood fills the capillaries in the lungs making them expand to provide support for the alveoli to open. It also aids lung fluid clearance from the alveoli. These changes allow the baby to breath effectively.
2. Increases the number of circulating red blood cells which carry oxygen. This increases the baby’s capacity to send oxygen around the body.
This transfer of blood volume from placenta to lungs takes place over a number of minutes following birth. Textbooks will tell you 3-7 minutes, but I have felt cords pulse for a lot longer than that. While these changes take place, oxygen continues to be provided by the placenta until the baby is ready to begin breathing.
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