Before we dig into some of the lighter biochemical details of oxytocin and its role in labor, we need to cover a basic premise of human organ and hormone function. Oxytocin is a hormone - a chemical that stimulates certain tissues and organs to perform various functions.
In order for a hormone to affect a tissue or organ, there must be a receptor for that hormone on the target tissue or organ. Like a lock and key - if the proper receptor is not present, the hormone will not be able to “unlock” the cells of the tissue or organ to carry out its intended task.
Most people know about synthetically produced oxytocin - pitocin, typically administered intravenously during labor, and some people know that oxytocin is naturally produced by the pituitary gland in the mother’s brain.
Interestingly, it has been known for some time within the scientific community that high levels of oxytocin in the bloodstream cause something called “receptor downregulation” in the uterus. What this means is that when there is a lot of oxytocin present in the blood, the receptors (the “locks”) “hide” themselves within the cells - making them unavailable to attach to oxytocin (the “keys”). If oxytocin is unable to attach to its receptors on the uterus, there is nothing stimulating uterine contraction.
What is less commonly known is the fact that oxytocin is actually produced by various tissues in the uterus such as the decidua, amnion, chorion, and placenta. The oxytocin produced by uterine tissues has been observed to have such a powerful effect on uterine contractions compared to oxytocin in the bloodstream that some question exists regarding the necessity of having any circulating oxytocin at all for labor to progress normally, especially given the receptor downregulation that tends to occur in response to high levels of circulating oxytocin.
The uterus is an amazing organ. It actually creates more oxytocin receptors in preparation for labor. So, at 35 weeks of pregnancy, you have significantly fewer oxytocin receptors on your uterus than you will have at 39 weeks. To accomplish this requires careful coordination of a cascade of chemicals and hormones beginning with oxytocin produced by the decidua, which causes the production of a chemical that slows the release of progesterone. This drop in progesterone is associated with an increase in the number of oxytocin receptors - all to prepare your uterus for the big event!
Now you know a little more about this fascinating hormone that helps you bring your baby into the world!
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