physiology in labour

Question 1

what hormones are involved in the initiation of labour?

Answer 1

^ in placental corticotropin-releasing hormone (PCRh)
^ in placental oestriol (PO)
^ prostaglandins
decrease in effectiveness of antiuterotonics (progesterone, relaxin, nitric oxide)

pressure from fetus against uterus and cervix stimulates secretion of prostaglandins and oxytocin

also maturation of fetal hypothalamic-pituitary-adrenal axis.

Question 2

physiology of contractions in labour

Answer 2

Labour is maintained by the uterotonic hormone oxytocin stimulated by neuroendocrine reflex called the fergusons reflex.
pressure on cervix causes sensory stretch neuroreflex to the hypothalamus then to the anterior pituitary which releases the oxytocin into the blood stream.
oxytocin receptors are stimulated causing the uterus to contract, and retract (keep tone between contractions) placing increasing pressure of the fetal presenting part to the cervix and causing dilation and effacement until 10cm dilation is reached and the cervix becomes continuous with the uterus.
This is a positive feedback mechanism until the baby is born.

Question 3

what are the mechanisms of placental separation and control of blood loss?

Answer 3

1. compression - pressure from the adjacent uterine wall as the uterus contracts and retracts.
2. living ligatures - criss-cross arrangement of muscle fibres intertwined with the spiral arterioles to stop bleeding.
3. hypercoagulation - fibrin plugs deposited in the torn ends of the blood vessels further prevent blood loss

uterine contraction can be enhanced by the neuroendocrine reflex of baby suckling at the breast to release oxytocin.