Physiology of Labour Flashcards
What is the uterus made from?
Uterine smooth muscle is myometrium.
How does the uterus contract?
Contractions are stimulated by calcium ions entering myometrial cells from the extracellular space.
Influx is increased by oxytocin, and the density of cell membrane receptors for oxytocin rises throughout pregnancy.
In the presence of calcium, actin and myosin form cross links to contract.
What are gap junctions?
There’s a donut shaped structure that spans adjacent cell membranes and allow ions such as Na+.
The more gap junctions, the greater free passage of ions and coordination of contractions.
What binds myometrial cells together?
Dense bands.
The purpose is to increase coordination of contractions. The number of dense bands increases throughout pregnancy.
What counteracts the tendency of the myometrium to contract?
the cAMP pathway - by promoting quiescence
How does cAMP promote quiescence of the myometrium?
- inhibits actin-myosin cross linkage through an enzyme called myosin light chain kinase, found in cytoplasm
- influences transcription of genes in the nucleus - results in reduction in formation of gap junctions and manufacture of prostaglandins
What is the uterus composed of?
Outer peritoneal covering.
Inner endometrial lining comtaining stromal cells and glands.
Bulk is made of myometrial cells bound together in bundles, about 100microns diameter.
What are Braxton Hicks contractions?
Women are aware of uterine activity but cervix hasn’t started to dilate. They may stimulate blood flow to the uterus promoting O2-ated supply to the maternal side of the placenta and facilitating removal of fetal O2.
What pressures can be generated in the uterine cavity with labour contractions?
100 mmHg or more
What effects do progesterone and oestrogen have on labour?
Oestrogen is pro-contractile while progesterone is the opposite.
At the initiation of labour, the myometrium may become resistant to the effects of progesterone, allowing oestrogenic effects to dominate.
What is the first stage of labour?
Presence of uterine contractions leading to progressive cervical dilatation. Ends when the cervix is fully dilated to 10cm - the approx diameter of the bony pelvis.
How to contractions affect the cervix?
- contractions reduce the length of the cervix (effacement)
- then the aperture of the cervix increases (cervical dilatation)
What is a partogram?
Record of labour.
Maternal BP, pulse, fetal HR (every 15 mins), number of contractions and cervical dilatation are recorded.
Usually 0.5cm per hour increase in cervical dilatation.
What is the second stage of labour?
The progressive descent of the foetus through the pelvis to achieve delivery once the cervix is fully dilated
What happens at the end of the second stage of labour?
The anterior shoulder descends below the symphysis to deliver, then the posterior shoulder follows.
