4. Placental function Flashcards
How is glucose moved across the placenta?
Facilitated diffusion
What is moved across the placenta by active transport?
Primary = iron, iodine, calcium and phosphate
2nd = amino acids
What is moved across the placenta by bulk transport?
Sodium and water
What is the immune/infection role of the placenta?
Creates a physical barrier to infection
Prevents foetal rejection = reduced IgG (only Ig that transfers across the placenta), reduced killer T cells and reduced cell mediated immunity.
Where does gas transfer occur in the placenta and what type of transport is it?
In the intervillous (mum) spaces by diffusion
How much of the O2 delivered to the placenta is utilised?
30%
What promotes transfer of oxygen from mum to baby?
High maternal foetal gradient
Good placental perfusion
Double Bohr effect: baby offloads CO2 to mum, pH rises, promoting O2 uptake (L shift)
Mum uptakes CO2, pH decreases, promotes O2 offloading (right shift)
Hbf = lower levels of 2-3DPG = higher O2 affinity (left) becomes a disadvantage after birth.
How does CO2 transfer across the placenta and in what form?
Simple diffusion
8% dissolved (20x more soluble than O2), 62% HCO3 and 30% carbamino Hb
What effect promotes CO2 transfer from baby to mum?
Double haldane effect
O2 uptake by baby promotes CO2 release
Decreased O2 in mum promotes CO2 uptake
What are the endocrine changes brought about by the placenta?
- hCG production = peaks at 8-10 weeks making the CL produce progesterone to maintain foetal viability
- Progesterone production starting at 8w to take over from CL. Continues to rise during pregnancy and then drops sharply at labour.
- Human placental lactogen = increases throughout pregnancy. Causes: increased lipolysis, increased gluconeogenesis and insulin resistance.
- Oestrogens (4 different types) = causes uterine expansion
- TSH
- Prostaglandins
- Hypothalamic inhibitory and excitatory factors
Describe what types of molecules freely cross the placenta and why?
Placenta is electrically charged and has a tight blood barrier.
Small (<600 daltons, > 1000 impermeable), unionised, lipophillic molecules diffuse freely.
What factors effect movement of molecules across the placenta e.g. drugs?
Increase and decrease
Increase =
lipid soluble
decreased molecular weight: under 600 daltons are free, over 1000 = impermeable. Rate of transfer is inversely proportional to weight.
increased placental surface area: decreased by: HTN, abruption, infarction, infection and DM.
pKa around 7.4.
Decreased:
increased diffusion distance (things that decrease surface area)
highly ionised
protein bound: either can’t leave the circulation or placenta directly binds to some drugs e.g. lithium and heavy metals, like cadmium in cigarette smoke.
low maternal foetal gradient: no auto regulation, so highly linked to BP. Decreased flow during contractions, so drugs given at beginning will have less effect.
Metabolism: placental metabolites e.g. O2, glucose, LDL’s can bio transform some drugs.
Describe the movement of opiates across the placenta?
Pethidine = freely crosses - baby peak 2-3h post IM
Morphine = weakly protein bound so moves freely
Fentanyl = protein bound but highly lipid soluble so free movement
Alf/remi = free movement.
Does heparin cross the placenta and why?
Highly ionised so no transfer.
Do LA’s cross the placenta and why?
Low molecular weight and highly lipid soluble, so diffusion.
Bupivicaine more protein bound, so less movement.
