The placenta is
- Organ that provides interface between mother and foetus
- Disk shaped
- ~500g
Uteroplacental blood flow is via
- Paired umbilical arteries \(\to\) Placental villi \(\to\) umbilical vein
- Uterine arteries \(\to\) spiral arteries \(\to\) Maternal sinuses \(\to\) uterine vein (no resistance arterioles)
- Uterine blood flow \(\approx\) 10% maternal CO at term \(\approx\) 2x foetal blood flow
and is determined by
- Remodelling of uterine vessels during pregancy \(\to\) reduced resistance
- No maternal arterioles \(\to\) no autoregulation; Q\(\propto \Delta\)P, and decreased perfusion \(\to\) increased oxygen extraction ratio.
- Extrinsic vasodilators: (\(\beta_2\) agonists, oestrogen, adenoside, bradykinin, NO
- Extrinsic vasoconstrictors: \(\alpha_1\) agonists, nicotine, vasopressin, thromboxane A2
Uteroplacental membrane consists of
- Syncitiotrophobalst, cytotrophoblast (atrophies in late pregnancy \(\to\) thinner membrane), villous stroma, foetal endothelium
- 15\(\text{m}^2\) SA
Membrane facilitates maternal\(\to\)foetal transfer of
- O2
- Glucose, amino acids, FFAs, nucleotides
- Substrates for growth (iron, vitamins etc)
- IgG via pinocytosis \(\to\) passive immunity transfer
and foetal\(\to\)maternal transfer of
- CO2
- Urea, uric acid, creatinine
Equilibration of body fluids across membrane \(\to\) foetal homeostasis
- ECF ions (Na, K, Ca)
- Osmolality
Foetal\(\to\)maternal O2 flux is accomplished by
- A 20mmHg pressure gradient (materal PO2 50mmHg, foetal PO2 30mmHg)
- High [Hb] in foetus \(\to\) large CaO2 despite low PaO2
- Foetal OHDC left shift by FHb
- Maternal OHDC right shift by \(\uparrow\)2,3DPG
- Double Bohr effect (at the membrane, CO2 moves foetal\(\to\)maternal circulation, locally left-shifting foetal and right-shifting maternal OHDC)
Placental has endocrine effects
- Secretes oestrogen \(\to\) uterine growth, mammary gland hypertrophy
- Early in pregnancy, releases HCG \(\to\) progresterone release from corpus luteum
- After six weeks, directly secretes progresterone
- Progesterone \(\to\) muscle relaxation, prevents labor onset