Placental Function & Blood Flow

Functions of the Placenta

• Organ of foetal + maternal origin; supports developing foetus
• Low pressure, low resistance AV shunt that provides metabolic nutrients necessary for growing foetus
• Functions
  • Transfer: gas exchange; nutrient + waste exchange; drugs; heat
  • Immunological barrier
  • Metabolic
  • Endocrine

1. Transfer

a. Gas Exchange

• Diffusion dependent on Fick’s Principle

• Maternal placental flow ~600ml/min at term (2x foetal flow) → ↑diffusion by ↑concentration gradient for solutes
• Molecules <600Da more readily diffuse down concentration gradient
• O2 diffusion
  • PO2 entering placenta via uterine artery = 18mmHg (SpO2 45%)
  • PO2 leaving placenta via uterine vein 28mmHg (SpO2 70%)
  • Foetus able to absorb large enough vol O2 despite low PO2 because:
    • Foetal Hb
      • 2 y subunits cf β → prevent binding of 2,3-DPG → L shifted OHDC → favours O2 loading at ↓PaO2
      • [FHb] 50% > maternal [Hb]
    • Double Bohr effect
      • Describes Bohr effects happening on either side of placental gas exchange in mother + foetal circulations
      • Accounts for 2-8% of O2 transfer
      • Bohr effect 1: placenta: foetus unloads CO2 in placenta → ↑placental CO2 → RIGHT shift HbA OHDC → ↓HbA affinity for O2 → ↑placental O2 unloading
      • Bohr effect 2: foetus: foetus unloads CO2 → ↓foetal CO2 → LEFT shift HbF OHDC → ↑HbF affinity for O2 → ↑foetal O2 binding/ uptake
• CO2 diffusion
  • Foetal PaCO2 50mmHg; intervillous PCO2 37mmHg
  • CO2 offloading favoured in foetus by:
    • High foetal [Hb] ↑s amount of CO2 that can be carried as carbaminoHb
    • Double Haldane effect
      • Haldane effect: describes how ∆O2 sat of Hb affect CO2 transport: deoxyHb has ↑affinity for CO2 than oxyHb
      • Double Haldane: describes Haldane effects happening across the placenta
      • Accounts for 45% of CO2 transfer between maternal and foetal circulation
      • Haldane 1: placenta: placenta unloads O2 → ↓placental O2 → deoxyHbA has ↑affinity of CO2 → ↑placental CO2 uptake from foetus
      • Haldane 2: foetus: foetus takes up O2 → ↑HbF O2 → ↓HbF affinity for CO2 → ↑HbF CO2 release to placenta

b. Nutrient delivery

• Nutrient diffusion
  • High foetal caloric requirements in late pregnancy
  • Facilitated diffusion of glucose via carrier molecules in trophoblasts
  • Active transport for amino acids, Ca2+, Fe, folate, vit A and C

c. Waste removal – Urea, Uric acid, Creatinine, Br

d. Heat transfer

2. Immunological function

• permeable to IgG via pinocytosis → allows maternal abs to provide passive immunity to foetus
• Trophoblast cells lose many cell surface MHC molecules → making them less immunogenic; also cells cover themselves in mucoprotein which
  disguises them from maternal immunie system
• Chorionic cells act as immunological barrier – preventing maternal T cells and abs from reaching foetal circulation
  • Some bacteria (listeria) and viruses (rubella, parvovirus B19, HIV) can cross into foetal circulation
• Progesterone + alpha-foetoprotein produced by yolk sac act as maternal immunosuppressive agents

3. Metabolic

• synthesis of glycogen, cholesterol, FA, enzymes

4. Endocrine function

• synthesis of 4 main hormones:
  • bHCG
  • hPL: human placental lactogen (human chorionic somatomammotrophin)
  • Oestriol
  • Progesterone
• Synthesis of other hormones and growth factors
  • Placental corticotrophin
  • Human chorionic somatostatin
  • Human chorionic thyrotropin
  • Epidermal growth factor
  • Somatomedin

Placenta

• Placenta:
  • temporary fetal organ that begins developing from the blastocyst shortly after implantation
  • ~22cm length, 2.5cm thickness. ~500gm
  • Connects to fetus by umbillical cord
• “Chorionic villus” is the basic structural unit of the placenta → it is a vascular projection of foetal tissue that is bathed by maternal blood within the “Intervillous space”. It consists of:
  • Foetal connective tissue containing foetal capillaries
  • Chorion → outermost layer of foetal tissue that is made of 2 layers →
    • (a) Syncytiotrophoblast (directly contacts maternal blood in intervillous space) and
    • (b) Cytotrophoblast (b/t syncytiotrophoblasts and foetal CT)

Placental Circulation

• Placental circulation = Two circulation systems in parallel – maternal and the fetal
• Maternal circulation system
  • Uterine Arteries (600ml/min, 100mm Hg) → arcuate arteries → radial arteries → spiral arteries (70 mm Hg) & basal arteries (myometrium and deciduas)
  • Spiral arteries → intervillous spaces (10mm Hg) → uterine veins that are arranged in the periphery of the intervillous space.
  • Blood in the intervillous space is exchanged 2-3 times per minute.
• Fetal circulation system:
  • Two villus Umbilical Arteries (50mmHg) → finer vessels that cross through the chorionic plate → villus capillaries (30mmHg) → leaves placenta through umbillical vein (20mmHg)
  • Their supply amounts to approximately 40% of the fetal heart blood volume per minute.
  • The pressure in the fetal vessels and their villus branches always lies over that of the intervillous space. This protects the fetal vessels from collapse.
• Substances traverse between foetal and maternal blood via the following layers:
  • Maternal blood (intervillous space) ↔ Chorion (2x layers of trophoblasts) ↔ Foetal connective tissue ↔ Endothelium of foetal capillaries ↔ Foetal blood

Determinants of Placental Blood Flow

Uteroplacental blood flow

• At term → uteroplacental BF is 500-700 mL/min (10% maternal C.O.) of which:
  • 70-90% of this BF enters the intervillous space (via the spiral arteries) → NOT autoregulated as blood flow is “pressure-dependent” (see factors below)
  • 10-30% of this BF supplies the myometrium/deciduas (via the basal arteries) → autoregulated blood flow
• Blood flow to the intervillous space (which participates in substance exchange with foetal blood) is affected by the following factors:

• Uterine arterial pressure (UAP):
  • Maternal arterial BP → ↓ MABP (Ie. due to SNS block 2° neuraxial block,
    hypovolaemia, supine hypotension syndrome, Etc.) causes ↓ UAP → ↓ UBF
• Uterine venous pressure (UVP):
  • Uterine tone and contractions → ↑ tone/contractions (Ie. due to contractions,
    oxytoxics, ketamine, Etc.) causes ↑ UVP → ↓UBF
• Uterine vascular resistance (UVR):
  • Uterine arteriolar tone → ↑ vasoconstriction (Ie. a/w essential HT and PET, α-adrenoceptor stimulation (by endogenous SNS innervation, catecholamines or sympathomimetics), and vasopressin) causes ↑ UVR → ↓ UBF

Umbilical blood flow

• At term → umbilical BF is 360 mL/min (25-50% of foetal C.O. (≈ 1000 mL/min))
• It is “autoregulated” (cf. uterine BF) → involves vasodilators (PCI-2/NO) derived from vascular endothelium
• BF is ↓ with severe hypoxia, ↑ BGL, catecholamine and cord compression

Kerr / Bianca 2020

Answer by Bianca, Kerr