Mechanisms of the feto-maternal exchanges

The placental exchange processes occur via classic membranous transport mechanisms:

• Passive transport (without energy consumption)
  
  Simple diffusion: non-polar molecules and fat-dissolvable substances follow concentration gradients. They diffuse from the side with the higher concentration to the side with the lower concentration, until a balanced condition is achieved, whereby no energy is used up in this process (e.g., diffusion of oxygen, carbon dioxide, fats and alcohol). Water enters the placenta through specialized pores (see osmosis).
  
  Osmosis: The diffusion of soluble substances through a membrane with selective permeability (e.g. cellular membrane). Water, a strongly polarized molecule, cannot penetrate through this double lipid layer of the cellular membrane. But it crosses the placenta through specialized pores, the aquaporines or water channels, proteins localized within the plasma membrane.
  
  Simplified transport: transition from the side with higher concentration to the one with lower concentration with the help of transport molecules (e.g., glucose).
  
  
• Active transport: Transport through the cellular membrane against a concentration gradient using energy (Na+/K+ or Ca++).
  
  
• Vesicular transport (Endocytosis / Exocytosis): Macro-molecules are captured by microvilli and absorbed in the cells or repelled (immunoglobulin).

The placental exchange surface is enlarged from 5m² at 28 weeks to roughly 12 m² shortly before delivery!.