Functional significance of the placenta
Optimal fetal development relies on three basic functions of the placenta:
- Metabolic function: establishing conditions for the transfer of nutrients, metabolites, and gases without direct contact between the circulatory systems.
- Barrier function: implemented via the placental barrier. This filter exhibits strict selectivity, allowing physiological substances to pass while blocking many hazardous compounds. It also provides immunological protection, preventing the maternal organism from rejecting the fetus.
- Synthetic function: the placenta is capable of independently producing structural proteins and biologically active components (hormones) that subsequently enter both bloodstreams.
Transport from Mother to Fetus
Maternal arterial blood reaches the site of exchange via the uterine arteries (a. uterina). Normally, components critically important for growth cross the barrier to the fetus:
- Trophic substances: amino acids for tissue synthesis, glucose as an energy source, as well as lipids, water, vitamins, and electrolytes.
- Oxygen: transferred exclusively via diffusion. This process is enabled by differences in the chemical properties of carrier proteins: fetal hemoglobin (HbF) has a higher oxygen affinity than maternal hemoglobin (HbA).
- Hormones: many regulatory molecules cross the barrier. Pituitary hormones are an important exception—they do not pass through the placenta.
The barrier is not entirely impassable. Damaging factors can enter the fetal bloodstream:
- Immunological: maternal antibodies that destroy fetal erythrocytes in rhesus incompatibility (hemolytic disease).
- Infectious: measles, rubella, smallpox, and hepatitis viruses can penetrate the defense and cause severe malformations (teratogenic effect).
- Toxic: xenobiotics, medications, alcohol, nicotine, and illicit drugs.
Transport from Fetus to Mother
Substances are cleared from the fetus via the umbilical arteries (a. umbilicalis), which carry venous blood to the placenta. After passing through the filter, waste products enter the maternal uterine veins (v. uterina).
Main eliminated substances:
- Carbon dioxide ($CO_2$).
- Nitrogenous wastes (specifically urea).
- Bilirubin.
In addition to catabolic waste products, certain fetal hormones pass into the maternal circulation. They provide feedback humoral regulation, influencing physiological processes in the maternal organism.
Cellular transport mechanisms
Substances move across the plasma membranes of barrier cells (including the syncytium) via various pathways. It is important to note transport polarity: mechanisms for the same substance may differ at the apical and basal membranes.
- Simple and facilitated diffusion: the main pathway for gases ($O_2$, $CO_2$), water, electrolytes, and lipids (including steroid hormones). Extracellular transport in the villous stroma also occurs predominantly via simple diffusion.
- Active transport: used to transfer glucose and amino acids against a concentration gradient.
- Exocytosis: required for the secretion of large molecules such as protein hormones and antibodies.
Vascular network features: to understand the exchange scheme, it is important to remember that umbilical vessels are named according to the direction of blood flow relative to the fetal heart. Deoxygenated blood with metabolites flows to the placenta via the a. umbilicalis, while oxygen- and nutrient-enriched arterial blood returns to the fetus via the v. umbilicalis.
Briefly: answers to topical questions
Which specific hormones are synthesized by the placenta itself?
The placenta acts as a temporary endocrine organ and independently synthesizes three groups of hormones.
- Pituitary hormone analogues — human chorionic gonadotropin (hCG), placental lactogen (hPL), ACTH, and chorionic somatomammotropin.
- Female sex hormones — estrogens and progesterone (taking over secretion due to ovarian inactivity).
- Relaxin — prepares the woman's body for labor (cervical dilation, ligament relaxation).
In the fetal part, the source of hormones is the chorionic epithelium (cytotrophoblast and syncytiotrophoblast), while in the maternal part, it is the decidual cells.
The "umbilical paradox": to avoid confusing fetal vessels, remember that everything in the umbilical cord works "in reverse" compared to the adult systemic circulation. The umbilical arteries carry venous blood away from the fetus, and the vein carries arterial blood toward the fetus.
Class and exam questions
Do pituitary hormones cross the placental barrier?
No, pituitary hormones are an important exception and do not cross the placenta.
How is oxygen transferred from mother to fetus?
Oxygen is transferred via diffusion due to the difference in hemoglobin affinities. Fetal hemoglobin (HbF) binds oxygen more readily, "stealing" it from maternal hemoglobin (HbA).
Which viruses can cross the placenta?
The placental barrier is permeable to rubella, measles, smallpox, and hepatitis viruses, which can lead to teratogenic effects and malformations.
What else to study on this topic
- Morphological structure of the placental barrier by layers
- Synthetic function: specific placental proteins and hormones
- Mechanisms of maternal immune tolerance toward the fetus
- Blood circulation features in the chorionic villi
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