Human Reproduction » Placenta Function

What you'll learn this session

Study time: 30 minutes

The structure and function of the placenta
How substances are exchanged across the placenta
The role of the placenta in supporting fetal development
How the placenta acts as a barrier against harmful substances
Placental hormones and their importance in pregnancy

Introduction to Placental Function

The placenta is an amazing temporary organ that forms during pregnancy. It connects the developing fetus to the mother's uterine wall, allowing for nutrient uptake, waste elimination and gas exchange through the mother's blood supply. Without the placenta, a pregnancy simply couldn't continue - it's that important!

Key Definitions:

Placenta: A temporary organ that develops in the uterus during pregnancy, connecting the developing fetus to the maternal blood supply.
Umbilical cord: The cord connecting the fetus to the placenta, containing blood vessels that transport nutrients and waste.
Diffusion: The movement of substances from an area of higher concentration to an area of lower concentration.
Active transport: The movement of substances against a concentration gradient, requiring energy.

🌱 Placental Structure

The placenta is a disc-shaped organ that typically measures about 20 cm in diameter and 2.5 cm in thickness when fully developed. It has two surfaces:

Maternal side: Rough and divided into lobes, attached to the uterine wall
Fetal side: Smooth and covered by the amnion, where the umbilical cord attaches

The placenta contains many tiny finger-like projections called chorionic villi that extend into the mother's blood spaces. These villi massively increase the surface area for exchange of substances between mother and fetus.

🔗 The Placental Barrier

The placenta isn't just a simple connection - it forms a selective barrier between maternal and fetal blood. This barrier consists of:

Fetal capillary endothelium
Connective tissue
Trophoblast cells (outer layer of the chorionic villi)

Importantly, maternal and fetal blood never mix - they remain separated by this barrier at all times. This prevents potential immune reactions while still allowing for essential exchange.

Exchange of Substances Across the Placenta

The placenta's main job is to facilitate the exchange of vital substances between mother and fetus. This exchange happens through several transport mechanisms:

🟢 Diffusion

Most substances cross the placenta by simple diffusion, moving from areas of high concentration to low concentration. This includes:

Oxygen (from mother to fetus)
Carbon dioxide (from fetus to mother)
Water
Some ions and small molecules

🟥 Facilitated Diffusion

Some larger molecules need help crossing the membrane. Carrier proteins in the placental barrier assist with the transport of:

Glucose (main energy source for the fetus)
Amino acids
Some vitamins

🟤 Active Transport

Some substances move against their concentration gradient, requiring energy from ATP:

Some amino acids
Calcium ions
Iron
Certain vitamins

Protective Functions of the Placenta

The placenta isn't just an exchange surface - it's also a protective barrier that shields the developing fetus from many (but not all) harmful substances.

What Can and Cannot Cross the Placenta

The placenta is selective about what it allows to pass through:

✅ What Can Cross

Nutrients: Glucose, amino acids, fatty acids, vitamins, minerals
Gases: Oxygen, carbon dioxide
Antibodies: Particularly IgG antibodies that provide passive immunity to the fetus
Waste products: Urea and other metabolic wastes from the fetus
Hormones: Various hormones that regulate pregnancy

❌ What Cannot Cross (Usually)

Blood cells: Red and white blood cells typically cannot cross
Large proteins: Most large proteins cannot pass through
Bacteria: Most bacteria cannot cross (with some exceptions)
Many maternal hormones: The placenta metabolizes many maternal hormones

Case Study Focus: Harmful Substances That Can Cross the Placenta

Unfortunately, the placental barrier isn't perfect. Several harmful substances can cross from mother to fetus:

Alcohol: Can cause Fetal Alcohol Spectrum Disorders (FASD), affecting brain development and growth
Nicotine and carbon monoxide from cigarettes: Can reduce oxygen supply to the fetus and impair growth
Some medications: Certain drugs can cross the placenta and affect fetal development
Some viruses: HIV, rubella and cytomegalovirus can cross the placenta and infect the fetus
Drugs of abuse: Substances like cocaine and heroin can cross the placenta and cause addiction in the newborn

This is why pregnant women are advised to avoid alcohol, smoking and certain medications during pregnancy.

Hormonal Functions of the Placenta

The placenta isn't just an exchange organ - it's also an endocrine gland that produces several important hormones essential for maintaining pregnancy and preparing for birth.

💊 Key Placental Hormones

Human Chorionic Gonadotropin (hCG): Maintains the corpus luteum in early pregnancy, which produces progesterone until the placenta takes over. This is the hormone detected in pregnancy tests.
Progesterone: Maintains the uterine lining, prevents uterine contractions and helps prepare the breasts for milk production.
Oestrogen: Stimulates uterine growth, develops breast tissue and increases blood flow to the uterus.
Human Placental Lactogen (hPL): Modifies the mother's metabolism to ensure the fetus receives adequate nutrition, particularly glucose.

📈 Hormone Changes During Pregnancy

Placental hormone production changes throughout pregnancy:

First trimester: hCG levels rise rapidly, peaking around 8-10 weeks
Second trimester: Progesterone and oestrogen levels continue to rise steadily
Third trimester: Hormone levels reach their highest point, preparing the mother's body for labour and delivery

These hormonal changes are responsible for many of the physical symptoms experienced during pregnancy, from morning sickness to breast tenderness.

Adaptations for Efficient Exchange

The placenta has several structural adaptations that make it incredibly efficient at exchanging substances:

Large surface area: The chorionic villi provide an enormous surface area for exchange - approximately 11-14 square metres by full term!
Thin exchange barrier: The placental membrane is extremely thin (just 3.5 micrometres) in places, reducing the diffusion distance
Dense capillary network: Both maternal and fetal sides have extensive blood vessel networks to maximize blood flow
Concentration gradients: The placenta maintains concentration differences that facilitate diffusion in the appropriate direction
Counter-current flow: Maternal and fetal blood often flow in opposite directions, maximizing the concentration gradient throughout the exchange area

Interesting Fact: Placental Variations

The human placenta is classified as a "hemochorial" placenta, where maternal blood directly contacts the chorion (fetal tissue). Other mammals have different placental types:

Epitheliochorial placenta: Found in pigs and horses - has more tissue layers separating maternal and fetal blood
Endotheliochorial placenta: Found in cats and dogs - maternal epithelium is removed, but endothelium remains
Hemochorial placenta: Found in humans and other primates - most invasive type with direct contact between maternal blood and fetal chorion

The human placenta's structure makes it particularly efficient at nutrient and gas exchange, supporting our relatively large and metabolically demanding brains during development.

Summary: The Vital Role of the Placenta

The placenta is a remarkable organ that performs multiple crucial functions during pregnancy:

Provides oxygen and nutrients to the developing fetus
Removes waste products from the fetal blood
Produces hormones that maintain pregnancy
Provides some protection against harmful substances
Transfers antibodies from mother to fetus for passive immunity

Without this amazing temporary organ, human development as we know it would be impossible. After birth, the placenta is expelled during the "third stage" of labour, having completed its vital role in bringing a new life into the world.

🧠 Test Your Knowledge!