Fetal Circulation

Introduction

• Welcome to this lecture about how a baby's blood system works before it's born (fetal circulation) and how it changes at birth. Charles Wood will be explaining these cool adaptations.

• This talk will last about 30 minutes.

• We'll learn how the baby's heart and blood vessels completely change to help it grow and develop inside the mother.

Learning Objectives

• We'll discover the main features of how blood flows in a fetus.

• We'll also see how this special circulatory system changes to meet the needs of a growing baby.

Fetal Growth

• Let's look at a graph showing how human babies grow:

  • The solid red line shows how the baby's weight increases. It grows really, really fast (exponentially) towards the end of the pregnancy, meaning it gets much heavier quicker as it gets closer to birth.

  • The dotted red line shows how the baby's length increases. This growth is more steady and even (linear) compared to how its weight shoots up.

• As the baby grows inside, important body parts get ready for life outside. These include:

  • Lungs (for breathing)

  • Gastrointestinal tract (stomach and intestines for digesting food)

  • Brain (for thinking and controlling the body)

• As the baby's body tissue gets bigger and more developed, it needs more blood. This extra blood delivers oxygen and nutrients (like food) and also carries away waste products.

Anatomy of Fetal Circulation

• Fetal circulation is special because:

  • Unlike in adults where the two sides of the heart pump one after the other (first to the lungs, then to the body), in a fetus, both sides of the heart pump blood at the same time but in different directions.

  • In adults, the heart's right side pumps blood to the lungs to pick up oxygen.

• How the fetus adapts:

  • Most of the blood that the right side of the heart pumps away from the heart usually goes to the lungs. But in a fetus, the lungs aren't used for breathing, so most of this blood goes through a special bypass or shortcut called the ductus arteriosus.

  • The ductus arteriosus connects the main artery to the lungs (pulmonary artery) directly to the body's main artery (aorta). This sends blood largely around the lungs and into the rest of the baby's body.

Oxygen Supply in Fetuses

• How fetuses get oxygen:

  • Adults get oxygen when they breathe air into their lungs, where tiny air sacs (alveoli) help bring oxygen into the blood.

  • In a fetus, the lungs are not working for breathing; they're actually filled with fluid. So, the baby needs another way to get oxygen.

• Umbilical placental circulation:

  • The placenta is a really important organ, acting like the baby's temporary lungs and digestive system. It passes blood to the baby, but the mother's blood and the baby's blood do not directly mix.

  • Instead, oxygen from the mother's blood simply moves across the placenta into the baby's blood.

• Ductus venosus:

  • This is another shortcut, a special vessel that helps carry oxygen-rich blood from the placenta directly to the baby's main vein (inferior vena cava) and then to its heart.

• Foramen ovale:

  • This is an oval-shaped opening or hole inside the baby's heart, between the two upper chambers (right and left atria). It allows blood to skip the lungs by flowing directly from the right side to the left side of the heart because of different pressures.

Blood Flow Dynamics

• Combined ventricular output in the fetus:

  • Because the blood flow in a fetus is so different (the two sides of the heart don't pump equal amounts of blood in a simple sequence), we talk about the "combined ventricular output." This means the total amount of blood pumped by both sides of the heart together.

• Blood flow percentages:

  • The right side of the baby's heart actually pumps about twice as much blood as the left side.

  • Only about $7\%$ of all the blood pumped by the baby's heart actually goes through its lungs.

  • Roughly $59\%$ of the blood pumped from the right side of the heart goes through the ductus arteriosus right into the body's main circulation.

• How much blood flows (in milliliters per minute per kilogram of body weight):

  • The right side of the baby's heart pumps around $300 \text{ mL/min/kg}$.

  • The left side of the baby's heart pumps around $150 \text{ mL/min/kg}$.

  • To compare, in adults, each side of the heart pumps about $100 \text{ mL/min/kg}$. So, babies need much more blood flow.

  • Fetuses need this much higher blood flow because their blood doesn't carry as much oxygen as an adult's blood. They need to pump more blood to deliver enough oxygen to their growing tissues.

Oxygen Saturation Levels

• For adults, the blood leaving the lungs is almost $100\%$ saturated with oxygen (meaning it's carrying almost as much oxygen as it possibly can).

• For a baby:

  • Blood in the right side of the heart is about $55\%$ saturated with oxygen.

  • Blood in the left side of the heart is slightly higher, about $65\%$ saturated.

• The left side has a bit more oxygen because the most oxygen-rich blood coming from the placenta tends to go straight to the left side of the heart.

Maturation of Fetal Circulation

• As babies grow and their body parts get more developed, the amount of blood going to certain organs changes:

  • The lungs and the digestive system need more and more blood flow as they get closer to being ready for life outside the womb.

• The amount of blood flowing to the placenta, shown as a percentage of the baby's total blood output, actually goes down as the baby's own organs mature and become more capable.

Transition to Postnatal Life

• When a baby is born, its circulation completely changes:

  • Losing the placenta means the baby's lungs immediately have to take over and start bringing in oxygen.

  • The baby's first breath is a huge effort! It involves creating a very strong sucking-in pressure, about $-60 \text{ cm H}_2\text{O}$, just to expand its lungs for the first time.

• How lung pressures change before and after birth:

  • Those first few breaths need a lot of pressure to inflate the lungs because they are stiff and collapsed.

  • Luckily, the baby's lungs already produce something called surfactant. This is like a natural soap that coats the inside of the lungs and makes it much easier for subsequent breaths by reducing the stickiness inside the air sacs.

• Changes in lung blood pressure:

  • Before birth, the pressure in the blood vessels going to the lungs is quite high, around $40-50 \text{ mmHg}$.

  • After birth, these pressures drop dramatically, allowing the lungs to receive normal blood flow and start working properly.

Clinical Significance

• Persistent pulmonary hypertension of the newborn:

  • This is a serious condition where, after birth, the blood vessels in the baby's lungs remain tight (high resistance). This makes it hard for enough blood to flow through the lungs to pick up oxygen.

• Ductus arteriosus after birth:

  • The ductus arteriosus, that shortcut we talked about, usually stays open (patent) for a few days after birth, meaning a small amount of blood can still bypass the lungs.

  • However, as the body's main artery (aorta) pressure increases compared to the lung artery pressure, the ductus arteriosus gradually closes up. This finishes the change to an adult-like circulatory system.

• If this normal closure doesn't happen, it can lead to problems with the baby getting enough oxygen and can even cause heart failure.

Key Concepts

• Fetal circulation involves incredible changes in the body's structure and how it works.

• The ductus arteriosus and foramen ovale are vital shortcuts in the baby's blood system before birth.

• Before birth, the placenta is essential for getting oxygen to the baby. After birth, the baby's body must quickly adapt to use its own lungs effectively.

• Understanding these changes is super important for doctors to manage any problems babies might have right after birth.