Changes in Circulation at Birth

Overview of Fetal Circulation Before Birth

Before birth, the fetus lives in a liquid environment and does not use its lungs for breathing. Oxygen and nutrients are supplied directly by the mother through the placenta and the umbilical cord. Because the fetal lungs are filled with fluid and collapsed, they require very little blood.

To bypass these non-functioning lungs and the liver, the fetal circulatory system features three unique vascular structures:

• Ductus venosus: A blood vessel that shunts about half of the oxygenated blood from the umbilical vein straight past the liver and into the inferior vena cava.
• Foramen ovale: A flap-like opening between the right and left atria that allows oxygen-rich blood coming from the placenta to cross directly into the left side of the heart, pumping it preferentially to the brain and upper body.
• Ductus arteriosus: A muscular blood vessel connecting the pulmonary artery to the descending aorta, diverting most of the blood away from the fluid-filled lungs and toward the systemic circulation.

The First Breath and Pulmonary Vascular Resistance

The dramatic transition from intrauterine to extrauterine life begins with the baby's first breath. As the newborn takes that initial breath, the lungs expand, and the fluid inside them is rapidly absorbed into the surrounding lung tissues and lymphatic system.

This expansion introduces oxygen to the pulmonary alveoli. Oxygen is a powerful natural vasodilator. As oxygen levels rise in the alveoli, the tiny blood vessels in the lungs—the pulmonary arterioles—relax and open up. This causes a massive drop in pulmonary vascular resistance. Blood can now easily rush into the capillary beds of the lungs to pick up oxygen, setting the stage for the permanent closure of fetal shunts.

Closure of Fetal Shunts

Once breathing starts and the placenta is disconnected, pressure changes inside the newborn's heart and blood vessels force the temporary fetal shunts to close.

1. Foramen ovale: Closes functionally within minutes of birth. The drop in pulmonary resistance increases blood flow back to the left atrium from the lungs, raising pressure on the left side of the heart and slamming the flap shut against the right atrium.
2. Ductus arteriosus: Constricts and closes due to a sudden rise in blood oxygen levels and the removal of placental prostaglandins. Functional closure usually occurs within the first 24 to 48 hours of life.
3. Ductus venosus: Closes soon after the umbilical cord is clamped and blood flow from the placenta stops, eventually turning into a fibrous ligament over the first few weeks.

Role of Cord Clamping in Hemodynamic Stability

The timing of umbilical cord clamping directly influences the baby's blood volume and overall circulatory stability during the transition period.

Delayed cord clamping—waiting for at least one to three minutes after birth—allows gravity and uterine contractions to push extra blood from the placenta into the newborn. This placental transfusion provides the baby with a rich supply of iron stores and helps establish a stable blood volume and blood pressure.

Conversely, immediate cord clamping cuts off this reserve abruptly. While often necessary if the newborn requires immediate resuscitation, immediate clamping can sometimes lead to lower blood pressure and reduced circulating red blood cells if the baby's transition is sluggish.

Long-Term Structural Remodeling of Vessels

Functional closure of the fetal shunts is quickly followed by permanent anatomical changes. Over weeks and months, the temporary tissue of these vessels undergoes physical degeneration and turns into fibrous scar tissue.

• The ductus arteriosus transforms into the *ligamentum arteriosum*.
• The ductus venosus becomes the *ligamentum venosum*.
• The umbilical vein becomes the *ligamentum teres* (round ligament of the liver).
• The distal parts of the umbilical arteries become the medial umbilical ligaments.

These anatomical remnants remain inside the adult body as silent testaments to the complex circulatory journey every human makes at birth.