The umbilical cord is the temporary organ connecting a mother and the developing fetus, serving as the conduit for all necessary exchange. The structure typically houses three vessels: two umbilical arteries and one umbilical vein. This network sustains the fetus by managing the flow of blood, nutrients, and waste products across the placental barrier.
Anatomy and Direction of Flow
The umbilical cord contains two umbilical arteries and a single, larger umbilical vein, all bundled together within a protective layer of connective tissue known as Wharton’s jelly. These vessels exhibit a circulatory pattern that is the reverse of what is found in adult circulation regarding oxygenation status. The umbilical arteries carry deoxygenated and waste-laden blood away from the fetus to the placenta. In contrast, the umbilical vein carries oxygenated and nutrient-rich blood toward the fetus from the placenta.
Specific Substances Transported
The umbilical arteries transport blood that has already circulated throughout the fetal body. The primary function of this blood flow is to deliver the waste products of fetal metabolism back to the placenta. This waste transfer is essential because the fetal liver, kidneys, and lungs are not yet fully functional.
The main substances carried by the arteries are carbon dioxide (\(\text{CO}_2\)) and nitrogenous metabolic waste. \(\text{CO}_2\), a byproduct of cellular respiration, is transported from the fetal tissues to the placenta, where it diffuses into the maternal circulation. Similarly, nitrogenous wastes, such as urea and uric acid, are carried by the arteries for transfer and eventual elimination by the mother’s kidneys.
Fetal Blood Pathway and the Placenta
The two umbilical arteries originate in the fetal pelvis, branching off the internal iliac arteries. They course along the anterior abdominal wall before exiting the fetus at the umbilicus and entering the umbilical cord. This anatomical origin means the blood they carry is a mix of systemic blood from the lower half of the fetal body and some blood from the upper circulation.
Once inside the umbilical cord, the two arteries spiral around the umbilical vein before reaching the placenta. Upon entering the placenta, the arteries divide into numerous smaller branches called chorionic vessels. These vessels form an intricate capillary network within the chorionic villi, which are the functional units of the placenta. Within these villi, the waste products in the fetal blood diffuse across the thin placental membrane into the nearby maternal blood sinuses.
Post-Birth Transformation
The role of the umbilical arteries ends abruptly with the severance of the umbilical cord and the onset of independent neonatal breathing. The arteries undergo functional closure within minutes after birth, primarily due to the contraction of the smooth muscle within their walls. This muscular contraction is triggered by the cessation of blood flow and the resulting change in oxygen tension within the vessel.
Although the functional closure is rapid, the anatomical obliteration of the vessels takes several months as fibrosis occurs. The distal portions of the umbilical arteries become non-functional cords of connective tissue known as the medial umbilical ligaments. The proximal segments remain open, giving rise to the superior vesical arteries, which supply blood to the urinary bladder.