Question

Name the fetal structure with each of the following functions: (pp. 307-308) a. Permits blood to flow from the right atrium to the left atrium b. Carries blood from the placenta to the fetus c. Permits blood to flow from the pulmonary artery to the aorta d. Carry blood from the fetus to the placenta e. Carries blood from the umbilical vein to the inferior vena cava

Short answer

Answer: Foramen ovale b. Which structure carries blood from the placenta to the fetus? Answer: Umbilical vein c. What fetal structure permits blood flow from the pulmonary artery to the aorta? Answer: Ductus arteriosus d. What structures carry blood from the fetus to the placenta? Answer: Umbilical arteries e. What structure carries blood from the umbilical vein to the inferior vena cava? Answer: Ductus venosus

Step-by-step solution

a. Permits blood to flow from the right atrium to the left atrium

The fetal structure that allows blood to flow from the right atrium to the left atrium is the foramen ovale. It is an opening in the atrial septum that facilitates the bypass of the fetal lungs, which are not yet functional for oxygen exchange.

b. Carries blood from the placenta to the fetus

The structure responsible for carrying blood from the placenta to the fetus is the umbilical vein. Blood in the umbilical vein is oxygenated and nutrient-rich, as it has been exchanged with the maternal blood in the placenta.

c. Permits blood to flow from the pulmonary artery to the aorta

The fetal structure allowing blood to flow from the pulmonary artery to the aorta is the ductus arteriosus. The ductus arteriosus is a small blood vessel that connects these two arteries and helps bypass the lungs since they are not yet functioning.

d. Carry blood from the fetus to the placenta

The structures that carry blood from the fetus to the placenta are the umbilical arteries. There are two umbilical arteries, and they transport deoxygenated, nutrient-depleted blood from the fetus back to the placenta for oxygen and nutrient replenishment.

e. Carries blood from the umbilical vein to the inferior vena cava

The structure connecting the umbilical vein to the inferior vena cava is the ductus venosus. The ductus venosus functions as a shunt, allowing oxygenated blood from the umbilical vein to bypass the liver and merge with the blood in the inferior vena cava, which then goes back to the heart.

Key concepts

Foramen Ovale

The foramen ovale serves a critical function in fetal circulation, as it provides a pathway for blood to flow directly from the right atrium to the left atrium. Since a fetus's lungs are not yet in use for oxygen exchange, it's important that blood can bypass this non-functional system. Functioning like a built-in shortcut within the heart, the foramen ovale ensures that oxygenated blood received from the mother can be swiftly distributed throughout the fetus's body.

After birth, when the newborn takes its first breath, the lungs expand, and the foramen ovale usually closes, ensuring that blood now passes through the newly operational lungs for oxygenation. If the foramen ovale does not close properly after birth, it can result in a patent foramen ovale (PFO), which might require medical attention.

Umbilical Vein

The umbilical vein is the lifeline between the mother and the developing fetus, carrying oxygen and nutrient-rich blood from the placenta directly to the fetus. This vein is unique to fetal circulation and presents as a single vessel, unlike other veins, indicating its primary importance. It delivers the essentials for fetal growth and development and plays a role in fetal waste management.

After birth, the umbilical vein closes and becomes a ligament as part of the natural transition from fetal to independent neonatal circulation. Understanding its role aids in appreciating the intricate developmental adaptations that occur in utero.

Ductus Arteriosus

Connecting the pulmonary artery to the aorta, the ductus arteriosus is a vital component of the fetal circulatory system. Its main function is to divert blood away from the non-functioning fetal lungs, facilitating efficient distribution of oxygenated blood to the rest of the body. This temporary vascular channel is a crucial adaptation to life in the womb where the lungs are filled with amniotic fluid and are not yet breathing air.

Typically, within minutes or days after birth, the ductus arteriosus closes as the lungs begin to function and oxygenate blood. This closure transforms the ductus arteriosus into the ligamentum arteriosum. In cases where it fails to close, it's known as a patent ductus arteriosus (PDA), which is a condition that might need to be corrected medically.

Umbilical Arteries

Contrasting the umbilical vein, the umbilical arteries (there are usually two) return deoxygenated, waste-laden blood from the fetus to the placenta. They are pivotal in the circulatory exchange process, ensuring that the fetal blood disposes of carbon dioxide and metabolic wastes back to the maternal system for elimination.

In the fascinating transition from intrauterine life to the outside world, the umbilical arteries close off and form the medial umbilical ligaments. This transition underscores how the fetal structures are perfectly designed for the in-utero environment and how they evolve post-birth.

Ductus Venosus

As a special vascular channel, the ductus venosus acts as a shortcut that allows oxygenated blood from the umbilical vein to bypass the liver and flow directly into the inferior vena cava. This shortcut is highly efficient, ensuring that the best-oxygenated blood reaches the heart, which then propels it to the brain and other critical organs of the fetus.

Following birth, the ductus venosus closes and turns into the ligamentum venosum, remnants of its past role in fetal circulation. It’s fascinating how the fetal body has mechanisms like the ductus venosus, designed for a time when the liver's full capabilities are not yet needed, yet they adapt quickly after birth when different physiological demands arise.