Question

Explain how blood circulates to and from the placenta and the fetus. How is blood shunted away from the lungs? (pages 401,403 )

Short answer

During fetal development, the umbilical cord plays a crucial role in blood circulation. Oxygenated blood from the placenta is transported to the fetus via the umbilical vein, while deoxygenated blood returns from the fetus to the placenta through the umbilical arteries. Within the fetus, oxygenated blood bypasses the liver through the ductus venosus and enters the right atrium of the heart. To bypass the lungs, which are not yet functional, blood is diverted from the right atrium to the left atrium via the foramen ovale, and from the pulmonary artery to the aorta through the ductus arteriosus. Deoxygenated blood then returns to the placenta through the umbilical arteries, completing the circulation process.

Step-by-step solution

Introduction

During fetal development, circulation of blood between the fetus and the placenta is essential for supplying oxygen and nutrients to the growing fetus, and for removing waste products. Additionally, fetal blood bypasses the lungs as they are not yet fully developed and functional.

Structures in Fetal Circulation

Some key structures involved in this circulation process include the umbilical cord, umbilical vein, and umbilical arteries. Inside the umbilical cord, there is one umbilical vein and two umbilical arteries. The umbilical vein carries oxygen-rich blood from the placenta to the fetus, while the umbilical arteries return deoxygenated blood from the fetus to the placenta.

Blood Circulation from Placenta to Fetus

The following steps illustrate the circulation of blood between the placenta and the fetus: 1. Oxygenated blood (rich in nutrients and oxygen) from the mother's blood supply diffuses into fetal blood vessels present in the placenta. 2. This oxygenated blood is transported through the umbilical vein towards the fetus. 3. A portion of the blood is directed towards the liver, bypassing it via the ductus venosus, and the remaining blood enters the inferior vena cava. 4. The blood from the inferior vena cava enters the right atrium of the fetal heart.

Shunting Blood Away from the Lungs

Since the fetus receives oxygenated blood from the placenta, and the lungs are not fully developed and functional, there are special structures that help bypass the lungs. These structures are: 1. The foramen ovale: A small hole in the septum between the right and left atria of the fetal heart. The blood entering the right atrium from the inferior vena cava is diverted through the foramen ovale to the left atrium, bypassing the pulmonary circulation. 2. The ductus arteriosus: A small connection between the pulmonary artery and the aorta. The blood that does enter the right ventricle is pumped into the pulmonary artery, but most of it is diverted away from the lungs through the ductus arteriosus and into the aorta, further bypassing the pulmonary circulation.

Blood Circulation from Fetus to Placenta

After oxygen and nutrients have been used by the fetus, the deoxygenated blood (rich in carbon dioxide and waste products) follows this path: 1. Blood from the fetal body drains into the superior and inferior vena cavae, entering the right atrium of the heart. 2. Blood from the right atrium passes into the right ventricle and is pumped into the pulmonary artery. 3. Most of the deoxygenated blood is shunted through the ductus arteriosus into the aorta, bypassing the lungs. 4. Deoxygenated blood is returned to the placenta through the two umbilical arteries present in the umbilical cord. This continuous circulation process between the placenta and the fetus allows for the exchange of oxygen, nutrients, and waste products until the fetus is born. At birth, the lungs fill with air and start functioning, the foramen ovale closes, and the ductus arteriosus constricts, transitioning the baby to a normal circulatory system.

Key concepts

Placenta

The placenta plays a vital role during pregnancy, acting as the lifeline between the mother and the developing fetus. This temporary organ is attached to the wall of the uterus and establishes a crucial connection via the umbilical cord. The placenta's most important functions include the exchange of nutrients, gases, and waste products.
Oxygen and essential nutrients from the mother’s bloodstream are transferred to the fetus through the placenta. Waste products such as carbon dioxide are sent back into the mother’s circulation to be eliminated.

• The placenta produces hormones like human chorionic gonadotropin (hCG), progesterone, and estrogen to support pregnancy.
• Structural adaptations like the villi increase the surface area for efficient nutrient and gas exchange.

Overall, without the placenta, the fetus would not have a direct source of oxygen and nutrients, crucial for growth.

Umbilical Vein

The umbilical vein is an essential component of fetal circulation. It is unique in that it carries oxygenated blood, which is rich in nutrients, from the placenta directly to the fetus. Unlike postnatal arteries and veins, which typically carry blood in the opposite direction relative to oxygen content, the umbilical vein serves the crucial task of delivering vital resources.
Upon reaching the fetus, the umbilical vein follows a specialized circulation route to optimize the delivery of oxygen and nutrients to the developing tissues.

• The blood from the umbilical vein is partially circulated through the liver, with the rest being redirected by the ductus venosus.
• The ductus venosus connects the umbilical vein to the inferior vena cava, effectively allowing oxygen-rich blood to reach the heart and consequently the rest of the fetal body.

This vital process ensures that the growing fetus receives the materials needed for development.

Ductus Arteriosus

The ductus arteriosus is an essential fetal structure that assists in bypassing the non-functional fetal lungs. It is a short vessel that connects the pulmonary artery to the descending aorta. The reason for this shunt is that the lungs are not used for oxygen exchange in utero; instead, this task is fulfilled by the placenta.
The presence of the ductus arteriosus ensures that most of the blood flow is diverted away from the pulmonary circulation.

• During fetal life, the ductus arteriosus remains open due to higher blood pressure in the pulmonary artery compared to postnatal conditions.
• It closes shortly after birth as the newborn's lungs expand, the blood flows through the lungs for oxygenation, and normal circulation is established.

Until birth, the ductus arteriosus plays its part in maintaining the efficiency of fetal circulation.

Foramen Ovale

The foramen ovale is a tiny opening in the heart's septum, situated between the right and left atria. It plays a crucial role in bypassing the pulmonary circulation during fetal development. As the fetus receives oxygenated blood from the placenta and not the lungs, this opening allows some of the blood to flow directly from the right atrium to the left atrium.
The foramen ovale acts as an effective shunt to streamline circulation while avoiding the developing lungs.

• It plays a significant role in the distribution of oxygenated blood to vital organs like the brain and upper body.
• After birth, the increased blood flow to the lungs causes the foramen ovale to close, ensuring the blood circulates normally through the lungs for gas exchange.

Thus, the foramen ovale is essential for fetal life, adapting the circulatory requirements before the newborn transitions to independent breathing.