FETAL BLOOD AND CIRCULATION

Introduction

Throughout the fetal stage of development, the maternal blood supplies the fetus with O₂ and nutrients and carries away its wastes.
- These substances diffuse between the maternal and fetal blood through the placental membrane.
- They are carried to and from the fetal body by the umbilical blood vessels.
Adaptations of fetal blood and vascular system.
The concentration of hemoglobin in fetal blood is about 50 % greater than in maternal blood.
Fetal hemoglobin is slightly different chemically and has a greater affinity for O₂than maternal hemoglobin.
- At a particular oxygen partial pressure, fetal hemoglobin can carry 20-30% more O₂ than maternal hemoglobin.

Fetal Circulation OH-98

In the fetal circulatory system, the umbilical vein transports blood rich in O₂ and nutrients from the placenta to the fetal body.
- The umbilical vein enters the body through the umbilical ring and travels along the anterior abdominal wall to the liver.
  - About 1/2 the blood it carries passes into the liver.
  - The other 1/2 of the blood enters a vessel called the ductus venosus which bypasses the liver.
- The ductus venosus travels a short distance and joins the inferior vena cava.
  - There, the oxygenated blood from the placenta is mixed with the deoxygenated blood from the lower parts of the body.
  - This mixture continues through the vena cava to the right atrium.
- In the adult heart, blood flows from the right atrium to the right ventricle then through the pulmonary arteries to the lungs.
  - In the fetus however, the lungs are nonfunctional and the blood largely bypasses them.
- As the blood from the inferior vena cava enters the right atrium, a large proportion of it is shunted directly into the left atrium through an opening called the foramen ovale.
  - A small valve, septum primum is located on the left side of the atrial septum overlies the foramen ovale and helps prevent blood from moving in the reverse direction.
- The rest of the fetal blood entering the right atrium, including a large proportion of the deoxygenated blood entering from the superior vena cava passes into the right ventricle and out through the pulmonary trunk.
  - Only a small volume of blood enters the pulmonary circuit, because the lungs are collapsed, and their blood vessels have a high resistance to flow.
    - Enough blood reaches the lung tissue to sustain them.
- Most of the blood in the pulmonary trunk bypasses the lungs by entering a fetal vessel called the ductus arteriosus which connects the pulmonary trunk to the descending portion of theaortic arch.
  - As a result of this connection, the blood with a relatively low O₂ concentration which is returning to the heart through the superior vena cava, bypasses the lungs.
  - At the same time, the blood is prevented from entering the portion of the aorta that provides branches leading to the brain.
- The more highly oxygenated blood that enters the left atrium through the foramen ovale is mixed with a small amount of deoxygenated blood returning from the pulmonary veins.
  - This mixture moves into the left ventricle and is pumped into the aorta.
    - Some of it reaches the myocardium through the coronary arteries and some reaches the brain through the carotid arteries.
- The blood carried by the descending aorta is partially oxygenated and partially deoxygenated.
  - Some of it is carries into the branches of the aorta that lead to various parts of the lower regions of the body.
  - The rest passes into the umbilical arteries, which branch from the internal iliac arteries and lead to the placenta.
    - There the blood is reoxygenated.

The Newborn

The initial inflation of the lungs causes important changes in the circulatory system.
Inflation of the lungs reduces the resistance to blood flow through the lungs resulting in increases blood flow from the pulmonary arteries.
- Consequently, an increased amount of blood flows from the right atrium to the right ventricle and into the pulmonary arteries and less blood flows through the foramen ovale to the left atrium.
In addition, an increased volume of blood returns from the lungs through the pulmonary veins to the left atrium, which increases the pressure in the left atrium.
- The increased left atrial pressure and decreased right atrial pressure (due to pulmonary resistance) forces blood against the septum primum causing the foramen ovale to close.
- This action functionally completes the separation of the heart into two pumps--right and left sides of the heart.

The closed foramen ovale becomes the fossa ovalis.
- The ductus arteriosis, which connects the pulmonary trunk to the systemic circulation, closes off within 1-2 days after birth.
  - Once closed, the ductus arteriosus is replaced by connective tissue and is known as the ligamentum arteriosum.

If the ductus arteriosus does not completely close it is said to be patent.
- This is a serious birth defect resulting in marked elevation in pulmonary pressure because blood flows from the left ventricle to the aorta, through the ductus arteriosus to the pulmonary arteries.
- If not corrected, it can lead to irreversible degenerative changes in the.heart and lungs.
- The fetal blood supply passes to the placenta through two (2) umbilical arteries from the internal iliac arteries and returns through an umbilical vein which passes through the liver, ductus venosus, and joins the inferior vena cava.
  - When the umbilical cord is cut, no more blood flows through the umbilical arteries and vein and they degenerate.