Fetal Circulation

Transport Across the Placenta

The primary function of the placenta in all species is to promote selective transport of nutrients and waste products between mother and fetus. Such transport is facilitated by the close approximation of maternal and fetal vascular systems within the placenta. It is important to recognize that there normally is no mixing of fetal and maternal blood within the placenta. Entry of small amounts of fetal blood into the maternal circulation does occasionally occur, and can evoke an immune response (AFE) in the mother that affects that fetus after birth or fetuses in subsequent pregnancies that are sired by the same father. The placenta is a complex tissue and should not be envisioned as simple permeable membrane. In addition to transporting some molecules unaltered between fetal and maternal blood, it also consumes a large fraction of certain types of cargo - glucose and oxygen being good examples. Additionally, a number of molecules that cross the placenta are metabolized to other things during passage. *DIFFUSION:* is the net passive movement of particles from a region in which they are in HIGHER concentration to regions of LOWER concentration. It continues until the concentration of substances is uniform throughout. *Transport of Gases* Gases like oxygen and carbon dioxide diffuse through and across tissues in response to differences in partial pressure. In late pregnancy, the mean partial pressure of oxygen (P02) in maternal blood is considerably higher than in fetal blood. As a consequence, oxygen readily diffuses across the placenta from maternal to fetal blood. Despite its low PO2, fetal blood is able to transport essentially the same quantity of oxygen to tissues as maternal blood. This is because the hemoglobin concentration in fetal blood is about 50% higher than in maternal blood, and the majority of hemoglobin in the fetus is fetal hemoglobin, which has a higher oxygen carrying capacity than adult hemoglobin. Carbon dioxide is produced abundantly in the fetus, and the PCO2 of fetal blood is higher than maternal blood. Carbon dioxide therefore diffuses from fetal blood, through the placenta, into the maternal circulation, and is disposed of by expiration from the mother's lungs. *Nutrients* Glucose is the major energy substrate provided to the placenta and fetus. It is transported across the placenta by facilitated diffusion via hexose transporters that are not dependent on insulin (GLUT3 and GLUT1). Although the fetus receives large amounts of intact glucose, a large amount is oxidized within the placenta to lactate, which is used for fetal energy production. Amino acid concentrations in fetal blood are higher than in maternal blood. Amino acids are therefore transported to the fetus by active transport. A family of at least 10 sodium-dependent amino acid transporters have been identified in placenta that serve this function. There is substantial metabolism of some amino acids as they cross the placenta - for example, much of the serine taken up by the placenta is converted to glycine prior to delivery to the fetus. There is much more variability among species in placental permeability to fatty acids than to glucose or amino acids. In some animals, there is little transport of fatty acids from mother to fetus, while in others a significant amount of transport takes place.

Fetal Circulation

Fetal circulation differs from adult circulation due to the presence of certain shunts and vessels. These shunts will close after birth and most of the vessels will be seen as remnants in the adult circulation. FUNCTION OF SHUNTS: is to direct *oxygen-rich VENOUS blood to the systemic circulation* and to ensure that *oxygen-depleted venous blood BYPASSES the underdeveloped pulmonary circulation* The lungs finish their development AFTER birth. Prior to birth, the lung function is taken over by the *placenta* during fetal life.

Fetal Circulation: Oxygenation

In the extrauterine world, oxygenation occurs in the lungs and oxygenated blood returns via the pulmonary veins to the left side of the heart to be ejected by the left ventricle into the systemic circulation. In contrast, fetal circulation oxygenation occurs in the PLACENTA, and the fetal lungs are nonfunctional as far as the transfer of oxygen and carbon dioxide is concerned. For oxygenated blood derived from the placenta to reach the fetus's systemic circulation, it has to travel through a series of SHUNTS to accomplish this. Thus, fetal circulation involves the *circulation of blood from the placenta to and through the fetus, and back to the placenta*. A properly functioning fetal circulation system is essential to sustain the fetus. Before it develops, *nutrients and oxygen DIFFUSE* through the *extraembryonic coelom and the yolk sac* from the placenta. As the embryo grows, its nutrient needs INCREASE and the amount of tissue easily reached by diffusion INCREASES.

Fetal Blood Flow

1. Oxygenated blood is carried from the placenta to the fetus via the *umbilical VEIN.* 2. Half of this blood passes through the *hepatic capillaries [liver]* and the rest flows through the *ductus venosus* into the *inferior vena cava* 3. Blood enters the *right atrium.* However, most of it flows through the *foramen ovale* into the *LEFT atrium.* 4. Blood then passes into the *LEFT ventricle.* 5. Blood then passes into the *ascending aorta* - the large artery coming from the heart. 6. From the aorta, most of the blood is sent to the *heart muscle* itself and to the *BRAIN and upper body [arms].* This allows the fetal *coronary circulation* and the brain to receive the blood with the HIGHEST level of oxygen. 7. After circulation there, the deoxygenated blood returns to the *RIGHT atrium* through the *superior vena cava.* Very little of this less oxygenated blood mixes with the oxygenated blood (Also, the blood that did NOT go to the brain travels down to the right atrium and right ventricle). 8. Instead of going back through the foramen ovale, it goes into the *RIGHT ventricle.* 9. This less oxygenated blood is pumped from the *RIGHT ventricle* into the *pulmonary artery.* Bc of HIGH pulmonary vascular resistance (: refers to the arteries that supply blood to lungs. If the pressure is high, the right ventricle must work harder to move the blood forward past the pulmonic valve), only a small percentage (5-10%) of the blood in the pulmonary artery flows to the *lungs.* 10. The majority of the blood is shunted through the patent *ductus arteriosus* to the *descending aorta*. 11. This blood enters the *umbilical arteries (2)* and flows into the *placenta* In the placenta, carbon dioxide and waste products are released into the mother's circulatory system. Oxygen and nutrients from the mother's blood are released into the fetus' blood. SIMPLIFIED OVERVIEW: 1. Umbilical VEIN --> placenta (oxygenated blood) 2. Half goes to hepatic capillaries (liver), rest flows through ductus venosus into inferior vena cava 3. Enters RIGHT ATRIUM --> Foramen ovale shunts most of it into LEFT ATRIUM 4. Left ventricle 5. Ascending aorta --> HEAD/BRAIN, heart muscle itself, upper body 6. Deoxygenated blood from superior vena cava flows to RIGHT ATRIUM 7. Right Ventricle 8. Part goes to Pulmonary Artery (only small amount of blood goes to LUNGS) 9. Majority of blood from Right Ventricle is shunted through ductus arteriosus --> descending aorta 10. Two umbilical veins carry unoxygenated blood from descending aorta back to the PLACENTA

Fetal & Newborn Circulation: Pressures

Fetal circulation is characterized by *low systemic vascular resistance (SVR)* with *high systemic blood flow* and *high pulmonary vascular resistance* with *low pulmonary blood flow*. Given the low oxygen tension in the fetus, fetal circulation allows for preferential flow of the most oxygenated blood to the heart and brain, two of the three "vital organs."1 With the placenta rather than the lungs being the organ of gas exchange, most of the right ventricular output is diverted through the patent ductus arteriosus (PDA) to the systemic circulation. The fetal lungs are essentially nonfunctional bc they are filled with fluid, making them resistant to incoming blood flow. They receive only enough blood for proper nourishment. *AT BIRTH:* At birth, the umbilical cord is clamped and the baby no longer gets oxygen and nutrients from the mother - the foramen ovale, ductus arteriosus, ductus venosus, and umbilical vessels are no longer needed. *Closure of Foramen Ovale:* With the first breaths of life, the lungs start to expand. As the lungs expand, the alveoli in the lungs are cleared of fluid. When the lungs inflate it leads to an increase in blood flow to the lungs from the right ventricle (due to a reduction in pulmonary resistance/pressure). This increase raises the pressure in the LEFT atrium, causing a one-way flap on the left side of the foramen ovale (called the *septum primum*) to press against the opening, creating a functional separation between the two atria. Blood flow to the lungs increases bc blood entering the right atrium can no longer bypass the right ventricle (with foramen ovale, blood entering right atrium was shunted to left atrium rather than traveling to right ventricle). As a result, the right ventricle pumps blood into the pulmonary artery and on to the lungs. Typically the foramen ovale is functionally closed within 1-2 hours after birth. Permanent closure occurs by 6 months. *Closure of Ductus Venosus:* The ductus venosus, which links the inferior vena cava with the umbilical vein, usually closes with the clamping of the umbilical cord and inhibition of blood flow through the umbilical vein. This structure closes by the end of the FIRST week. *Closure of Ductus Arteriosus:* An increase in the baby's blood pressure and a major reduction in the pulmonary pressures reduce the need for the ductus arteriosus to shunt blood. The ductus arteriosus constricts partly in response to the higher arterial oxygen levels that occur after the first few breaths. This closure prevents blood from the aorta from entering the pulmonary artery. Functional closure of the ductus arteriosus in a term infant usually occurs within the first 72 hours after birth. Permanent closure occurs at 3 to 4 weeks of age. Frequently a *functional or innocent murmur* is auscultated by the nurse when there are delayed fetal shunt closures, but they are usually not associated with a heart lesion.

Additional Fetal Shunts Info

The fetal circulatory system uses 3 shunts. These are small passages that *direct blood that NEEDS to be oxygenated*. The *purpose of these shunts is to bypass the lungs and liver.* That's because these organs will not work fully until after birth. *The shunt that bypasses the lungs is called the FORAMEN OVALE.* This shunt moves blood from the right atrium of the heart to the left atrium. The *DUCTUS ARTERIOSUS* moves blood from the pulmonary artery to the aorta. Oxygen and nutrients from the mother's blood are sent across the placenta to the fetus. The enriched blood flows *through the umbilical cord to the liver and SPLITS into 3 branches.* The blood then reaches the inferior vena cava. This is a major vein connected to the heart. Most of this blood is sent through the *DUCTUS VENOSUS*. This is also a shunt that *lets highly oxygenated blood bypass the liver to the inferior vena cava and then to the right atrium of the heart*. A small amount of this blood goes straight to the liver to give it the oxygen and nutrients it needs. *Waste products* from the fetal blood are transferred back across the placenta to the mother's blood.

Umbilical Cord and Placenta

The fetus is connected by the umbilical cord to the placenta. This is the organ that develops and implants in the mother's uterus during pregnancy. Through the blood vessels in the umbilical cord, the fetus gets all needed nutrition and oxygen. The fetus gets life support from the mother through the placenta. Waste products and carbon dioxide from the fetus are sent back through the umbilical cord and placenta to the mother's circulation to be removed.

Fetal Shunts

The most significant difference between fetal and newborn circulation is that *oxygen is received from the PLACENTA* during fetal life and via the lungs after birth. In addition, the *fetal liver does NOT perform the metabolic functions* that it will after birth bc the mother's body performs these functions. 3 Shunts are also present during fetal life: 1. *Ductus venosus:* connects the UMBILICAL VEIN to the INFERIOR VENA CAVA 2. *Ductus arteriosus:* connects the main PULMONARY ARTERY to the AORTA 3. *Foramen ovale:* anatomical OPENING between the right and left ATRIUM NOTE: Fetal circulation functions to carry highly oxygenated blood to VITAL areas [e.g., heart, brain] while *first shunting it away* from LESS important ones [e.g., lungs, liver]. The placenta essentially takes over the functions of the *lungs and liver* during fetal life. As a result, *large volumes of oxygenated blood are NOT needed.*

Adult & Newborn Blood Flow

The right and left sides of the heart work together! RIGHT SIDE: 1. Blood enters the heart through two large veins, *the inferior and superior vena cava,* emptying oxygen-POOR blood from the body into the *right atrium.* 2. Blood flows from your right atrium into your *right ventricle* through the open *tricuspid valve*. When the ventricles are full, the tricuspid valve shuts. This prevents blood from flowing backward into the atria while the ventricles contract (squeeze). 3. Blood leaves the heart through the *pulmonic valve*, into the *pulmonary artery* and to the lungs. How Does blood flow through the Lungs? 4. Once blood travels through the pulmonic valve, it enters your lungs. This is called the pulmonary circulation. From your pulmonic valve, blood travels to the pulmonary artery to tiny capillary vessels in the lungs. Here, oxygen travels from the tiny air sacs in the lungs, through the walls of the capillaries, into the blood. At the same time, carbon dioxide, a waste product of metabolism, passes from the blood into the air sacs. Carbon dioxide leaves the body when you exhale. Once the blood is purified and oxygenated, it travels back to the left atrium through the pulmonary veins. LEFT SIDE: 5. The *pulmonary vein* empties oxygen-rich blood, from the lungs into the *left atrium*. 6. Blood flows from your left atrium into your *left ventricle* through the open *mitral valve*. When the ventricles are full, the mitral valve shuts. This prevents blood from flowing backward into the atria while the ventricles contract (squeeze). 7. Blood leaves the heart through the *aortic valve*, into the *aorta* and to the *body*. This pattern is repeated, causing blood to flow continuously to the heart, lungs and body. SIMPLIFIED OVERVIEW: 1. Inferior and Superior Vena Cava [deoxygenated blood] 2. Right Atrium --> Tricuspid valve 3. Right Ventricle --> Pulmonic Valve 4. Pulmonary Artery 5. LUNGS [oxygenated blood] 6. Pulmonary Veins 7. Left Atrium --> Mitral [Bicuspid] Valve 8. Left Ventricle --> Aortic Valve 9. Aorta 10. Body