Overview of fetal circulation

Overview of Basic Heart Anatomy & Physiology

The heart is primarily composed of muscle, made up of  4 chambers, two upper and two lower, separated by one-way valves.

On the right side the upper chamber, the Right Atrium, receives “used”, or de-oxygenated, blood from the body through the Superior Vena Cava (SVC) and Inferior Vena Cava (IVC). The blood partially runs and partially is pushed into the Right Ventricle through the Tricuspid Valve (TV) during diastole, between contractions. Then, when the heart contracts in systole, the Tricuspid Valve, a one-way valve tethered to the floor of the Right Ventricle, closes and the Pulmonic Valve (PV) is pushed open. The blood is pumped through the Pulmonic Valve and into the Pulmonary Artery, toward the lungs, to be re-oxygenated. When the blood has been reoxygenated it is returned to the Left Atrium through the Pulmonary Veins, where it flows into the Left Ventricle through the Mitral Valve during diastole. When the heart contracts in systole the Mitral Valve, tethered to the floor of the Left Ventricle, closes, and the blood is pushed through the opened Aortic Valve (a small portion of which is seen just to the left of the Mitral Valve) into the Aorta, then on to the body.  At the end of the contraction the Aortic Valve closes.

Understanding Fetal Circulation

In utero fetal lungs are ‘nonfunctional’ as they are not required for gaseous exchange, as this role is performed by the placenta (Kain & Mannix 2023). In utero the systemic vascular system  has high blood flow low resistance, the pulmonary vascular system low blood flow high resistance.

Fetal circulation differs greatly from postnatal circulation as it contains structures that are not required in extrauterine life. 

Blood Flow in Utero

The umbilical vein carries oxygenated blood from the placenta toward the liver. The ductus venosus is a fetal vessel that connects the umbilical vein to the inferior vena cava, allowing approximately 45-50% of blood to bypass the liver (Kain & Mannix 2023). In the inferior vena cava oxygenated blood carried from the placenta mixes with the deoxygenated blood returning from the fetus’s lower limbs, this mixed blood then enters the right atrium (Kain & Mannix 2023).  A large portion of the blood entering the right atrium is shunted across the foramen ovale into the left atrium due to the higher-pressure gradient of the in the left side (Kain & Mannix 2023). It then enters the left ventricle and leaves the heart via the aorta. The blood remaining in the right atrium passes to the right ventricle and leaves the heart via the pulmonary artery. Approximately 10% of this blood enters the lung as the majority flows into the aorta via the ductus arteriosus (Kain & Mannix 2023). As the lungs are not aerated in utero the pulmonary vascular resistance is high. The ductus arteriosus protects the lungs from overloading and allows the right ventricle to strengthen. The placenta has a low resistance, therefore in utero, there is low systemic vascular resistance. (Hillman et al 2012, Marty et al 2023) Via the descending aorta the blood circulates the fetus, and deoxygenated blood then returns to the placenta for re-oxygenation and waste removal via the umbilical arteries (Hillman et al 2012, Marty et al 2023)

Transition to Postnatal Circulation

Major circulatory changes occur at birth. Although most full-term infants achieve this without support, careful nursing/midwifery assessment is required to ensure that infants do make the transition safely.

What happens within the lungs

Transition from fetal to postnatal circulation depends on a number of factors.

These can be divided into:

1. Factors that contribute to a postnatal increase systemic vascular resistance (SVR), removal of the placenta, catecholamine surge post birth, & cold extra-uterine environment.
2. Factors that contribute to a postnatal decrease pulmonary vascular resistance (PVR), expansion of air-filled lung, establishment of alveolar ventilation and oxygen tension, & successful clearance of fetal lung fluid

At the onset on labour there is a release of catecholamines causing lung fluid production to cease (Hillman et al 2012). Following birth, the fluid in the lungs is replaced with air. Commencement of breathing is dependent on chemical, thermal and sensory stimulation. As the lungs become aerated, there is fall in the pulmonary vascular resistance and an increase in pulmonary blood flow (Hillman et al 2012, Marty et al 2023)

At birth, the umbilical cord is clamped removing the placenta from the circularity system causing a rise in systemic vascular resistance and blood pressure (Bernardo & Dawson as cited in Kain & Mannix 2023). Clamping the umbilical cord stops the flow of blood through the umbilical vein to the ductus venous, enabling blood to flow through the portal system, the umbilical vessels close functionally within minutes (Marty et al 2023), the anatomical remnant of the ductus venosus is the ligamentum venosum (Hillman et al 2012, Marty et al 2023)

The increased pulmonary blood flow along with increased systemic pressure causes the pressure in the left atrium to be greater than the right atrium, which causes the foramen ovale to functionally close by pressing the valve of the foramen ovale against the septum secundum. This prevents blood flow from the right to the left atrium and ensures blood flow into the right ventricle into the pulmonary trunk. There are two separate forms of foramen ovale closure: functional and structural. functional closure begins at the first breath and is rapid, structural (anatomical) closure is much slower and generally occurs before the end of the first year forming the fossa ovalis (Hillman et al 2012, Marty et al 2023).

Alterations in pressure within the heart also decrease the blood flow across the ductus arteriosus. Closure results from an increase in oxygen (PaO₂) levels after birth. During the initial inflation of the lungs a substance called bradykinin is released. Bradykinin is dependent on high PaO₂ levels causing smooth muscle to contract. High prostaglandin levels in utero help to maintain duct patency (Hillman et al 2012). With increased blood flow to the lungs an increased amount of prostaglandin is metabolised aiding with the ducts closure. For the first 24-48 hours of life there is often a shunt of blood across the ductus arteriosus from the aorta to the pulmonary trunk. In the healthy term infant ducts are functionally closed by 96 hours. Anatomical closure is much slower occurring by 2–3 weeks after birth (33% of infants), by 2 months (90% of infants) and by 1 year (99% of infants) forming the ligamentum arteriosum (Hillman et al 2012, Marty et al 2023)

Test your understanding