Foetal Circulation - Anatomy & Physiology
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Prior to birth the foetus is not capable of respiratory function and thus relies on the maternal circulation to carry out gas, nutrient and waste exchange. The foetal and maternal blood never mix, instead they interface at the placenta. Consequently the liver and the lungs are non-functional, and a series of shunts exist in the foetal circulation so that these organs are almost completely by-passed.
Shunt 1: The Ductus Venosus
Oxygenated blood travels from the placenta via the umbilical vein and most of it bypasses the liver by way of the ductus venosus. The ductus venosus links the umbilical vein to the caudal vena cava and the flow of blood is controlled by a sphincter, enabling the proportion travelling to the heart via the liver to be altered.
Shunt 2: The Foramen Ovale
The foramen ovale is an opening between the two atria enabling blood to be channelled directly into the systemic circulation thereby bypassing the lungs. The septum secundum directs the majority of the blood entering the right atrium through the foramen ovale into the left atrium. Here it mixes with a small volume of blood returning from the non-functional lungs via the pulmonary veins.
Shunt 3: The Ductus Arteriosus
The ductus arteriosus connects the pulmonary artery to the aorta and allows equivalent ventricular function in the foetus. The blood from the right ventricle is pumped to the pulmonary trunk where, due to the high resistance in the collapsed foetal lungs, a larger volume passes through the ductus arteriosus to the caudal aorta. Most of the blood in the aorta is then returned to the placenta for oxygenation through the umbilical arteries. The ductus arteriosus empties blood into the aorta after the artery to the head has branched off thus ensuring that the brain receives well-oxygenated blood.
Circulatory Changes at Birth
Important circulatory changes occur at birth due to the replacement of the placenta by the lungs as the organ of respiratory exchange. When an newly born animal takes its first breath, the lungs and pulmonary vessels expand thereby significantly lowering the resistance to blood flow. This subsequently lowers the pressure in the pulmonary artery and the right side of the heart. On the other hand the removal of the placenta causes an increase in the resistance of the systemic circulation and hence an increase in the pressure of the left side of the heart.
The birth of the animal also triggers the closure of the foetal shunts:
Closure of the Ductus Venosus
The ductus venosus is weakly responsive to prostaglandin E2 (PGE2) and prostacyclin (PGI2) which behave as vasodilators. This influence is lost with the improved pulmonary clearance resulting from the absence of an umbilical blood supply. This loss of blood supply also causes the sphincter in the ductus venosus to constrict thereby diverting blood to the liver. Closure of the ductus venosus becomes permanent after two to three weeks. The remnants of the ductus venosus form the ligamentum venosum.
Closure of the Foramen Ovale
In the foetus the foramen ovale is kept open by the higher pressure of blood in the right atrium compared to the left atrium. At birth the blood pressure in the right atrium decreases due to termination of blood flow from the placenta, whilst pressure in the left atrium increases due to increased pulmonary flow. As a result, the flap of the septum primum presses against the septum secundum closing the foramen ovale. In most individuals, the foramen ovale closes a few months after birth. A scar remains between the two atria once the foramen ovale has closed and this is termed the fossa ovalis.
Closure of the Ductus Arteriosus
The ductus arteriosus is a muscular artery and immediately after birth, contraction of the musculature closes the shunt. Factors which may contribute to the physiological closure of the ductus arteriosus include the increased oxygen content of the blood passing through it and the production of bradykinin, which causes smooth muscle contraction. This physiological closure causes blood to be directed from the pulmonary arteries to the now functioning lungs. Anatomical closure takes about two months and occurs by infolding of the endothelium and proliferation of the subintimal connective tissue layer. The residual ligament is termed the ligamentum arteriosum.
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