Prior to birth the foetus is not capable of respiratory function due to the lungs being full of amniotic fluid, subsequently the foetus relies on the maternal circulation for gas, nutrition and waste exchange. However, the foetal and maternal blood never mix, instead they interfere at the placenta. The foetus' lungs and liver are not functional.
Oxygenated blood travels from the placenta to the foetus via the umbilical vein, within the umbilical cord. Most of it bypasses the liver as the umbilical vein is connected to the caudal vena cava by the ductus venosus. The flow of blood to the heart is controlled by a sphincter.
Due to the foetus' lungs being dis functional, oxygenated blood does flow through the right ventricle into the pulmonary circulation, but bypasses this route by going from the right atrium straight to the left atrium via the foramen ovale (an opening between the two atria). This enables blood to flow directly into the systemic circulation. 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.
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.
The two cardiac shunts I have talked about are:
- Connection between the right and left atria via the foramen ovale
- Connection between the pulmonary pulmonary artery and aorta via the ductus arteriosus
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.
With birth a change from parallel flow to a serial one requires the following changes to take place:
- Gas exchange takes place in lungs in place of the placenta
- By cutting the umbilical cord, the placental circulation system is switched off
- The foetal heart shunts become closed
With the activation of breathing the lungs becomes distended, the capillary network dilated and their resistance is reduced drastically so that a rich flow of blood can take place. As a consequence, the pressure in the right atrium sinks in comparison with that of the left one. This pressure turnaround in the atria causes the septum primum to be pressed against the septum secundum and the foramen secundum becomes functionally closed. Additionally, the pressure in the aorta is now higher than that in the pulmonary artery thus decreasing blood flow in the wrong direction. After a few weeks, the shunt via the ductus arteriosus is obliterated.
Diseases which result from defects in this process
- Pre-eclampsia: It is a combination of hypertension and proteinurea. It can potentially progress to a condition called eclampsia, which can cause fits or convulsions. Symptoms include: severe headaches, problems with vision, severe pain just below ribs, heart burn and rapid swelling of face, hands and feat. The exact cause of pre-eclampsia remains unknown but research indicates that genetics and the placenta could be factors in the development of pre-eclampsia. High blood pressure is very dangerous during pregnancy as it may interfere with the placenta's ability to delivery oxygen and nutrients to the foetus. It also can cause abnormal renal and liver functioning. Also, it can cause destruction of red blood cells and platelets which can result in spontaneous bleeding as clotting factors decrease. The high blood pressure can also cause the placenta to begin to pull away from the uterus wall ( placental abruption). This can cause severe bleeding an even death. Also, the mother can experience seizures which puts the foetus at risk of deprived oxygen.
- Uterine rupture: It is when the scar from your previous caesarian section tears open. It is possible for your scar to gape slightly (scar dehiscence) but this is unlikely to cause problems for the mother or the baby. Uterine rupture, however, can be life threatening for the foetus as it puts it at risk of oxygen deprival. This is common in african women as caesarians are not carried out properly, putting the uterus at a higher risk of rupturing.
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