LACRIMAL PUNCTUM

FOETAL CIRCULATION 

To have an indept understanding of the foetal circulation, it is essential to understand the post natal circulation. Blood flows from the RIGHT ATRIUM to the RIGHT VENTRICLES then it is pumps out into the PULMONARY ARTERIES which takes blood to the LUNGS where it gets rid of CO2 and picks up oxygen, then it comes out of the LUNGS through the pulonary veins into the LEFT ATRIUM, then moves to the LEFT VENTRICLE and then out into the AORTA, where all the oxygenated blood is taken to the body, where it is used and the blood then picks up the by-products of respiratory, such as CO2 and these are then taken back via the SUPERIOR and INFERIOR VENA CAVA back into the right hand side of the heart. 

Hence, the objective of postnatal heart circulation is to take blood from the right side of the heart➡️Lungs➡️ Left Heart➡️Body then to return back to the right side of the heart.

However, the objective of the foetal circulation is to take blood back to the PLACENTA instead of the lungs to get rid of waste products and  to pick up oxyegen and Nutrients 

From the picture in this post,  the DUCTUS ARTERIOSUS, DUCTUS VENOSUS and FORAMEN OVALE represent the 3 foetal shunts and they are essential.

How does blood flows through the foetal circulation? 

Starting from the aorta, the blood is pumped out of the heart, so you have oxygenated blood from the heart going to the body, a portion of the blood flows through the UMBILICAL ARTERIES into the PLACENTA and the placenta is where the the nutrients from maternal blood transfer into the fetal blood and all the waste product from fetal blood can transfer into the maternal blood, once all these have taken place, the blood flows out through the UMBILICAL VEIN.

The umbilical vein is connected to a shunt (Ductus venosus), 20-30% of the oxygenated blood from the umbilical vein flows through the ductus veosus into the inferior vena cava and straight to the right side of the heart. Then the remaining 70-80% of the rest of the blood joins with the portal vein going into the liver, then come out through the hepatic vein and then joins the inferior vena cava.

Now to the second shunt THE FORAMEN OVALE, It is a septum between the two atrium. It allows the oxygenated blood to flow from the right atrium, bypassing the lungs and right ventricle, straight into the left atrium where it can go into the left ventricles and then to the rest of the body. Not all the blood goes through the foramen ovale, some of them goes via the right ventricles into the pulmonary artery. 

Now it is at this point, the third shunt, THE DUCTUS ARTERIOSUS does its own role. The ductus arteriosus is a connection between the pulmonary trunk and the pulmonary arteries and the aorta, which mean that once again blood can bypass the lungs and go straight from the right ventricle into the pulmonary trunk the to the aorta and down to the rest of the body.

Hence, in the fetal circulation, there are two ways (Foramen ovale and ductus arteriosus) for blood to bypass the lung in the heart because lungs aint really doing much in the fetus until they are born.

These shunts are aided by the fact that in the foetus, the pulmonary vascular resistance is very high, and this is because the alveoli are all closed and everything in the lung is sort of squeezed so blood flow will prefer the path of least resistance, so instead of flowing into the pumonary arteries, it flows into the aorta

At birth, when the baby take the first breathe, all the alveoli in the lung expand thereby decreasing the vascular resistance in the pulmonary vessels. There is also a drop in the circulating prostaglandin response to the increased oxygenation of the blood from taking that first breath. Prostaglandins are important for the ductus arteriosus to stay open, when there is a drop in the corculating prostagladin, the ductus arteriosus closed and becomes the ligamentum arteriosum and at this point, the foramen ovale closes and becomes a sort of notch in the righ and left atrium called the fossa ovalis and this allows normal adult circulation to happen after that first breath.

Finally, the ductus venosus will close automatically to form ligamentum venosus because at birth, the umbilcal cord is cut and there is no circulation coming through the umbilical vein.