Fetal Circulation and Transition To Extra-Uterine Life

The amelioration of a human baby from a single cell (one ovum unites with one sperm to form the zygote, the name given to fertilized ovum) and the subsequent transition from intra-uterine to extra uterine life is one of the greatest wonders of creation. It is to be remembered that the fetus cannot get atmospheric oxygen but is nourished by oxygen from the mother through the placenta. In order to accomplish this, the fetus has a mechanism which would be extremely abnormal in extra uterine life. To understand this, first we need to understand the general circulation in adults.

The heart is a four chambered organ. The two upper chambers are called atria-left and right. The two lower ones are called ventricles -left and right. The 2 atria are thoroughly separated by a structure called inter-atrial septum. Similarly the ventricles are separated by the inter-ventricular septum. Thus blood in the left side of the heart is thoroughly separated from the blood in the right side. The right atrium receives blood from all the organ of the body through two main blood vessels-the superior vena cava and the inferior vena cava and then pumps that blood into the right ventricle. This blood is depleted of oxygen.

The right ventricle pumps this blood into the lungs through a blood vessel called pulmonary artery, which divides into two one each for a lung. The main artery goes on dividing and subdividing till it reaches the sac like structures in the lungs called alveoli. The total covering area of alveoli is practically equal to the size of a tennis court. In the alveoli gas transfer takes place-carbon-dioxide is thrown out and oxygen is taken up by the blood. The minuscule blood vessels carry oxygenated blood into progressively larger blood vessels and ultimately this blood comes to the left atrium through four pulmonary veins. From the left atrium oxygenated blood enters the left ventricle through the left atrio-ventricular valve. The left ventricle then pumps blood into a big blood vessel called aorta. This gives off branches and supplies oxygenated blood to all the cells of the body-the whole process is no less complicated than the water contribute and drainage system of a town! Blood from the cells is again returned to the right atrium thus completing the cycle. The cycle goes on and on till the end of life.

This can never work in a fetus because there is no direct access to atmospheric oxygen. Nature has devised a breathtaking mechanism to contribute oxygen to the fetus. The umbilical cord which forms the link between the mother and the fetus and is cut after birth, contains a blood vessel called umbilical vein. This vein carries oxygenated blood from the placenta into fetus. This divides into two inside the body of the fetus one field goes to the liver and the other called ductus venosus joins inferior vena cava, which carries deoxygenated blood to the right atrium. From this point the fetal circulation is different. As this oxygenated blood has to be supplied to all the fetal cells it has to go to the left side of the heart. There is no point in pumping it to lungs, which cannot carry out gas exchange. The inter-atrial septum in the fetus consists of two overlapping layers. They overlap in such a way that blood from the right atrium can enter the left atrium but the reverse cannot take place (valve like mechanism). The pressure in the right side heart in the fetus is higher than the left-exactly opposite to that after birth. Hence most of this blood goes into the left atrium, then to the left ventricle and pumped into aorta.

The right atrium also receives deoxygenated blood from the upper parts of the body through the superior vena cava. This blood mixes with blood from the inferior vena cava (which carries blood with higher oxygen concentration) enters the right ventricle and then pumped into the pulmonary artery. In the fetus the pulmonary artery is related to the aorta by a blood vessel called ductus arteriosus. Because there is no point in pumping blood into the lungs most of the blood in the pulmonary artery in shunted over the ductus arteriosus into the aorta. This blood contains less oxygen than that pumped by the left ventricle and supplies the lower parts of the body. The umbilical cord also contains two umbilical arteries. These arteries consist of deoxygenated blood from the fetus into the placenta completing the cycle.

Soon after birth the umbilical cord is clamped and cut. This increases resistance to systemic blood flow and raises the pressure in the left side of the heart. At the same time pulmonary pressure falls as air enters the lungs of the baby with the first breath. This stops the right to left shunting of blood over the atria. As the pressure in the aorta goes above that of pulmonary artery the shunt over ductus arteriosus gets reversed and some blood flows from the aorta into the pulmonary artery. But the ductus arteriosus starts shrinking and functionally closes by about 72 hours of life and anatomically closes within a few weeks. Thus the mixing of blood between the two sides of the heart thoroughly stops and the usual adult type of blood circulation is established.

Another unique feature of the fetus is the proximity of a separate type of hemoglobin called fetal hemoglobin. This differs from the adult hemoglobin in that it has higher affinity for oxygen than the adult hemoglobin. Hence it is able to take up hemoglobin from the maternal hemoglobin and deliver it to fetal cells. It can be observed that fetal cells get blood with lower attentiveness of oxygen than adults do because of mixing of blood in the two sides of the heart. The fetus is able to survive in spite of low oxygen attentiveness because the maternal body takes care of many functions and the vigor requirement is lower. After birth the attentiveness of fetal hemoglobin falls rapidly and that of adult hemoglobin increases. Thus nature ensures that the fetus develops inside the uterus by getting oxygen from the mother and soon after birth starts utilizing atmospheric oxygen through a complicated mechanism. It is actually breathtaking that for the vast majority of newborns the transition is smooth.

Circulation - Cardiovascular and Lymphatic principles

Blood circulates throughout the body in the cardiovascular system, which consists of the heart and the blood vessels. This principles forms a continuous circuit that delivers oxygen and nutrients to all cells and carries away waste product.

The Heart

The heart is placed between the lungs, with its point or apex directed toward the left. The thick muscle layer of the heart wall is the myocardium, which is lined on the inside with a thin endocardium and covered on the face with a thin epicardium. The heart is contained within a fibrous sac, the pericardium.

Each of the upper receiving chambers of the heart is an atrium. Each of the lower pumping chambers is a ventricle. The wall separating the two ventricles is the interventricular septum; the wall dividing the two atria is interatrial septum.

The right atrium receives blood low in oxygen from all body tissues straight through the classic vena cava and the inferior vena cava. The blood then enters the right ventricle and is pumped to the lungs straight through the pulmonary artery. Blood returns from the lungs high in oxygen and enters the left atrium straight through the pulmonary veins. From here it enters the left ventricle and is pumped into the aorta to be distributed to all tissues. Blood is kept involving in a forward direction by one-way valves-the bicuspid valve, regularly called mitral valve. And the tricuspid valve.

Each contraction of the heart, termed systole is followed by a leisure phase, diastole, while which chambers fill. Each time the heart beats, both atria ageement and immediately thereafter both ventricles contract.

The Vascular System

The vascular principles consists of arteries that carry blood away from the heart, arterioles-small arteries that lead into the capillaries, capillaries, the smallest vessels, straight through which exchanges take place between the blood and the tissues, veins, that carry blood back to the heart and venules - the small veins that receive blood from the capillaries and drain into the veins. Nervous principles stimulation can cause the diameter of a vessel to increase (vasodilation) or decrease (vasoconstriction).

The Lymphatic System

The fluid carried in the lymphatic principles is called lymph, and the role of the lymphatic principles in the circulation is to return excess fluid and the proteins from the tissues to the bloodstream. Other functions of the lymphatic principles contain absorption of digested fats from small intestines and protecting the body from invading microorganisms. Along the path of the lymphatic vessels are small masses of lymphoid tissue, the lymph nodes. Their function is to filter the lymph as it passes through. Other organs and tissues of the lymphatic principles contain the tonsils, the thymus and the spleen.

Physiology of Circulation

The heart receives all of the blood from colse to the body into the right atrium. The thoracic duct receives the lymph fluid from the lymph circulation and it empties into the previous vena cava just before it empties into the upper right chamber or the heart (right atrium). Here the blood then goes straight through the tricuspid or right A/V valve to the right ventricle. The wall of this ventricle is quite thin because the pressure under which it works is very low as compared to the other side of the circulation and the heart.

The blood then goes straight through the pulmonary valve into the pulmonary artery where it goes straight through progressively smaller vessels until it travels straight through the capillaries that surround the air sacs (alveoli) of the lungs where the oxygen that is brought in straight through the respiratory ideas combines with the hemoglobin of the red blood cell and the dissolved carbon dioxide is expelled straight through the air sacs of the lungs and exhaled straight through the larger and larger tubes of the lungs called bronchioles and bronchi and ultimately the trachea or wind pipe. The oxygenated blood is then brought straight through larger and larger veins from the pulmonary circulation to the pulmonary veins that associate and the well oxygenated blood is then emptied into the left upper chamber (left atrium) of the heart. The blood then travels straight through the bicuspid or mitral valve and enters the left lower chamber of the heart (the tricuspid valve) where it is then pumped out straight through the aortic valve into the aorta. Here the first vessels that receive the riches oxygenated blood are the coronary arteries, or the arteries to the heart.

The heart does all of the work or circulation and so it needs the best blood that the body can produce. This is why the coronary arteries come off the aortic arch right after the blood is pumped from the left ventricle. The wall of the left ventricle is many times thicker than the wall of the right side of the right ventricle. The blood pressure is many here in the left ventricle because of the force that has to be exerted by the lower left chamber of the heart to pump the blood back out to the whole body where the circulation becomes progressively added and added from the center of the heart and the size of the vessels becomes smaller and smaller in order to get the used blood back for resupplying the hemoglobin molecules with oxygen. The whole process of circulation takes roughly one dinky from the right side of the heart back to the right side of the heart again. This process repeats itself thousands of times an hour and millions of times in a day. Without the allowable circulation in all of the limbs and the extremities happening regularly, the cells and tissues of the body will shrivel up and die. The blood also carries all of the nutrients the discrete cells of the body need and transports waste that is produced by the cells from general functioning to the liver where it is detoxified and eliminated in the bile or in the urine as nontoxic waste straight through the bowel or straight through the kidneys and the urine.