Learning Objectives:
· In a developing fetus, the lungs (organs of the respiratory system) and digestive system are not yet functional.
· All nutrient, excretory, and gaseous exchanges occur through the placenta.
· Nutrients and oxygen move across placental barriers from the mother’s blood into fetal blood. Carbon dioxide and other metabolic wastes move from fetal blood supply to the mother’s blood.
· Before we can understand how fetal circulation differs from post-natal circulation, we need to understand the flow of blood in the post-natal organism.
· Here we will be discussing the flow of blood in mammals (including humans).
• In mammals the heart consists of four chambers; 2 atria(atrium) on the top of the heart and 2 ventricles on the bottom of the heart (Figure 10.1).
• The heart has four valves that prevent blood from flowing backwards; the bicuspid valve (lies between the left atria and left ventricle), the tricuspid valve (lies between the right atria and right ventricle), pulmonary semilunar valve (lies between the right ventricle and the pulmonary trunk) and the aortic semilunar valve (lies between the left ventricle and the aorta).
• Figure 10.1 shows the four chambers, the four valves and the direction of blood flow through the heart.
• NOTE -when viewing the anterior surface of the heart the HEARTS right side is on YOUR left side. In anatomy we always refer to the structures left or right which is often times the reverse of your left and right.
• The sinoatrial (SA) node is a region in the right atria that initiates an electrical impulse that passes to all the other cells of the heart.
• The SA node is responsible for initiating contraction of the heart muscles.
• This is the hearts natural pacemaker.
• The brain is not required to tell the heart to beat. However, the brain and hormones can tell the heart to beat faster or slower.
Flow of Blood
• Blood always enters into the heart through a vein(easy to remember, the word vein has IN right in it - vein).
• That is what all veins have in common, they all carry blood, whether it is oxygenated or deoxygenated, into the heart.
• From veins blood flows into an upper atrium (either the right or left atria.)
• Deoxygenated blood enters the right atria via the largest vein in the body, the vena cava. The superior vena cava carries blood returning from the head and neck. The inferior vena cava carries blood returning from the rest of the body.
• Oxygenated blood enters the left atria via 4 small pulmonary veins.
• As the heart beats the blood moves from the upper atria into the larger lower ventricles (left or right ventricle).
• The blood is then pushed out of the ventricles into large arteries (either the aorta or the pulmonary trunk).
• All arteries carry blood away from the heart (arteries - away).
• That is one thing that all arteries have in common, they all carry blood,whether it is oxygenated or deoxygenated, away from the heart.
• The largest artery of the body, the aorta, carries oxygen rich bloodout from the left ventricle to the body.
• The second largest artery, the pulmonary trunk, carries deoxygenated blood from the right ventricle to the lungs.
• Notice that there is no relationship between deoxygenated/oxygenated and being an artery or a vein!
• The heart is really 2 side by side pumps.
• The right side of the heart receives deoxygenated blood via the vena cava and pumps it to the lungs via the pulmonary trunk and then the left or right pulmonary arteries.
• This blood is rich in carbon dioxide (a waste product of cellular respiration) and low in oxygen.
• In the capillary bed of the lungs the carbon dioxide is removed from the blood and oxygen is added. This is called the pulmonary (which means lung) circuit.
• The now oxygenated blood returns to the left side of the heart via the pulmonary veins.
• The left side pumps the blood out via the aorta to the entire system(from head to toes).
• This is called the systemic circuit.
• As blood travels through the systemic circuit it drops off oxygen to cells that need it (so they can carry out cellular respiration) and picks up the carbon dioxide (a waste product of cellular respiration).
• The deoxygenated, carbon dioxide rich, blood then again returns to the pulmonary circuit via the vena cava.
• Figure 10.2 shows the two side by side circuits and the flow of blood through them.
Click on the image below to trace the path of a red blood cell through the post-natal (adult) cardiovascular system.
• Because fetal lungs are nonfunctional There is no need to send most of the blood through the pulmonary circuit.
• The lungs are collapsed, and the fetus is floating in amnionic fluid. If it could breathe it would inhale this fluid instead of air.
• Two shunting mechanisms ensure that blood almost entirely bypasses the lungs (Figure 10.3).
• Much of the blood entering the right atrium is shunted into the left atrium through the foramen ovale.
• The foramen ovale is a flaplike opening in the interatrial septum.
• The left ventricle then pumps the blood out the aorta to the systemic circulation.
• Blood that does enter the right ventricle and is pumped out of the pulmonary trunk encounters another shunt called the ductus arteriosus.
• This is a short vessel connecting the pulmonary trunk and the aorta.
• Because the collapsed lungs present an extremely high-resistance pathway, blood more readily enters the systemic circulation through the ductus arteriosus.
• The umbilical cord contains three blood vessels: two smaller umbilical arteries and one large umbilical vein (Figure 10.4).
• The Umbilical vein carries blood rich in nutrients and oxygen to the fetus. (again, a vein carrying something IN)
• The Umbilical arteries carry carbon dioxide and waste-laden blood from the fetus to the placenta.
• The umbilical arteries meet the umbilical vein at the umbilicus (navel, or bellybutton) and wrap around the vein within the cord en route to their placental attachments.
• Newly oxygenated blood flows in the umbilical vein superiorly toward the fetal heart.
• Some of this blood perfuses the liver, but the larger proportion is ducted through the relatively nonfunctional liver to the inferior vena cava via a shunt vessel called the ductus venous, which carries blood to the inferior vena cava.
• The aorta carries blood to the tissues of the body; this blood ultimately finds its way back to the placenta via the umbilical arteries.
• The only fetal blood vessel that carries highly oxygenated blood is the umbilical vein.
• All other vessels contain varying degrees of oxygenated and deoxygenated blood.
• At birth, or shortly after, the foramen ovale closes and becomes the fossa ovalis.
• The ductus arteriosus collapses and is converted into the fibrous ligamentum arteriosum.
• Lack of blood through the umbilical vessels leads to their eventual obliteration, and the circulatory pattern becomes that of the adult.
• Remnants of umbilical arteries persist as the medial umbilical ligaments on the inner surface of the anterior abdominal wall.
• The umbilical vein persists as the ligamentum teres (or round ligament) of the liver.
• The ductus venous becomes a fibrous band called the ligamentum venosum on the inferior surface of the liver.
Click on the image below to trace the path of a red blood cell through the fetal cardiovascular system.
