SEM_07_02_Circulatory pathways. Arterial system. Venous system. Lymphatic system_PARTE1.docx

Circulatory pathways. Arterial system. Venous system. Lymphatic system Learning objectives Understand the vitelline and allantoic circulatory pathways and their contribution to the formation of the placental circulation. Discuss the differences in the prenatal and post-natal circulations. Descri...

Circulatory pathways. Arterial system. Venous system. Lymphatic system Learning objectives Understand the vitelline and allantoic circulatory pathways and their contribution to the formation of the placental circulation. Discuss the differences in the prenatal and post-natal circulations. Describe the origin of the aorta and the development of the major arteries. Describe the origin of the venous system. Understand the lymphatic system formation. Consider briefly some congenital abnormalities in the development of the blood circulation. Embryonic blood circulation circuits In the embryo, as in the adult, the blood circulation is closely associated with the organs where the metabolic activity takes place. The differences in the blood circuits between the embryo/foetus and the new-born are due to the fact that the centres of metabolic activity are differently located before and after birth. During prenatal life, the organs for the digestion and absorption of food, securing of oxygen, and elimination of waste products are located outside of the embryo. Accordingly, the prenatal circulatory system provides a connection between the embryo and its main metabolic organs: the yolk sac and allantois. The vitelline circulation carries blood to the yolk sac, where it is enriched with nutrients, and then returns the food-laden blood to the heart to be distributed within the embryo. - Vitelline blood circuit The allantoic circulation establishes a circulatory connection between the chorioallantoic membrane and the embryo, complementing the respiratory and nutrient capabilities of the vitelline circulation. In birds, both extra-embryonic circuits, the vitelline and allantoic circulation, are complementary and must persist until hatching. In mammals, the allantoic circulation is used to form the placental circulation which, depending on the efficiency of the placenta, can fully assume the full support of the foetus until birth; thereby, the need of a vitelline circuit wanes early in gestation. In addition to these two extra-embryonic circuits (the vitelline and the allantoic circulation), a third intra- embryonic circuit conveys and collects blood from all the various organs of the body. - Allantoic blood circuit The intra-embryonic circulation includes all the distributing arteries that bring oxygen and food material to the embryonic tissues (arterial system), as well as the collecting veins that carry CO2 and waste material back to the heart (venous system). As the complexity of the embryonic organs increases during development, the blood circulatory system must be accordingly redesigned until it reaches its adult arrangement. Therefore, the development of the permanent vessels is a gradual and complex process that requires not only the formation and growth of new vessels but also the remodelling of early vascular layouts that became obsolete over evolution. In summary, embryonic blood circulation involves three circulatory arcs of which the heart is the common centre and pumping station. Two of these circuits, the vitelline and allantoic/placental circulation, carry blood between the embryo and the extra-embryonic membranes. A third circulatory circuit, the intra-embryonic circulation, is confined to the interior of the body and constitutes the origin of the permanent circulation in the adult. - Intra-embryonic blood circuit Vitelline circulation Vitelline circulation is the earliest circulation to develop and refers to the system of blood flowing from the yolk sac to the embryo and back again. Blood is conveyed to the wall of the sac by the right and left vitelline arteries which enter the umbilical cord from their origin at the end of the paired dorsal aortas. After circulating through a wide-meshed vitelline capillary plexus, the blood is returned inside of the embryo by the right and left vitelline veins, which drains blood into the atrium of the heart. This constitutes the vitelline circulation, and by means of it, the nutritive component of the yolk is absorbed and conveyed to the embryo. In laying-egg animals, where a large amount of yolk is present in the sac, the vitelline circulation is of prime importance in supplying the growing embryo with nutritive materials; thereby, the vitelline circulation must persist until hatching in reptiles and birds. In mammals, since little yolk is present, the vitelline circulation does not assume the same important role of carrying food supply. In some mammals like horses, carnivores and rabbits, the vitelline circulation is associated with the development of a temporary vitelline placenta. In other species like ruminants, pigs or humans, the vitelline circulation is rudimentary and is never established as a completely functional circuit. Whether functional or not, the vitelline circulation regresses and disappears during the early stages of gestation in mammals, but its remnants within the embryo will be always fundamental in the formation of some major components of the permanent vascular system. https://sway.office.com/VKE6vIU0DRejS3Dg#content=uSKneoFiZQcXvm - Vitelline circulation Remodelling of the vitelline circulation While the regression and eventual disappearance of the extra-embryonic portions of the vitelline vessels are associated with the regression of the yolk sac, the intra-embryonic portions of the vitelline vessels contribute to the formation of some permanent blood vessels related to the gut and liver. To be assimilated into the permanent blood circuit, the original layout of the vitelline circulation must undergo profound transformations. In summary: The distal portions of the right vitelline vein between the umbilicus and the gut are transformed into the intestinal veins, which largest branch is the cranial mesenteric vein. On entering the liver, the vitelline veins form the portal vein which conveys blood from the gut and placenta into the liver. Within the liver, the vitelline veins are engulfed by the developing hepatic tissue where they contribute to forming a capillary plexus (liver sinusoids). The part of the right vitelline vein between the liver and the heart contributes to the formation of the caudal vena cava (hepatic portion). Most portions of the left vitelline vein involute and disappear. The right vitelline artery becomes the main intestinal artery in the adult, the cranial mesenteric artery. The left vitelline artery normally degenerates. Incomplete degeneration of the left vitelline artery can result in a fibrous band that may cause colic by entrapping a segment of the intestine. https://sway.office.com/VKE6vIU0DRejS3Dg#content=I730fzWoBEQ4aW - Regression of the vitelline circulation The regression of the vitelline blood vessels provides some of the major vessels of the gut and liver Allantoic and placental circulations As the allantois sac leaves the umbilical cord and fuses with the chorion, the allantoic vessels ramify throughout the chorion. In birds, the chorioallantoic membrane lies directly beneath the porous shell membrane, allowing the allantoic circulation to interchange gases with the outside. In mammals, the vascularised chorioallantoic is intertwined with the endometrium forming the placenta, and the allantoic circulation is transformed into the placental circulation. In some species, the placenta becomes so efficient that makes the allantoic useless as an embryonic reservoir for waste disposal. In these species, humans, for example, the allantoic sac becomes rudimentary and never expands outside the embryo, but even in such species, the formation of an allantoic stalk is always necessary for the development of the umbilical vessels and the expansion of the allantoic vessels throughout the placenta. In allantoic circulation, blood is brought toward the placenta by the right and left umbilical arteries. The umbilical arteries arise at the caudal end of the dorsal aorta and leave the embryo through the umbilical cord. Within the umbilical cord, the umbilical arteries are closely associated with the allantoic duct (also called urachus). They terminate in the chorioallantoic membrane where they branch into the allantoic arteries which carry deoxygenated blood to the chorion. After circulating through a rich plexus of chorionic capillaries, where placental interchange takes place, the blood is collected from the chorion and returned toward the embryo by the allantoic veins. On leaving the placenta, the allantoic veins gradually converge and fuse into the right and left umbilical veins which enter the umbilical cord to bring blood enriched with nutrients and oxygen toward the heart. Initially, the paired umbilical veins empty directly into the sinus venosus of the heart. Further, with the development of the liver, the course of the umbilical veins undergoes an important remodelling. The nearest portions to the heart of the umbilical veins involute and atrophy; thus, the oxygenated blood, after passing the liver, gets into the sinus venosus of the heart via the caudal vena cava. Thereby, inside the caudal vena cava, the oxygenated blood from the placenta mixes with deoxygenated blood from the caudal part of the body before being delivered into the heart. The middle portion of the umbilical veins become quickly included within the developing liver, where they contribute to forming the capillary plexus of the liver along with the remnants of the vitelline veins. The portion of the right umbilical vein between the umbilicus and the liver involutes and disappears leaving the left umbilical vein as the only vessel from the placenta to enter the embryo. Consequently, the right umbilical vein is diverted toward the left umbilical vein within the umbilical cord. The precise point where the two umbilical veins converge into one varies between species. In horses and pigs, the two umbilical veins fuse before entering the umbilical cord; therefore, three umbilical vessels are present within the umbilical cord: two umbilical arteries and one (left) umbilical vein. In ruminants and carnivores, the two umbilical veins fuse on entering the abdominal wall of the embryo; therefore, four umbilical vessels are present along the umbilical cord: two umbilical arteries and two umbilical veins. https://sway.office.com/VKE6vIU0DRejS3Dg#content=30msvOx0sgIF4c - Allantoic and placental circulations Because the foetus is not eating, and because the placenta is able to detoxify the blood, it is more desirable for the placental blood to bypass the foetal liver and reach the heart sooner. A shunt inside the liver, the ductus venosus, develops between the left umbilical vein and the caudal vena cava. The ductus venosus diverts blood from the left umbilical vein toward the caudal vena cava without passing through the liver sinusoids. Not all species of mammals possess a ductus venosus throughout gestation. It disappears at an early stage of gestation in horses and pigs. At birth, when the umbilical cord is physiologically cut, the umbilical vessels occlude inside the embryo. Within a week of birth, the neonate's umbilical vein is completely obliterated and is replaced by a fibrous cord called the round ligament of the liver. - In the fetus, the ductus venosus (Arantius' duct) shunts a portion of umbilical vein blood flow directly to the caudal vena cava. Thus, it allows oxygenated blood from the placenta to bypass the liver. If the ductus venosus fails to occlude after birth, it remains open, and the individual is said to have a patent ductus, that is, an intrahepatic portosystemic shunt. This condition is hereditary in some dog breeds (e.g. Irish Wolfhound). Initially, a patent ductus venosus may be unnoticed because it usually does not cause a significant change in the condition of the new-born. Nevertheless, in the long term, a patent ductus venosus results in a congenital portosystemic shunt that allows toxic digestive products to bypass the liver. These toxic agents typically affect the brain resulting in a neurologic disorder at some time during life (hepatic encephalopathy). - Anatomy of dog normal liver and of livers with intra- and extrahepatic portosystemic shunts. Normal development of the liver circulation. Blood flows through the hepatic sinusoids. Intrahepatic portosystemic shunt. Blood bypasses the liver sinusoids and is therefore not subjected to hepatic metabolism. The intrahepatic shunt represents an abnormal connection of the portal vein with the systemic circulation, which is seen inside the liver. Extrahepatic portosystemic shunt. The aberrant connection is located outside the liver. Circulation before and after birth Foetal circulation During foetal development, enriched blood from the placenta reaches the embryo through the left umbilical vein which is the blood vessel with the highest oxygen saturation (80–90%). Blood flows through the left umbilical vein towards the liver where most of it is shunted through the ductus venosus, bypassing the liver, to enter the caudal vena cava. In the caudal vena cava, the blood from the placenta (highly oxygenated) mixes with the blood from the caudal portions of the body (deoxygenated) before entering the right atrium. Therefore, blood entering the heart from the caudal vena cava has a relative oxygen saturation (55-65%). The right atrium of the heart also receives blood from the cranial part of the body with the lowest oxygen saturation (20-40%) via the caudal vena cava. In the right atrium, the two flows of blood, one from the caudal vena cava, other from the cranial vena cava, follow different paths. Most of the relatively-oxygenated (55-65%) blood coming from the caudal vena cava is guided towards the foramen ovale due to haemodynamic factors: pressure in the right atrium is greater than in the left one; thus, most of the relatively oxygenated blood bypasses the right ventricule and reaches the left atrium passing through the foramen ovale. This deflection occurs because the lungs are not functional in the fetus and hence, the best oxygenated blood do not reaches the pulmonary circulation. Subsequently, the relatively- high-oxygenated blood is pumped from the left atrium to the left ventricle and thence into the ascending aorta. This way, the better-oxygenated blood is delivered to the brain and coronary circulation thus ensuring that blood with the highest possible oxygen concentration is delivered to these vital structures. Most of the less-oxygenated blood (20-40%) returning to the heart from the cranial vena cava is directed across the tricuspid valve into the right ventricle. From the right ventricle, the less oxygenated blood is pumped towards the pulmonary trunk but does not reach the lungs, since these are not functional, but is diverted through a fetal blood vessel called the ductus arteriosus into the descending aorta. From the descending aorta, the relatively-deoxygenated blood (40-50% oxygen saturation) is sent to supply the caudal part of the body, including the placenta, since the umbilical arteries are branches of the descending aorta. The placenta, therefore, receives relatively deoxygenated blood from the fetal systemic arterial circulation and its function is to return the blood with the highest oxygen levels to the fetal venous circulation. - Foetal blood circulation before birth Circulation before birth is designed in a way that the oxygen-filled blood from the placenta goes to the most important parts of the body without passing through the lungs. To do this, the better oxygenated blood passes from the right atrium to the left through the foramen ovale so that the left ventricle can pump relatively well oxygenated blood towards the ascending aorta, whose first branches are directed towards the most vital organs (brain and heart ). The less oxygenated blood reaches the right ventricle and is pumped to the pulmonary artery, although only a small portion of this blood circulates through the lungs as most of it flows through the ductus arteriosus into the aorta, preventing pulmonary circulation. Changes in circulation at birth Suddenly, at birth, the environment changes abruptly. By cutting the umbilical cord the placental circulation is switched off. After the rupture of the umbilical cord, contraction of smooth muscle and elastic fibres in the wall of the umbilical vessels closes their lumina to prevent bleeding. The interruption of the placental circulation quickly results in increased levels of CO2 in the newborn's blood. This stimulates receptors in the respiratory centre in the brain and thus the first inspiration. 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. The cut of the umbilical cord and, therefore, the end of the fetal circulation, and the beginning of breathing, and therefore of the pulmonary circulation, determine crucial changes in the arterial pressure which in turn determines the closure of the foetal circulatory shunts: Closure of the foramen ovale: On one hand, the pressure in the right side of the heart decreases as a result of the rupture of the umbilical cord. On the other hand, the first inspiration expands the collapsed lungs and the pressure in the left side of the heart increases as more blood is returned from the lungs via the pulmonary veins to the left atrium. This pressure turnaround in the atria causes the thin septum primum to be compressed against the septum secundum and the foramen ovale becomes functionally closed. Over time, the functional closure of the foramen ovale grows permanent but the closed foramen ovale will remain visible in the right atrium as indentation called the fossa ovalis. If the foramen ovale fails to close, this results in a malformation called patent foramen ovale, which has already considered in the previous chapter. Closure of the ductus venosus: After cutting the placental input, reflexive contraction of the musculature of the wall of the ductus venosus closes this foetal shunt. This closure becomes permanent after 2-3 weeks. Closure of the ductus arteriosus: Reflexive muscle contractions are also responsible for the closure of the ductus arteriosus, immediately after birth. Thus, all the output from the right ventricle is directed to the functioning lungs. After a few weeks or months, this shunt is definitively obliterated, and the remnant is known as the ligamentum arteriosum (arterial ligament). Persistence or patency of the ductus (patent ductus arteriosus) is a congenital anomaly that has already considered in the previous chapter. - Changes in blood circulation at birth At birth, the umbilical circulation is disrupted and the newborn no longer receives oxygen and nutrients from the placenta. With the first breaths, the lungs begin to expand leading to a significant reduction in the pulmonary pressures so contraction of the musculature of the wall of the ductus arteriosus closes this foetal shunt. These changes increase the pressure in the left atrium of the heart and decrease the pressure in the right atrium. This shift in pressure stimulates the foramen ovale to close. - After birth blood circulation in the newborn After birth, pulmonary and systemic circulations become completely separated. Pulmonary circulation uses the right chambers to carry blood between the heart and the lungs. Systemic circulation starts at the left ventricle and ends at the right atrium. It carries oxygen-filled blood to the rest of the body.