The vascular system, like all other parts of the animal body, attains its full perfection only by degrees. Its organization is very complex, presenting diverse appearances in successive stages of development. Several modern physiologists have endeavored to ascertain the laws according to which the formation of the heart and blood vessels is regulated.
In tracing the changes that occur in the vascular system, there are four principal points which it is desirable to determine: 1) The order according to which the different parts of this system first appear. 2) The arrangement these parts observe at their primitive origin. 3) The relation which exists at different periods between the organs of the greater and lesser circulation. 4) The proportion between the number and capacity of the different orders of vessels in the several epochs of life.
In attempting to ascertain what part of the vascular system is first developed, we are obliged to judge rather from the phenomena that occur in the process of incubation than from any positive observations that have been made in man or mammiferous animals. It may be stated with tolerable certainty that the vessels of the vesicula umbilicalis, which corresponds with the membrane of the yolk, are the first which become apparent.
The omphalo-mesenteric vein opens into the mesenteric vein of the embryo and communicates with the vena porta. The latter vessel constitutes the principal trunk of the venous system at this period and has an enlargement corresponding to the heart's position. This organ appears in the form of an imperfect ring, the left ventricle being first observed followed by the aorta.
<Callout type="important" title="Key Embryonic Development">The development of the vascular system is crucial for understanding fetal circulation and its unique features compared to postnatal life.</Callout>
In proportion as the heart's development proceeds, the auricle becomes divided by an imperfect septum permitting free communication between the two cavities through a large opening called the foramen ovale. The division of the ventricle is effected in a different manner: the right ventricle gradually extends towards the apex of the heart and communicates with the left ventricle, so that initially, the aorta connects to both ventricles.
The peculiarities which have been briefly pointed out in these remarks have a very striking influence on the circulation of the blood. The fetus derives its support during most of its existence from the mother through the placenta. The umbilical vein arising by numerous branches from this body passes in the umbilical cord to the abdomen and then to the liver, where it mixes with the vena porta's blood before circulating back to the heart.
<Callout type="risk" title="Fetal Circulation Risks">Understanding fetal circulation is critical for medical professionals dealing with premature births or complications involving placental function.</Callout>
The aortic circulation in the fetus bears considerable resemblance to postnatal circulation, but there is one great peculiarity: each internal iliac trunk gives off a large vessel called the umbilical artery by which most of the blood contained in the abdominal aorta is sent to the fetal portion of the placenta.
The peculiarities of the circulation of the fetus have evidently a reference to its dependent mode of existence. Thus, the nutritious fluid requires to be carried to and from the mother, explaining the necessity of umbilical blood vessels; as the placenta supplies the office of lungs, there is no occasion for blood passing to those organs, hence the provision of the foramen ovale and canalis arteriosus.
In proportion as the fetus approaches the term of its existence, the mechanism of the vascular system resembles that which is required for supporting extruterine life.
Key Takeaways
- The first part of the vascular system to develop is the vesicula umbilicalis, which corresponds with the yolk sac in birds.
- Fetal circulation differs from postnatal circulation due to unique structures like the foramen ovale and canalis arteriosus.
- Understanding fetal circulation is crucial for medical professionals dealing with premature births or placental complications.
Practical Tips
- Study embryonic development to better understand normal cardiovascular function in adults.
- Recognize the importance of umbilical vessels in providing nutrients from mother to fetus.
- Understand how placenta functions as a lung substitute for fetal respiration.
Warnings & Risks
- Fetal circulation can be dangerous if not properly understood, especially during premature births or placental issues.
- The unique features of fetal circulation may pose risks if medical professionals are unaware of them.
Modern Application
While the chapter focuses on historical embryonic development and fetal circulation, its insights remain relevant today. Understanding these processes is crucial for modern medical practice, particularly in neonatal care and obstetrics. Knowledge of how blood flows through a developing fetus helps doctors manage complications during pregnancy and delivery.
Frequently Asked Questions
Q: What part of the vascular system develops first according to this chapter?
The vesicula umbilicalis, which corresponds with the yolk sac in birds, is stated as the first part of the vascular system to develop.
Q: How does fetal circulation differ from postnatal circulation?
Fetal circulation differs due to unique structures like the foramen ovale and canalis arteriosus that allow blood to bypass the lungs until after birth.
Q: Why is understanding umbilical vessels important in fetal development?
Umbilical vessels are crucial as they provide a direct pathway for nutrients from the mother to the fetus, ensuring proper growth and development before birth.