List and describe the five phases of fetal lung development
Explain the function of pulmonary surfactant its role in lung development
Describe high-risk conditions associated with delivery and the potential complications that may occur to the infant
The development of the pulmonary system begins soon after conception and continues well into the pediatric years. Lung development takes place in 5 stages. The first stage is the embryonic period. This period of development covers the first six weeks of gestation. At roughly 21 days, the embryonic disc elongates and becomes broad at the cephalic end and narrow at the opposite end. The endoderm forms a tube-like structure, shaping the future gastrointestinal tract. The ectoderm is also developing into a cylindrical tube, forming the future central nervous system. As the gut develops, the upper portion forms the early oral and nasal openings, while the lower segment forms the pharynx and foregut. The nasal cavities now develop from the ectoderm. The pharynx begins development near day 21; the earliest development of the lung begins at 24 days following conception.
The second stage is the pseudoglandular period, which covers weeks 7-16. By week 7 of gestation, the tissue that will form the epiglottis is present. Above the epiglottal tissue, the arytenoid tissues begin developing, eventually becoming the opening to the lower airways. Continuing at week 7, the oropharynx disintegrates at the choana, opening the anterior nasal cavity to the pharynx. Failure of this membrane to disintegrate results in a blockage known as choanal atresia, which will be discussed in more detail later in the course. By week 11, cartilage begins to appear in the airways and continues to form from that time. The major lobes of the lungs are identifiable by week 12. The bronchial glands begin development during week 13 and complete their development by week 24.
The third stage is known as the canalicular period and covers weeks 17 through 26. During the canalicular period the terminal and respiratory bronchioles continue to multiply. It is during this period that the fetal lung undergoes a tremendous amount of vascularization. As the period advances, small outpouchings begin appearing along the walls of the respiratory bronchioles, eventually becoming the alveoli during the terminal sac period. In these primitive alveoli, the epithelial tissue is beginning to differentiate into its two separate types. Type I will form the alveolar capillary membrane, while the Type II cells will produce pulmonary surfactant. Capillaries are present in proximity to the alveolar cavity during week 20 to 21, but it is not until week 24 to 25 that they are close enough to allow for adequate gas exchange.
The fourth and fifth stages are termed the saccular and alveolar periods covering from week 27 to term (40 weeks) and into the post-term period. By week 24 to 26, the lungs have been completely formed. True alveoli, with the terminal airways called saccules, have not yet formed. True alveoli appear around weeks 32-34, developing from a thinning of the terminal air saccules. The number of alveoli continues to increase until the approximate age of 8 years old. It is during the alveolar period that pulmonary surfactant is produced in increasing amounts by the Type II alveolar cells.
Surfactant is the substance found on the alveolar wall that lowers surface tension. Due to its unique composition and the fact that the amount remains stable in the alveoli, surfactant exerts a varying influence on the alveoli as they enlarge and shrink. As the alveoli are stretched during inspiration, the surfactant thins on the surface and tension builds. This aids in the process of passive exhalation, allowing surface tension to constrict the alveoli back down to a small size. As the alveoli become smaller, the surfactant thickens on the alveolar surface, weakening surface tension and preventing the alveoli from collapsing.
Watch the following video to gain a better understanding of surface tension and the role of surfactant in the lung. (Total time 12:12 minutes)
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The embryologic development of the fetus is an extraordinary event that begins with two cells and finishes as a fully developed child with all the intricacies of the body systems in place. The lungs, which develop from the endoderm, begin development at 24 days, they undergo changes until most lung structures are developed by 40 weeks.
The fetal lung produces fluid throughout gestation, which helps maintain the size, shape, and patency of airways and spaces within the lung. Normally, the fluid is expulsed during vaginal delivery, however, excessive amount of lung fluid may be retained in the lungs following cesarean section deliveries. Many cesarean deliveries may lead to a syndrome called transient tachypnea of the newborn (TTN), which will be discussed later in the course.
High Risk Conditions
Placental abruption is the separation of the placenta from its implantation into the uterine wall before the birth of the infant. Partial or complete abruption is a serious condition because it results in the abrupt cessation of gas exchange between the infant and the mother. The mother is also at risk of life-threatening hemorrhage following an abruption.
Dystocia is the prolongation of labor secondary to uterine, pelvic or fetal factors. Dystocia is present when the first and second stages of labor exceed 20 hours. As the length of labor increases, the chances for infant morbidity and mortality are increased.
Prolapse of the Umbilical Cord
When the umbilical cord passes through the cervix into the birth canal ahead of the presenting infant, it is called prolapse of the umbilical cord. This problem is common during breech deliveries. A breech delivery is when the infant presents buttocks first.
Cesarean deliveries are common procedures at the hospital, with some physicians even allowing the mother to choose to have a cesarean delivery or a vaginal delivery. A cesarean delivery should only be performed in the presence of a prior cesarean delivery, dystocia, breech presentation, and fetal distress. Most cesarean deliveries result in the delivery of a healthy infant, however, there are many risks to the mother and infant as this is a surgical procedure. A common complication of a cesarean delivery is transient tachypnea of the newborn, this syndrome is thought to be caused by the retention of lung fluids.
One of our greatest assets in caring for the sick neonate is the ability to predict those fetuses at high risk. Early in the development of perinatology it becomes clear that certain factors both maternal and fetal were present whenever a distressed infant was born. Some high risk factors can be identified on the first office visit, as the pregnancy advances, during labor, or during the post delivery period. The ability to anticipate a distressed fetus will better prepare the practitioner to care for the infant.
For this assignment, provide detailed answers the following questions:
1. Discuss the developmental events of the respiratory system in utero.
2. Describe the process of which the cardiac system is developed in utero.
3. Discuss the role of the placenta in utero and potential complications.
4. Discuss the difference between fetal circulation and extrauterine circulation.
5. List and discuss two potential cardiac problems that may be present at birth due to improper closing of valves.
Provide your answer in 500 words or more. You must submit the assignment in IWG format including two peer reviewed references.
Please proceed to the remaining items for week one when you are ready:
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