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28 Cards in this Set
- Front
- Back
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What is the respiratory system derived from?
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primitive pharynx
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Components of the respiratory system
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1. Trachea
2. Bronchi 3. Larynx 4. Lungs |
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Development of the respiratory system
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The respiratory system begins to develop in the fourth week, when a groove called the laryngotracheal groove forms on the midline of the caudal part of the pharynx.
By the end of the fourth week, the groove elongates and evaginates, to form the respiratory primordium/diverticulum (respiratory diverticulum grows out of primitive pharynx) The laryngotracheal groove divides into two branches, which grow into the lung buds. he pharynx and larynx begin to split, with the development of tracheoesophageal folds, which fuse in the midline to form the tracheoesophageal septum, separating between the ventral respiratory system and the dorsal digestive system. |
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two layers of pharynx and laryngotracheal groove
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1. Endoderm, on the inside
2. Splanchnic mesoderm, on the outside |
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laryngeal inlet
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opening of the respiratory diverticulum, opens into pharynx. During swallowing, it's covered by the epiglottis, which is derived from the pharyngeal arches.
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Development of the Lungs
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By the end of the fourth week, the two bronchial buds growing out of the respiratory diverticulum have elongated to form the primary bronchi. By the end of the fifth week, secondary brochi (3 on the right and 2 on left) have formed. Each secondary bronchus gives rise to a lobe of the lung; the right lung has 3 lobes, and the left has 2 (to make room for the heart).
The secondary bronchi give rise to tertiary bronchi. More and more air cavities form in the growing lungs. |
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What space do the lungs grow in?
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The lungs grow inside the region of the intraembryonic coelum (in between somatic and splanchic mesoderm, forms thoracic and abdominal cavities) that will develop into the pleural cavity. The growing trachea and lungs occupy the middle of the pleural cavity, dividing it into two spaces, called the pericardioperitoneal canals. The lungs grow into the sides of the cavity, called the pericardioperitoneal canals.
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lining of lungs and pleural cavity
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Externally, the lungs are lined with splanchnic mesoderm, which forms the external coating of visceral pleura. The somatic mesoderm, which lines the inner body walls, forms the parietal pleura.
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What do lungs need to develop normally?
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In order to develop normally, the lungs need:
1. Adequate space 2. Amniotic fluid Oligohydramnios (deficiency of amniotic fluid) impairs development of the lungs 3. Movements on the part of the fetus that mimic breathing, through aspiration of the amniotic fluid. In a fetus, the lungs are about 1/2 full of amniotic fluid. The rate of fetal "breathing" accelerates, close to labor, and may be an indication of impending delivery. During delivery, some of the amniotic fluid is pushed out the mouth and nose, and the rest is absorbed in the lymph. |
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Lung Functioning
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Embryonic lungs need to go through a process of maturation before they will work properly. Factors that affect lung functioning include:
1. Long surface area 2. Thin surface area Long and thin surface area enhances the lung's ability to filter blood 3. Adequate vascularization 4. Production of surfactant |
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surfactant
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Surfactant is a complex mixture of phospholipids and some proteins and lipids, secreted by lung cells beginning by week 20. Surfactant forms an oily monolayer between the lungs and the surrounding fluid (alveolar fluid) that lowers surface tension between the air and the alveoli and allows the lungs to expand more easily.
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Maturation of the Lungs
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Maturation of the lungs can be divided into four stages:
1. Pseudoglandular period--weeks 5--17. 2. Canalicular period--weeks 16--25 3. Terminal sac period--weeks 24 to birth. 4. Alveolar period--late fetal period, until childhood. |
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pseudoglandular period -- lung development
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weeks 5--17. Involves development of the major structures. Respiration cannot take place during this stage of development, and a fetus born during this period will not be able to breathe.
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canalicular period- lung development
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weeks 16--25 (note that the four stages of respiratory development overlap each other, chronologically). More smaller airways develop in the lungs. The terminal bronchioles, which later branch into respiratory bronchioles, appear. Terminal sacs, which will develop into the alveoli, appear on the bronchioles. Vascularization increases. Respiration might be possible, towards the very end of this period. From the 20th week onwards, small amounts of surfactant are produced.
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terminal sac period - lung development
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weeks 24 to birth. Many terminal sacs form on the walls of the respiratory bronchioles. The endoderm cells lining the air cavities differentiate into 2 types of cells:
i. Type I alveolar cells--line the air cavities and are the most abundant cell type ii. Type II alveolar cells--secrete surfactant Vascularization increases and the capillaries start to bulge into the air spaces. Two cell layers (type I alveolar cells and the vascular endothelium) separate the blood from the air. The lungs are functional during the terminal sac period, and a fetus born during this time is more likely to survive. |
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alveolar period - lung development
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late fetal period, until childhood. Babies are born with a low number of mature alveoli. During the first years of childhood (until about year 8) the lungs continue to mature as the alveoli increase in number. Septa form across the terminal sacs, splitting them into multiple sacs.
The most important element of lung development is the presence of surfactant and vascularization. Surface area is less important, and continues to increase into childhood, indicating that it is not vital for survival. |
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Esophageal Atresia/Tracheoesophageal Fistula
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When the tracheoesophageal septum fails to develop properly, the trachea and esophagus remain connected.
The most common form of esophageal atresia is when the lower part of the esophagus communicates with the trachea, and the upper part is disconnected. Polyhydramnios (too much amniotic fluid) occurs, because the embryo will not be able to swallow. After birth, a baby with esophageal atresia will not be able to drink without choking. This condition can be diagnosed by insertion of a catheter into the esophagus. Gastric reflux into the lungs will result in pneumonia. Most of these cases can be corrected. Narrowing of the esophagus or trachea occurs, but is rare. |
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Respiratory Distress Syndrome
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Insufficient production of surfactant (common in premature babies) is characterized by rapid, labored breathing. If RDS occurs in a pre-term baby, it is less problematic (because it does not indicate abnormal development). RDS can be treated by administration of thyroxine or steroids (usually steroids), which accelerate surfactant production.
If a doctor expects a mother to deliver prematurely, she may be treated with steroids while pregnant. RDS in a non-premature baby tends to be more severe. Causes may include: 1. Genetic mutations 2. Hyaline membrane disease--a defect in the type II alveolar cells, which causes them to secrete a high-protein fluid, which forms a membrane-like structure that makes the airways more rigid and causes the lungs to be underinflated. |
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Agenesis of the Lungs
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Agenesis of the lungs is characterized by failure of one or both of the lungs to develop. Bilateral agenesis is not compatible with extrauterine life.
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Congenital Lung Cysts
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Characterized by the appearance of fluid or air filled cysts in the lungs, usually found at the periphery, probably caused by some disturbance in bronchial development during late fetal life.
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Lung Hypoplasia
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Markedly reduced lung volume is a life-threatening condition. Most lung hypoplasia is caused by a hole in the diaphragm (congenital diaphragmatic hernia) through which the abdominal viscera protrude upwards and take over the space needed for the lungs. In such a case, in utero surgery can sometimes correct the problem.
Another possible cause of lung hypoplasia is insufficient amniotic fluid (oligohydramnios), which again compresses the space needed for the lungs. |
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Development of the Diaphragm
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The diaphragm is part of a septa that forms in the intraembyronic coelum, dividing it into different cavities.
The intraembryonic coelum forms in the third week. Before folding, the intraembryonic coelum communicates with the extraembryonic coelum. During the head fold, the intraembryonic coelum, which was originally dorsal to the heart, becomes ventral to it, and forms the pericardial cavity. The developing heart sinks into this cavity. In the lateral fold, the two caudal limbs of the intraembryonic coelum come together in the midline and fuse to form the peritoneal cavity. Initially, a layer of ventral mesoderm separates the peritoneal cavity along the midline, but this mesoderm degenerates over time. After folding (still in the fourth week) the pericardial and peritoneal cavities have formed. The pericardioperitoneal canals allows for communication between these two cavities. The pericardialperitoneal canal will give rise to the pleural cavity. A mass of mesenchyme, initially located cranially to the heart, called the septum transversum is relocated to the chest during the head fold, and develops into the diaphragm. |
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3 cavities that develop from intraembryonic coelum
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1. Pericardial
2. Peritoneal 3. Pleural (from the pericardioperitoneal canals) |
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development of separation between pericardial and pleural cavities
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As the lung buds develop, they grow laterally, into the body walls. As they grow, they push into the body walls and divide them into two regions:
1. The lateral thoracic walls 2. Two folds that develop out of the upper part of the pericardioperitoneal canal, called the pleuropericardial membrane. As the lungs grow, the folds lengthen, meet in the midline, and fuse to form a complete separation between the pericardial and both pleural cavities. This process is completed between the 7th and 8th weeks. |
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parts of the diaphragm
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1. The septum transversum, on the ventral aspect
2. The pleuroperitoneal membranes, which have grown and fused ventrally with the septum transversum, and medially with the mesentery of the esophagus, separating between the pericardial and peritoneal cavities 3. The mesentery of the esophagus Develops into the crura, which connect the diaphragm to the body walls 4. Muscular ingrowths from the body wall, which begin to develop from about week 12. Eventually, this muscle replaces most of the pleuroperitoneal membranes. |
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Position of the Diaphragm
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Initially, the diaphragm is very high, at the level of the cervical somites. When it is at this level, myoblasts from the somites migrate into the diaphragm and form muscle tissue.
By about the 6th week, it has descended to the 6th T vertebra. By the 8th week, it's at the level of the lumbar vertebra. |
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Congenital Diaphragmatic Hernia
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Congenital diaphragmatic hernia occurs when holes in the diaphragm fail to close. Organs from the abdominal cavity move into the thorax and compress the heart and lungs.
The holes are usually caused by failure of the pericardioperitoneal canals to close, resulting in holes in the posterolateral aspect of the diaphragm. |
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Eventration of the Diaphragm
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A very rare condition, in which the muscles of the diaphragm are too weak. Viscera from the intestines push the diaphragm up into the thorax and compress the lungs.
This condition is usually less severe than herniation of the diaphragm. |
