Foetal Lung Development
A brief outline of the stages of structural development of the pre and postnatal lung:
0 - 7 weeks
- Budding from the foregut
- Developing of primitive pulmonary vein from left atrium
- Developing of pulmonary artery
8 - 16weeks
- Airway division commences
- Adult pattern of vascularization has begun
- Terminal bronchioles
- Complete set of vessels that lead to respiratory bronchioles and terminal sacs
17 - 27 weeks
- 3 generations of respiratory bronchioles
- Primitive saccule formation with type l and ll epithelial cells (Surfactant)
- Capillarization
28 - 35 weeks
- Transitional alveolar ducts formed
- 3 generations of saccules formed
- 36 weeks' Terminal saccules formed
Postnatal 2 months
- 5 generations of alveolar ducts
- Alveoli form with septation
6 - 7yrs of age
- Airways are remodelled
- Alveolar sac budding occursAlveoli form with septation
Upper Airways
- Heterogeneous
- Conduct the airflow
- Do not participate in gaseous exchange
- Made up of nose, mouth, pharynx, and larynx
Their function is to conduct,humidify,warm and filter air into the lungs and maintain patency.
Lower Airways
Made up of conducting airways of the intrathoracic trachea.- Gas exchange parts of the terminal and respiratory bronchioles and alveolar ducts
- Several functions of airway smooth muscle; controls airway lumen; appears to have the ability for airway reactivity in the presence of barotrauma
- Narrowing of the airways increases the resistance to airflow which results in an increase in the work of breathing
Thoracic and respiratory muscles
3 groups of muscles make up the 'respiratory pump' that moves air in and out of the lungs; diaphragm; intercostals and accessory muscles; abdominal muscles.
The diaphragm is innervated by the phrenic nerve and the resultant contraction pulls the muscle downwards and displaces the abdomen outwards and lifts up the thoracic cage.The thoracic cage is then pulled inward. The resulting pressure changes are a reduction in intrapleural pressure and an increase in intra abdominal pressure.
Respiratory muscles fatigue when the energy consumption exceeds the energy available. Fatigue will occur when the Work of Breathing is increased. When there is respiratory muscle fatigue an increase in carbon dioxide and apnoea result.
Postnatal development
Lung size, surface area and lung volume grow for about 2 months postnatally. Control of breathing and neural maturation also continues to about 2 months postnatally. Volume and structural growth continues into early adolescence.
Control of Breathing
This is controlled by neural and chemical factors.
The respiratory centre, in the medulla of the brain, controls breathing. Increases the rate and depth of breathing are detected by stretch receptors in the lungs. The expiratory centre inhibits the inspiratory centre resulting in stimulation of the expiratory muscles.
The respiratory centre sends nerve impulses via the phrenic and intercostal nerves, to the respiratory muscles. Inspiration results from this stimulus. When the stimulus stops, expiration occurs.
An increase in CO2 is detected by the respiratory centre and results in an increase in the rate and depth of breathing.
An increase in blood pressure is detected by baroreceptors in the aorta and carotid arteries resulting in a decrease in the rate of breathing.
Proprioceptors in the muscles respond to movement and stimulate the respiratory centre, increasing the rate and depth of breathing.
The Hering - Breuer Reflex
Stretch receptors in the bronchi and bronchioles respond during excessive Inspiration by sending messages to the respiratory centre to inhibit inspiration.
