5: Respiratory System - Pt. 2

Respiratory System Organization

• The respiratory system is divided into two portions: conducting portion and respiratory portion.
• The conducting portion includes the nasal cavity, larynx, trachea, bronchi, and bronchioles.
• The respiratory portion includes respiratory bronchioles, alveolar ducts/sacs, and alveoli.

Lecture Learning Objectives

• Recognize and describe respiratory epithelium and its structural features (e.g., nasal concha, epithelial type).
• Visually identify olfactory epithelium.
• Visually identify the trachea, bronchi, bronchioles, respiratory bronchioles, alveolar ducts, and alveoli.
• Describe the development of each segment of the lower respiratory system, including structure, terminology, tissue origins, and sequence of events.
• Explain how left and right lungs differ during development.
• Associate major events in respiratory system development with weeks/periods.
• List requirements for normal lung development and explain fetal breathing.
• Describe tracheoesophageal fistulas including anatomical cause, terminology, and symptoms.

Identifying Characteristics

• Conducting portion: nasal cavity, larynx, trachea, bronchi, and bronchioles.
• Respiratory portion: respiratory bronchioles, alveolar duct/sac, and alveoli.
• Additional questions related to identifying characteristics include: What type of epithelium is present? Is cartilage present? Is cartilage continuous?

Upper Respiratory Tract Histology

• Nasal cavity is structured to condition and filter air.
• Nasal conchae increase rotational movement of air to warm and moisten.
• Respiratory epithelium is pseudostratified ciliated columnar epithelium that filters air.
• Goblet cells produce mucin for moisture.
• Lamina propria is highly vascularized for warmth.
• Olfactory epithelium is for special senses.

Trachea

• Function is conduction only.
• Lined with respiratory mucosa- pseudostratified ciliated columnar epithelium and numerous goblet cells.
• Contains numerous seromucous glands that produce watery mucus in lamina propria.
• Supported by C-shaped hyaline cartilaginous rings for flexibility.

Bronchial Tree Histology Defining Characteristics

• Bronchi: epithelium is pseudostratified ciliated columnar, cartilage is hyaline and continuous, muscularis mucosa is present.
• Bronchioles: epithelium is simple ciliated columnar to simple cuboidal, cartilage is absent (mostly) -continuous walls
• Respiratory bronchioles: epithelium is simple cuboidal, cartilage is absent. Not continuous - opens to alveoli and alveolar ducts.
• Alveolar ducts: epithelium is simple squamous, cartilage is absent, Not continuous - walls open to alveoli
• Alveoli: epithelium is type I (simple squamous) and type II alveolar cells, cartilage is absent.

Conducting to Respiratory Transition

• Bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Respiratory Portion

• Respiratory bronchioles: openings in wall but thicker epithelium.
• Alveolar ducts: open spaces with thin walls.
• Alveolar sacs: border may be continuous/open.
• Alveoli: Functional part of respiratory system.

Respiratory Embryology

• Respiratory system develops between weeks 3 and 4.
• Foregut endoderm forms epithelial lining.
• Foregut mesoderm forms connective tissue, cartilage, and smooth muscle.
• Laryngotracheal groove appears on ventral wall.
• Laryngotracheal diverticulum becomes larynx, trachea, and lungs.
• Proximal portion becomes larynx, middle portion trachea, and distal portion lungs.
• Respiratory buds divide into primary bronchial buds that develop into primary, secondary, and tertiary bronchi.

Pleura

• All pleura is derived from lateral plate mesoderm.
• Visceral pleura is derived from splanchnic mesoderm.
• Parietal pleura is derived from somatic mesoderm.

Lungs

• Lungs mature relatively late (2-7 months gestation).
• Pseudoglandular period (wk 5-16): All portions of the bronchi and bronchial tree are formed but not respiratory bronchioles/alveoli,
• Canalicular period (wk 16-26): Airways increase in diameter and vasculature develops, primitive versions of respiratory portion develop.
• Saccule period (wk 26/4-birth): Terminal sacs develop with type I and type II cells.
• Alveolar/Prenatal-Childhood period: “Breathing” of amniotic fluid occurs in utero, alveoli mature from 0-3 years, and increase in number from 3-8 years.

Clinical Relevance- Tracheoesophageal Fistula (TEF)

• TEF = abnormal communication between trachea and esophagus.
• Occurs in 1/3000 to 1/4500 births, predominantly in males.
• Often associated with esophageal atresia.
• Causes incomplete division of cranial part of foregut.
• Symptoms include choking, coughing, regurgitation, reflux; may result in pneumonia or pneumonitis.

Requirements for normal lung development

• Adequate thoracic space.
• Adequate amniotic fluid volume.
• Fetal breathing movements.
• Fetal breathing movements occur in utero to stimulate lung development, with fetal lungs being ½ filled with fluid from the amnion and pulmonary glands. At birth, fluid moves out through nose and mouth—pulmonary arteries, veins, or lymph vessels.

Test your knowledge on the organization of the respiratory system, focusing on the conducting and respiratory portions. This quiz covers key structures, developmental aspects, and features of the respiratory epithelium. Challenge yourself to identify various parts and understand their functions.