Respiratory System Quiz: Bronchi and Alveoli

Respiratory Epithelium

• Bronchi divide into bronchioles, then membranous bronchioles, and finally terminal bronchioles
• Conducting to respiring epithelium
• The respiratory system can be split into two sections: conducting and respiring
• Conducting portion serves to get air to the respiring portion
• Conducting system includes: trachea, bronchi, and bronchioles
• Trachea has cartilage rings that are C-shaped
• Bronchi start with cartilage rings which transition to smooth muscle
• Bronchioles contain no cartilage and have smooth muscle, lining with respiratory epithelium

Respiratory Bronchioles

• Respiratory bronchioles are the transition zone between the conducting airways and the gas-exchanging alveoli
• Branching occurs to form alveolar ducts and sacs
• Cilia in the respiratory bronchioles disappear distally
• Respiratory bronchioles have alveoli along their walls. Alveoli increase in number as the respiratory bronchioles get more distal.

Alveolar Ducts and Sacs

• Back-to-back alveolar openings along the walls of alveolar ducts
• Smooth muscle between alveolar openings looks like knobs.
• Alveolar ducts open into alveolar sacs

Acinus

• The acinus is the functional unit of the lung.
• Consists of the terminal bronchiole, respiratory bronchiole, alveolar duct, and alveolar sacs

Alveoli

• Tiny air sacs at the end of the respiratory bronchioles
• Walls are thin to allow for gas exchange between air and blood
• Separated by interalveolar septae, which contain capillaries.
• Lined by Type I cells (thin, flat squamous cells), Type II cells (produce surfactant), and alveolar macrophages
• Type I cells cover 95% of alveolar surface, Type II cells cover 5% of alveolar surface

Surfactant

• Surfactant is made by Type II pneumocytes and decreases surface tension in alveoli. This prevents alveoli from collapsing during expiration.
• Decreases surface tension in alveoli, requiring less pressure to keep them open
• If there is no or low surfactant, alveoli collapse.
• Composed of phospholipids and proteins

Pulmonary Alveolar Macrophages

• Found on the surface of alveoli and within the lumen of alveoli
• Derived from blood monocytes
• Remove debris and particles that escape mucus and cilia in the conducting portion of the respiratory tract
• Also called "dust cells"

Interstitium

• Connective tissue between the alveoli
• Consists of the basement membrane of the endothelial and epithelial cells, collagen and elastic fibers, fibroblasts and a few mononuclear cells

Respiratory Blood Supply

• The lung receives a dual arterial supply
• Pulmonary arteries are low pressure and branch alongside the bronchi and bronchioles
• Pulmonary arteries divide into capillary networks in the alveolar wall
• Bronchial arteries are high pressure and originate from the aorta
• Bronchial arteries branch alongside the bronchi to the respiratory bronchioles

Parietal and Visceral Pleura

• The outer surface of the lung and the inner surface of the thoracic cavity are covered by the pleura
• Parietal pleura lines the thoracic cavity
• Visceral pleura covers the lungs
• Serous membranes consist of simple squamous epithelial cells called mesothelium plus a thin layer of connective tissue
• The pleural cavity contains serous fluid made by the pleura

Clinical Correlations

• Injury to Type I pneumocytes can lead to pulmonary edema
• Smoking can lead to "smokers’ macrophages"
• Fibrosis in ARDS can cause thickened hyaline membranes and interstitial fibrosis
• Bronchiectasis is a condition where the airways are dilated, mucus secretion and inflammation are increased.

Test your knowledge on the structure and function of the respiratory epithelium, including bronchi, bronchioles, and alveolar ducts. This quiz covers how air travels through the conducting and respiring portions of the respiratory system. Explore the key features from trachea to alveoli, and understand their transitions.