Respiratory System: Bronchi and Bronchioles

• The lower respiratory system consists of the larynx, trachea, bronchi, and lungs

Bronchi Anatomy

• Air passageways branch repeatedly (~23 times) to form the bronchial tree leading to the lungs
• The bronchi are divided into two zones
• One zone is pictured, for anatomical reference
• The other zone is where gas exchange occurs

Structure of Bronchiole Walls

• Main bronchioles share similarities with the trachea in structure
• As bronchioles get smaller, structural changes occur
• Cartilage rings are replaced by plates of cartilage
• Cartilage support is absent once reaching the bronchiole level
• Epithelium changes in smaller bronchioles, with fewer mucus-producing cells and cilia
• Debris removal relies on alternative mechanisms as you descend
• As passageways narrow, the amount of smooth muscle increases, becoming a complete layer in bronchioles

Respiratory Zone

• Gas exchange occurs in the respiratory zone of the lung
• The respiratory zone is characterized by thin-walled air sacs, known as alveoli
• This zone starts at the respiratory bronchioles

Respiratory Membrane

• Alveolar and capillary walls form the thin respiratory membrane
• Blood flows on one side of the membrane, while air flows on the other
• Gas exchange occurs across this membrane easily via diffusion
• Alveoli walls are very thin, only 15 times thicker than a sheet of paper
• The alveolar outside is covered by a 'cobweb' of pulmonary capillaries

Alveoli

• Alveoli consist of three major cell types
• Type I alveolar cells form the major part of the alveolar walls
• Type II alveolar cells are less abundant and secrete a substance
• Type II alveolar secretion plays a role in reducing surface tension of alveolar fluid
• Alveolar macrophages consume bacteria, dust, and debris from the air
• Dead macrophages are moved by ciliary currents to the pharynx
• About 2 million alveolar macrophages are cleared and swallowed per hour