Detailed Notes on Bronchioles, Alveolar Ducts, Sacs, and Alveoli

Bronchioles

Serve lung lobules.
Very small, microscopic (or nearly so).
Do not have cartilage in their walls.
Have smooth muscles that are acted upon by the autonomic nervous system to increase or decrease the depth of breathing.

Terminal Bronchioles:
- Walls of simple columnar ET.
- Last structures in the conducting zone.
Respiratory Bronchioles:
- Send air to alveolar ducts.
- First structures in the respiratory zone.
- Walls are made of simple cuboidal ET.
- Some alveoli branch directly off respiratory bronchioles.

Alveolar Ducts:
- Follow respiratory bronchioles.
- Terminate in alveolar sacs.
- Linear tubes of alveoli.
Alveolar Sacs:
- Clusters of alveoli.
- From the outside, look like a bunch of grapes or clusters of balloons.

Functional units of the lung.
Each alveolus is a microscopic gas exchange chamber.
Surrounded by blood capillaries and elastic fibers that stretch during inhalation and recoil during expiration.
The walls of an alveolus and the adjacent capillary walls form the respiratory membrane (aka alveolar-capillary membrane).

Type I Alveolar Cells:
- Make up most of the wall of an alveolus.
- Simple squamous epithelium.
- Very thin, allowing rapid diffusion of respiratory gases, facilitating quick and thorough gas exchange.
Type II Alveolar Cells:
- Produce surfactant.
- Surfactant prevents the moist inside surfaces of the alveolus from sticking together.
Alveolar Macrophages (aka dust cells):
- Resident phagocytes.
- Phagocytize (consume) tiny debris from the air.
- Seen in the microscope, alveolar macrophages usually have frothy cytoplasm and a distinct nucleolus.

When overworked (as happens with smoking), they rupture and release enzymes from their lysosomes.
This breaks down nearby alveolar walls and interstitial elastic tissue, reducing alveolar surface area and elasticity.
When this is rampant, the result is emphysema.