Respiratory Diseases and the Fire Service - IAFF

12 Chapter 1-1 • Anatomy
The wall of the alveoli is primarily made up of two types of cells, the type I
pneumocytes and type II pneumocytes. There are also alveolar macrophages
(involved with defense) found in the alveoli or attached to the wall. The cells
are described in detail below. Because the alveoli are designed to easily expand
when we breathe in and collapse when breathing out there is a risk that
the thin walls would stick together. To prevent this there is a layer of a protein
called surfactant coating the alveolar membranes. Surrounding the alveoli
is a complex network of capillaries that carry the blood and red blood cells
through the lungs to pick up oxygen and discard the carbon dioxide. Between
the capillaries and the alveoli cells is a layer of protein called the basement
membrane and the pulmonary interstitium. The latter contains a variety of
cells, collagen and elastic fibers that facilitate the expanse of the lungs.
Parenchyma
The definition of parenchyma is: The tissue characteristic of an organ, as
distinguished from associated connective or supporting tissues. The majority
of the lung tissue consists of the airways and gas exchange membranes as
discussed above. There is some interstitial tissue between the alveolar cells
and the capillary wall.
Cell Morphology and Function
There are many different types of cells found in the airways of the lung. A
variety of functions are performed by these different cells. For example some
cells are present for physical support, some produce secretions and others
defend the body against infection. Approximately 50 distinct cells have been
identified in the airways. 1 Below you will find a brief description of some of
the important cell types.
Type I pneumocyte: These are the flat epithelial cells of the alveolar wall
that have the appearance of a fried egg with long processes extending out
when seen under a microscope. They account for 95% of the alveolar surface . 2
Type II pneumocyte: These are the cells responsible for the production of
surfactant; the protein material that keeps the alveoli from closing off during
exhalation. They are rounded in appearance. The surfactant is stored in
small sacks called lamellar bodies. They are also involved with the regulation
of fluid in the lungs. 1
Alveolar macrophages: These are cells that clear the lung of particles such
as bacteria and dust. They enter the alveoli from the blood through small
holes in the wall called the pores of Kohn.
Smooth muscle cells: As discussed above the airways down through the level
of the terminal bronchioles contain bands of smooth muscle. The muscle
cells are controlled by the autonomic nervous system and chemical or hormones
released from other cells such as mast or neuroendocrine cells. 1 The
contraction of the muscle cells leads to a narrowing of the airways.
Ciliated epithelia cells: The lining of the majority of the airways is composed
of pseudostratified, tall, columnar, ciliated epithelial cells. The cilia
are hair-like projections on the surface of these cells that beat in rhythmic
waves, allowing the movement of mucus and particles out of the lungs. This
mechanism is also a defense mechanism against infection.