Download Cells of the Respiratory System

Cells of the Respiratory System
Epithelial Cell Types
Respiratory epithelial cells include three types: a) ciliated cells, b) goblet
cells, and c) basal cells. Ciliated cells have cilia on the surface, Goblet
cells, shaped like a wine goblet, secrete mucus. Basal cells, found at
the basal region of epithelial sheets, are stem cells used to regenerate
and maintain a healthy epithelial cell layer.
• Cilia
The apical surfaces of ciliated epithelia cells possess specialized
structures called cilia. Depending upon the tissue cilia may function
in either sensory or mechanical capacities. The cilia in the nasal
cavity and bronchial tree sweep mucus toward the pharynx to be
swallowed or expectorated. Swallowing delivers the microorganisms
and particles that were trapped in the mucus to the stomach where
the low pH of the stomach will destroy them.
•
Macrophages in the lung
The respiratory system interacts with the external environment
during gas exchange. This interaction can provide contact with
pathogens (viruses, bacteria, and other disease causing organisms),
atmospheric debris and other particulates. We have already covered
how the sticky mucus traps many pathogens and particles and
facilitates removal from the body. Yet the mucus does not trap all
inhaled particles. Particles that make it to the level of the alveoli are
typically removed by alveolar macrophages through the process of
phagocytosis.
•
Alveolar cells
In the alveoli there are specialized epithelial cells called alveolar
cells. Type I alveolar cells are very thin, simple squamous cells
through which gases easily diffuse. These cells are so thin that they
can only be seen through the use of an electron microscope. The type
I epithelial cells also make angiotensin-converting enzyme (ACE), an
important enzyme of the renin-angiotensin system used in the
control of blood pressure. Inhibition of this enzyme is one of the
methods used to control hypertension. Scattered among the
squamous cells are type II alveolar cells. Type II alveolar is cuboidal
epithelial cells with microvilli on their apical surface. These cells
make and secrete surfactant, which decreases the surface tension on
the alveolar surfaces. The alveoli themselves are so thin that without
the surfactant the force of surface tension created by the water on
the cell surfaces would cause the alveoli to collapse upon exhalation.
Mixed in among the types I and II alveolar cells are macrophages
that migrate within the sacs and clean up material that has entered
this part of the system. These cells may accumulate and sequester
non-biodegradable material that persists in the cells. Cells that
contain accumulated material are referred to as dust cells. The
alveoli are considered to be functionally sterile, due to the
combination of the mucus and cilia found through most of the
respiratory membranes and the macrophages in the alveoli.
Epithelial Types Throughout the Respiratory system
In the nasal cavities and upper respiratory tract, epithelial cells are
primarily ciliated psuedostratified columnar epithelium. The bronchial
tree is lined with pseudostratified, ciliated, columnar epithelia with
goblet cells dispersed among the columnar cells. At the terminal
bronchioles, epithelial become ciliated cuboidal cells without any goblet
cells.
Anatomy of the Respiratory-Cardiovascular Junction
There is intimate contact between respiratory tissue and the blood
supply in the lungs. Alveolar sacs in the lungs are wrapped in capillary
beds of the cardiovascular system. At the cellular level, the simple
squamous alveolar cells are in close contact with the capillaries, which
are also one endothelial cell thick.
Oxygen from the inhaled air diffuses from the alveoli to the hemoglobin
in the red blood cells. In order to do so, it has to diffuse through the
alveolar epithelial cell, the capillary endothelial cell, plasma in the
capillary, and into the red blood cell.