The Lungs 101
The Lungs 101
Lung functions
• Bring
oxygen into the
body and expel carbon dioxide.
Oxygen travels through the wind pipe (trachea),
bronchi and bronchioles to the alveoli (air sacs), where it diffuses through the
thin alveoli walls and adjacent pulmonary capillaries (mini blood vessels) into
the bloodstream; carbon dioxide travels the
reverse pathway, diffusing from the bloodstream into the alveoli and out of
the body via exhalation. This process requires the lung airways to expand and relax
thousands of times a day.pathway, diffusing from the bloodstream into the
alveoli and out of the body via exhalation. This process requires the lung airways
to expand and relax thousands of times a day.
• Defend
against infection by pathogenic airborne microbes and particulates:
• Hairs (cilia),
twisted pathways, mucus-producing Goblet cells.
Trap offenders
• Lysozymes in mucus
/ immune system macrophages. Trap and "eat" microbes,
particulates, escorting them them out of the airways for swallowing
• Antioxidants
defend against inhaled gases, particles and microbes.
Chronic exposure to these offfenders can dramatically increase oxidation in
the airways, with its resulting production of cell-damaging radicals. Lung-protective
antioxidants of note include:
ozone and nitrous oxide (both normally present in lung lining fluid},
glutathione (body's in-house water soluble antioxidant),
Ascorbate (vitamin C), uric acid, resveratol
(in red wine), and α-tochopherol (a form of
vitamin E)
The M2 protein of the flu virus damages cells of the lung's
inner lining, rendering them unable to remove fluid. This increases susceptibility
to pneumonia and other lung diseases. Human airways transfected with virus type
A protein, M2, increased damaging reactive oxygen species
(ROS) , but co-administration of an ester of the antioxidant
glutathione prevented M2 from causing
damage. M2 protein forms a tiny proton channel in the virion (complete infective
form of virus outside host cell) essential for infection / viral replication.(M2
is the target of anti-influenza drugs: amantadine and rimantadine)
FIndings of Sadis Matalon and colleagues, Univ. of Alabama's
Dept of Anesthesiology Pulmonary Injury and Repair Center.
The bronchotrachial tree
• The trachea
(windpipe) divides into 2 main bronchi, one for each lung
- The right primary
bronchus. Divides into 3 secondary bronchi to supply the lungs 3 lobes
- The left primary
bronchus. Divides into 2 secondary bronchi and then tertiary
bronchi
• The bronchi
branch off into smaller tubes called bronchioles
like tree branches (airways branch an average of
23 times). Each of these eventually ends with a microscopic
terminal bronchiole , which divides into the alveolar ducts ,
which end with alveoli (air sacs);
Pathway of Air: Nose => pharynx => larynx
=> trachea => primary bronchi => secondary bronchi => tertiary bronchi
=> bronchioles => terminal bronchioles => respiratory bronchioles =>
alveolar duct => alveoli
"Mucus Trap" / Airway lubrication
Mucus membranes line the inside of your respiratory tract
i.e nose, mouth, windpipe, bronchi, bronchioles. T heir purpose
is to produce mucus (containing mucin) to lubricate the airways and to entrap any
breathed in dust, particulates (e.g. carbon from smoking or long-term inhalation
of city air), or disease-causing organisms (transporting them back to the
pharynx for swallowing), and thus prevent them from accessing the lungs.
If the membranes become inflamed they overproduce mucus
The SMALLER bronchioles do not have cilia or mucus-producing cells, but they
do have another type of secretory cell (called clara cells).
Airway muscles expand / Contract for breathing
Muscle tone of bronchi / bronchioles.
The diameter and length depends on smooth muscle tone of the these airways;
- Bronchi.
Smooth muscle of bronchi contracts during expiration to expel air
- Bronchioles.
If bronchiole muscles contract strongly, they can be almost completely shut
(as can occur in an asthmatic attack);
Clean-up Crew - "Dust Cells"
A veritable "army"
of alveolar macrophages, also called dust cells (macrophage means "Big eater"
from Greek origin) reside in the lumen (inside)
of alveoli and in the connective tissue between them, where
they engulf and digest:
• Dust particles, particulates, and
microbial "escapees" that managed to avoid the "mucus trap" higher up in the bronchotrachial
tree
• Old blood cells from bleeding lungs
As many as 100 million macrophages each day carry their conquests out of the
airways to be swallowed and digested.
Alveoli - "Gas-filled airspaces"
The number of alveoli in an ADULT averages ~480 million.
Aveoli have a total surface area of ~75 square meters [Pubmed ];
their diameter is ~200µm (~2-3 times the width of an adult human hair), increasing
during inhalation
Each alveolus is surrounded by blood capillaries.
Supplied by small branches of the pulmonary artery
Alveolar Cells (aka. pneumocytes - responsible for
oxygen /carbon dioxide
exchange)
Type I - Wall structure cells
- Type I squamous alveolar
cells form the main structure of the alveolar wall. They
cover ~95% of alveoli surface
Type II - Repair and "non-stick" cells
- Comprise ~5% of alveoli
surface. However, there are twice as many as structural
wall cells; 80-90% phospholipids
- Can differentiate
into poor substitutes for type 1 cells. Unable to replicate,
and more susceptible to attack
- Two functions:
(1) Repair alveolar epithelium when
squamous cells are damaged
(2) Secrete "non-stick" surfactant
to coat inside wall of alveoli and smallest bronchioles.
Prevents aveolus walls clinging together as alveolus deflates on exhalation,
which would otherwise make it difficult for re-inflation on the next inhaled breath.
Respiratory membrane
Respiratory membrane separates the alveolus from the
pulmonary capillaries. This thin barrier (0.2-0.5µm)
is comprised of the alveolar epithelial wall, a shared basement membrane and the
capillary endothelium;
