The Air Conducting Pathways:

The upper air conducting passages:  nostrils (nares), nasal passages (nasopharynx), conchae (turbinates), buccal cavity, oropharynx, larynx (and additional structures associated with the passages... pharyngotympanic tube, tubal tonsils, pharyngeal tonsils (adenoids), palatine tonsils, lingual tonsils... noting the "ring" the tonsils form around the oropharynx.


The lower air conducting passages: ( trachea, bronchi, bronchioles )

The trachea divides into the left and right primary bronchi under the collar bone.  A primary bronchus enters each lung. Secondary bronchi branch from the primary bronchi and enter each of the 5 lobes of the lungs, 3 lobes on the right and 2 lobes on the left.  The lobes represent the functional parenchyma of the lungs and are separated by "septa." Tertiary bronchi branch from the secondary bronchi and enter the segments of the lobes.  Bronchioles have readily apparent cartilage.

The Lobes of the Lungs:

The tertiary bronchi lead to the primary bronchioles, and then to the terminal bronchioles.  The terminal bronchioles are known as such because they are the last of the purely "conducting" passages, and, as such, are the last of the air passages in the so-called "conducting zone."  No gas exchange of significance occurs in the air passages of the "conducting zone."

The terminal bronchioles now pass into the respiratory bronchioles.  The thin-walled respiratory bronchioles are the first air passages of the "exchange zone," as significant gas exchange occurs across the walls of the respiratory bronchioles (about 1% of total gas exchange).  The respiratory bronchioles enter the alveolar ducts, which are central airways within alveolar sacs, and lead to the individual alveoli within the alveolar sacsThe alveoli along with the alveolar ducts represent the major gas exchange area, with about 99% of gas exchange occurring within alveolar sacs.

Alveoli, Alveolar Sac, Respiratory Zone

The total volume and surface area of the conducting pathways are small relative to the respiratory (exchange) area. In fact, the entire volume of the conducting pathways (as far as the terminal bronchioles) is about 150 mL, less than half the volume of a 355 mL soda pop can.  Thus, inspired air travels quickly through the "conducting zone" and into the "respiratory zone."

The lungs are covered by pleural membranes (visceral pleura) and limited connective tissues provide support.  A pleural membrane lines the pleural portion of the thoracic cavity (the parietal pleura).

epithelial mucus/ciliary apparatus of the conducting pathways

Goblet and glandular cells beneath the epithelium produce mucus that lines the epithelial layer of the air conducting pathways.  Goblet cells are surrounded by epithelial cells with lots of cilia.  Mucus produced by the Goblet and gland cells is moved by the cilia from the bronchi to the trachea and towards the mouth.  The mucus is propelled through the conducting pathways as fast as 1 cm/min.  Mucus moves more slowly, or in the wrong direction in smokers as smoking eradicates the cilia population.  Smokers must clear most of this mucus by coughing it up!  Coughing due to the irritation of ciliary re-growth and subsequent cleaning of the airways may be associated with cessation of smoking; the cardiovascular and respiratory benefits of quitting are immediate and long-term.

When you breathe, you essentially inhale a lot of junk.  The ciliary system is required to remove "particulate matter."  Upon inspiration, the majority of particulate matter is deposited or caught on epithelial surfaces in the upper respiratory tract.  It is important to recognize that the surface area of the respiratory tract has more than 25 times the surface area of the skin... and so the respiratory tract is exposed to a lot of nasty stuff in the air moved in and out of the lungs.  Smaller particles may be deposited in the trachea and bronchi and very fine particles (dust) may reach the alveoli.  The upper respiratory tract has epithelial cells with cilia that move mucus back towards the mouth, from which the mucus can be swallowed.  Below the epiglottis, the cilia "sweep" towards the superior end of the trachea.  The cilia always move material away from the lung parenchyma.  If the cilia do not function properly there is increased disposition to lung disease; where the malfunction is associated with smoking there is also increased disposition to lung cancer.

Abrupt or large temperature changes slow cilia movement; you may have noticed that when you step outside on a really cold day, your nose starts to run as the cilia stop sweeping mucus back towards the mouth!  Drugs may affect ciliary action; caffeine stops ciliary movement.  If a smoker has not already destroyed their ciliated cells, smoking just a few cigarettes each day can keep their cilia paralyzed all day long!  Heavy smokers, like the one that lives on the hill below me, can be heard coughing, almost violently, every morning in an attempt to expel mucus accumulated in the air passages overnight.  The finest particles deposited in the alveoli are removed by phagocytic cells (macrophages) delivered to the alveoli by the bloodstream.  The phagocytes engulf particles and are then carried away by the blood or lymph.

Black Lung:  This term was originally used in the Appalachian Region to describe the appearance of black coal dust in the lungs of coal miners.  The appearance of coal dust in the lungs, and the resultant irritation of the lining of the lung, is referred to as anthracosis (in reference to hard anthracitic coal that gives off a lot of dust as it is broken apart).  The term black lung is now commonly used to refer to grossly visible particulate matter in the lungs.

While the effects of coal mining on the ecosystem and health have a questionable history, coal miners of the southern Appalachian region performed back breaking labor that brought a new level of prosperity to a previously isolated region.  Often sacrificing their health, miners paved the way for educational and industrial development in the region... and for the benefit of future generations.  While many coal miners lost their lives in mining accidents, many continue to suffer today with black lung.  Below is a picture taken in 1914, depicting just one of the hazards of coal mining!  Many of the coal shafts were not as large as the collapsed shaft below.  Many miners spent their days digging, eating, and using the washroom while lying flat in tiny digging shafts.


air flow patterns

Air flow patterns are somewhat like blood flow patterns in that they tend to be laminar... that is, moving in concentric layers through the airways, with slower layers of air towards the wall of the airways, and faster layers towards the center of the lumen.  Turbulent airflow tends to occur at areas of bifurcation of the conducting pathways, especially when the volume of air flow is increased.  Turbulent flow is primarily restricted to the larger conducting pathways, primarily the trachea and larger bronchi and is noted especially during vigorous exercise.  Generally, only laminar flow occurs in the smaller airways, but there are so many branches (bifurcations) that we see "eddies" (transitional areas) of turbulent/laminar flow around areas of bifurcation.  Laminar flow can become turbulent flow in some pathological conditions; for example, a tumor growing in a bronchus or the inflammation associated with bronchitis could increase the likelihood of turbulent air flow.  Snoring usually involves turbulent air flow and vibration around the uvula.

Poiseuille's Law:  We are fortunate to have Poiseuille's Law to better understand how increased resistance to laminar flow through the conducting pathways can result in turbulent flow.

R = 8nL/pr4 ...  which simplifies to R is proportional to 1/r4

R = resistance to air flow
n = viscosity of air (fairly constant)
L = length of the bronchus or bronchiole (fairly constant)
r = radius of the bronchus or bronchiole (changes)

We used a slightly different version of Poiseuille's Equation when we considered the effects of vessel diameter on blood flow.  Try Poiseuille's law with some imaginary bronchi and bronchiole lengths and radii.  Note the dramatic effects of radius on resistance to airflow.  Remember that resistance is inversely proportional to flow, such that increased resistance with decreased vessel diameter means reduced airflow.  Note again that, as resistance is inversely proportional to the radius to the 4th power, resistance to laminar flow is increased by a factor of 16 with a 50% decrease in radius of an air pathway.  For example, if a child was having an asthma attack and the primary bronchioles were constricted by 50%, there would be a 16-fold increase in resistance to airflow through their primary bronchioles.

Example:  Assign an arbitrary control value for airway radius of 1.0.  A 50% reduction in airway radius would mean that the new airway radius would be 0.5.  Now,  according to Poiseuille, that gives us...

R = 1/(0.5)4
R = 1/0.0625 = 16
Therefore, resistance to airflow is increased 16-fold with a decrease in airway diameter (and radius) of 50%.

Reynold's Number:  Experimentally, flow patterns of fluids or gases (eg. blood, air, water, etc.) in pipes (such as a bronchus or bronchiole) or  around airfoils (such as the wing of an aircraft or the "uvula" located on the roof of the mouth, at the superior anatomic margin between the buccal cavity and the oropharynx) are described by Reynold's Number.  Lower Reynold's numbers mean more perfect laminar flow, with Reynold's numbers greater than 3000 indicating turbulent flow!  Snoring is associated with Reynold's numbers in excess of 3000 in the regions bypassing the uvula.  In some "snoring clinics," a series of 2 laser surgeries are sometimes performed to correct snoring.

David Currie.
Copyright 2000. All rights reserved.
Revised: January 05, 2009