The Lower Respiratory System

We use our Respiratory System to breathe.  We obtain oxygen from the outside air by inhaling, and exhale carbon dioxide out of our body.  In essence, all our body’s cells need oxygen and sugar to generate their own energy, just like a fire needs oxygen & fuel to produce heat.  The fire’s waste products from its process are smoke & ash; the body’s are carbon dioxide and urine.

The Respiratory System is a system of passageways (Diagram A).  Air enters through the Nose or Mouth, continues down the back of the Throat, and then the Trachea.  The Trachea dead-ends, as air passes into both Lungs through the two main Bronchi (singular = Bronchus).  Inside each Lung, Bronchi branch out into small and smaller subdivisions, the smallest are called Bronchioles.  Bronchioles end at microscopic air sacs called Alveoli (singular = Alveolus).  On autopsy, this all looks like a giant upside-down broccoli, with the tiny flower buds representing Alveoli.  The Upper Respiratory Tract ends at the Trachea; the Lower Respiratory Tract consists of the Lungs (Diagram B).

Diagram A

NOTE — For diagrams and discussion of the Upper Respiratory System, click the link.

Diagram B

The lungs are covered by Pleura, a.k.a. Pleural Membranes, which attach to the Ribs and to the Diaphragm (the muscle separating chest from abdomen) (Diagram A).  We breathe as our chest muscles and the Diaphragm pull to stretch the lungs wider, so air rushes in.  When those muscles relax, the lungs recoil back to their original size, and air gets exhaled.

It’s in the Alveoli where oxygen passes into the bloodstream, to travel throughout the body to all our cells.  Carbon dioxide passes from the blood into the alveoli, to be exhaled.  In order to maximize the area for this to occur, alveoli have lots of folds.  Once years of smoking have damaged the alveoli, there’s less area available.  So the person never gets enough oxygen, & always retains too much carbon dioxide; that’s Emphysema, which is incurable (Diagram C).  Pneumonia is an infection of alveoli in just part of a lung; the most common kind, caused by bacteria, fills them with pus.  Covid-19, and Influenza (โ€œthe Fluโ€) when serious, also involve the alveoli.

Diagram C

A variety of diseases affect other parts of the respiratory system.ย  Asthma involves irritation of the bronchi, causing them to go into spasm, swell, & produce mucus, all of which cuts down on the space available for air to move in and out. Diagram D below depicts Bronchospasm,ย (which, when heard by stethoscope, has its special sounds)

Diagram D

Other diseases of the bronchi besides Asthma include Bronchitis (caused by viruses, acts like asthma), Bronchiolitis in small children (viral infections of the bronchioles), and Pertussis (whooping cough).  COPD (Chronic Obstructive Pulmonary Disease) can both irritate the bronchi (like asthma), and also cause chronic low-grade bacterial growth (all parts of the lung are normally completely sterile).

Some uncommon diseases make it hard for the lung to expand when trying to inhale.  There are many kinds of Interstitial Lung Disease in which the tissue surrounding the alveoli becomes tight and fibrous.  The space between the pleura can fill with fluid, called a Pleural Effusion, for a variety of reasons.  And a number of uncommon Neuromuscular Diseases, of muscle or nerves, can interfere with the ability of the diaphragm and chest to take deep enough breaths.

Finally, diseases of the nose and throat like the Common Cold, Allergies or Sinusitis, often cause a cough.  This can be uncomfortable, and make you worry that something’s wrong with the lungs.  But since the lungs aren’t involved, you’re always getting enough oxygen, even though you may feel like you can’t breathe because of coughing fits, or because the nose is so stuffed up.

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