The Air Connection: a Tour of the Lungs

Before birth the lungs are not inflated.  Oxygen and carbon dioxide enter and exit through the mother’s blood via the placenta. In the first 10 seconds after birth, a dramatic changeover begins, and from that point until the last breath is drawn the lungs connect us to this world.  The first gasping breath of a newborn baby signals the beginning of the body’s most vital  and most tenuous relationship with this world – that with air, or more correctly with the 21% of air that is oxygen.

Breathing is something we do without thinking, adjusting rate and depth of respiration to the demand for oxygen by the tissues.  Healthy hearts and lungs rise to the demand, increasing blood flow as needed.  In a sedentary person, exercising muscle has to be trained in order to be able to utilize the increased oxygen delivered.  Unhealthy lungs, however, fail to meet the increased demands of exertion, and may eventually fail to meet oxygen demands at rest. Then, breathing is always rapid and even speaking is laborious.

Decoding lung symptoms

A huge number of problems affect the airways, but only a limited number of symptoms result. They are: 1. shortness of breath, especially with exertion. 2.  cough – either dry and unproductive, or productive of sputum which is either clear, red and frothy, yellow or green, foul smelling, or bloody. 3. chest pain, often associated with the chest wall movement. 4.  wheezing.    In addition, lung tumors can cause a variety of peculiar symptoms outside the lungs – the most common is weight loss; the most exotic are peculiar neurological symptoms.

Because the lung is a living record of the air inhaled over time, a complete patient history is the first step in decoding symptoms. The second step is almost always an imaging study.  X-rays, CT and MRI scans are so good at revealing chest pathology that a skilled chest examination is a dying art. Technology also uncovers lung scars and nodules in patients who have no symptoms. Bronchoscopies and biopsies define abnormalities further. Often, but not always, such findings are benign, evidence of old encounters with fungus spores or of unknown cause. Autopsies also uncover unsuspected lung pathology like mild to moderate emphysema (see side bar) and evidence of long term exposure to pollution and smoke. Lungs of modern city dwellers are speckled with black particles from polluted air. Smokers’ lungs are black and shiny, as if smeared with tar. Smoking avoidance would be the single most effective way to improve many people’s respiratory problems, but a pristine lung at the end of a long life would still be a virtual impossibility.  Breathing is often very dirty work.

The bronchial tree – air’s roadway

The work of breathing begins with muscular contraction.  The diaphragm, a muscular sheet lying horizontally between the abdomen and the chest, contracts downward and expands the chest cavity. The short muscles between the ribs – the intercostals – assist by pulling the chest walls outward. Like a bellows, this chest expansion pulls air into the lungs by means of negative pressure. Air rushes from the nose into the trachea, down hollow tree-like branches called bronchi, into tinier branches called bronchioles, and finally into millions of tiny expandable sacs called alveoli. The bronchial tree is elastic, expanding on inhalation and collapsing back to baseline diameter on exhalation. In asthmatics, the fine muscles in these tubular walls respond irritably to allergens and constrict the passages. Inflammation from infection or inhalation of toxic substances and even rapid breathing of cold dry air can also trigger tightening of the bronchi, called bronchospasm.  Airflow resistance rises in bronchospasm, making the sufferers of asthma and bronchitis cough and wheeze.   Treatment of both conditions aims at dilating the airways by using drugs that relax the smooth muscle fiber and at reducing inflammation of the bronchial lining with steroids and antibiotics.

 The alveoli – where the work gets done

Air flow rushing down the bronchial tree dead ends in the alveoli, expanding them until they resemble very soft Styrofoam.  Fine capillaries course through the thin walls of the alveoli, picking up oxygen and delivering carbon dioxide for exhalation. Airborne particles settle out in the moist alveoli, though they only get that far by surviving the cough reflex and the cilia -the tiny hair-like projections that constantly sweep dust and foreign bodies up and out of the bronchial tree. Alveoli are also the site of pneumonia – infections with either bacteria or viruses that set up shop in thin alveolar fluid.  The organisms excite inflammation and increased secretions, which impair gas exchange and cause shortness of breath, fever and coughing. Patients with pneumonia look and feel terrible. Pneumonia can also involve the pleura, the lining over the lung, adding the misery of pleuritic pain with each breath, and causing fluid to accumulate between the lung and chest wall – a problem called a pleural effusion.

Fluid accumulation in the chest

Pleural effusions sometimes accompany heart failure. Less often, they are blood tinged accompaniments of pulmonary emboli – clots that reach the lung from leg and pelvic veins.  Small clots in the lung block oxygen exchange in their vicinity; large ones kill suddenly, with no warning, by blocking major blood vessels bringing blood to the lungs. Shortness of breath and coughing producing blood tinged sputum are warnings to be heeded, particularly if they occur after a prolonged period of bedrest or sitting without getting up – situations that promote clot formation in the legs.

Sputum- the stuff that gets coughed up

Blood-tinged sputum can be less serious a symptom, since it is a common result of a bloody nose above. But when associated with cough and weight loss, it is a telltale sign of lung cancer.  Yellow and green sputum usually indicate infection. If foul smelling, it may come from a lung abscess.  Pink frothy sputum is a sign of edema in the lungs, usually from heart failure, but also from altitude sickness, and is always accompanied by severe shortness of breath.

The last breath

At the end of life, breathing usually ceases not because the lungs fail, but because the drive to breathe that made the newborn gasp its first breath ceases. All those breaths in between are like footprints left behind. Use them well.


Addendum: Alveoli Gone Wrong

Over a lifetime, especially when chronic airway disease impedes airflow, the alveoli change. They merge making larger spaces with fewer capillaries for air exchange. When severe, even getting enough oxygen in at rest is difficult. This condition is called emphysema, most commonly a problem in chronic smokers, but also a genetic disease in rare people. Oxygen delivered by a mask helps the symptoms for a while, but the condition inevitably progresses.

If alveoli rupture, as they can in emphysema or in congenital conditions that produce enlarged pockets, they leak air into the chest cavity. Air builds up between the lung and the inner chest wall an emergency condition called a pneumothorax that is very painful and potentially life threatening, requiring insertion of a chest tube to drain the air and restore negative pressure to keep the lung inflated. Similarly, a penetrating wound in the chest wall sucks air into the chest cavity, collapsing the lung.  Covering the chest wound tightly to prevent air from entering can save a life.

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