The best way to explain how our lungs work is to think of them as an upside down tree. A long trunk, called the trachea, leads to thick branches. Off those branches, called bronchi, shoot smaller limbs, known as bronchioles. At the ends are the leaves called alveoli.
Through the airways, the trachea, bronchi, and bronchioles, passes air containing about eighteen percent oxygen. It moves down to the leaves, the alveoli, which are tiny, thin-walled sacs. Their thin walls and chemical properties allow them to act like exchange filters, soaking up oxygen and giving up carbon dioxide.
Breathe in, called inspiration, and your blood absorbs oxygen. Breath out (exhalation) and you expel one waste product of the metabolic processes, carbon dioxide. That inhalation process is possible because of two anatomical facts which are muscle strength and tissue elasticity.
How the Lungs Work Diagram
How the Lungs Work when you Breathe
The muscles of the diaphragm (a large sheet just above the stomach and below the rib cage) pull down as you breathe. That causes the lungs to expand, creating what is called negative pressure.
The pressure is not actually less than zero. What the phrase means is that the pressure inside the lungs is lower than it is outside, and air moves from high pressure to low-pressure areas.
Picture it like a balloon in reverse. Inside a balloon, the air pressure is higher than it is outside, so when you open the neck, air rushes out. In breathing, you have the negative situation.
That process in turn only works because the lungs, airways, and alveoli are all elastic. That is, they stretch under tension and return when the tension is lowered again. That circumstance can exist because the lung contains thousands of elastic fibers that surround the airways and line the walls of the alveoli.
Along with those large-scale physical parameters, there are smaller properties that keep the whole system functioning. The alveoli, which collectively would look like a big round sponge, are what are known as semi-permeable membranes. Specifically, they let gases pass back and forth, but keep fluid (the blood) from leaking into the airways.
That gas exchange can occur because the alveoli (remember, these are the tiny sacs that do the transfer) are surrounded by blood vessels. Those blood vessels, called capillaries, are very small-diameter tubes through which the blood passes.
Because of their narrow width, and the relatively large surface area provided by thousands upon thousands of alveoli, the carbon dioxide held in the blood diffuses across the membranes and into the airways. From there, simple exhalation expels the CO2 outside the body.
That cycle - oxygen in, CO2 out - is called respiration and it is a key component of the body's effort to feed all its complex biochemical reactions and dispose of an important waste product.
Lung impairment results when this exchange cannot be carried out efficiently, because of loss of elasticity, destruction of the alveoli, and other effects of lung disease. Optimized lung health comes from exercise, diet, lifestyle choices, and more by maximizing that elasticity and lung capacity.