Breathing, Part I

You’ve probably noticed already that breathing and respiration have a lot to do with chemistry, and that because you’ve most likely taken it for granted, it is surprisingly complex, dynamic, and interesting… This website is an overview of what happens as you breathe, starting with some basic anatomy and working into chemical principles that govern every breath you take.

Take a look at this picture. A “conducting zone,” or pipeline consisting of the nose, pharynx, larynx, trachea, and bronchi, and terminal bronchioles, allows ventilation (facultated by the diaphragm) and a “respiratory zone,” consisting of respiratory bronchioles, aleolar ducts, and alveoli, permits gas exchanges essential to respiration.

lower respiratory system

Are they like empty balloons?

Maybe you never even thought about it before…

The lungs are very elastic, spongy, soft, organs made out of connective tissue. Together they weigh about 2.5 pounds. As you can see from the illustration, the lungs are dense with tissue, bronchioles, alveoli, capillaries; however, they are mostly air spaces. Each lung, lobed and segmented, occupies its own pleural cavity. The pleura, two membranes separated and lubricated by pleural fluid, line the thoracic cavity (parietal pleura) and the lungs (visceral pleura) and allow smooth breaths. People suffering from pleurisy, often caused by a lack of this pleural lube, feel the excruciating pain of dry, scraping pleural surfaces. Ouch.

The lungs have a natural tendency to recoil to the smallest possible size — so why don’t they collapse?

The answer has mainly to do with the space between the visceral and parietal pleurae, the intrapleural space.

pleural cavity

This fluid-filled cavity is sealed and separate from both the alveoli and other body cavities, and maintains an intrapleural pressure. We know that atmospheric pressure is 760 mm Hg. The intrapleural pressure is 756 mm Hg. This negative (-4 mm Hg) value for pressure is the result of a tug of war between forces acting to pull the lungs away from and push the lungs against the thorax wall (rib “cage”). Because the walls of the intrapleural cavity are pulled in opposite directions, the volume is stretched and the intrapleural pressure decreases.



Surface tension created by pleural fluid Lung elastic recoil
Pressure in lungs is always greater than intrapleural pressure Surface tension within alveoli, which acts to reduce alveoli to their smallest size
Atmospheric pressure “caves your chest in” around your lungs

The lungs are absolutely held to the thoracic wall due to this pressure interplay. Lung collapse is prevented by the negative pressure of the thin, fluid-filled pleural cavity. Atelectasis, or lung collapse, occurs when a pleural cavity is punctured and air enters. The intrapleural pressure is driven up, surface tension is snapped, and the lung recoils to its smallest size. Luckily, the pleural cavities are separate; if only one lung is punctured, you still have one to breathe with.


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