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How Do the Lungs and Breathing Actually Work?

The Lungs, Pleura, and Chest Wall Work Together During Respiration

Sep 22, 2009 Matthew Miller

The process of respiration is more complicated than it appears. It involves many steps and relies on pressure variations and airtight seals to maintain steady breathing.

The function of the human respiratory system is based on a delicate pressure balance between the exterior environment, the interior of the lungs, and the pleural cavity. The lungs themselves contain no muscle tissue and thus their expansion and contraction are governed by the respective pressures of the volumes which surround them.

The Pleura and the Chest Wall Start the Respiration Process

The lungs are contained within the pleural cavity which is attached to the interior of the chest wall. The pleural cavity, like the lungs, has no capacity for independent expansion or contraction, although, unlike the lungs, the pleural cavity is filled with fluid, not air. This setup is akin to one balloon being inflated inside of another balloon that is already filled with water.

During inhalation, the chest muscles forcefully expand the volume of the chest cavity. This causes an increase in the size of the attached pleural cavity and an accompanying decrease in the pressure of the fluid inside it (Remember PV=RT?). This, in turn, causes the lungs to expand in order to reestablish the equilibrium between the pressure within the pleural cavity and the pressure of the air inside the lungs.

The Air Pressure Differences Between the Pleura, Lungs, and Outside Environment

The expansion of the lungs creates a pressure imbalance between the air inside the lungs and the outside environment. This relative pressure difference causes air from the outside environment to flow into the lungs until the pressures are equalized, completing the process of inhalation. During exhalation, the chest muscles are relaxed and the process is reversed.

Aside from the contraction and relaxation of the chest muscles, respiration is a passive process requiring very little energy. This level of efficiency is achieved through the use of relative pressure. This partially explains how such a complex series of events can be executed flawlessly thousands of times per day with little or no conscious effort.

What Causes a Collapsed Lung, also known as a pneumothorax?

The efficiency achieved during the process of respiration comes at a price. In order for it to work properly, every participating element must be completely airtight. A small hole or tear in the pleura, the lung wall, or the chest wall can result in the accumulation of air in the pleural cavity. Eventually, this pressure buildup will restrict the expansion of the lungs, resulting in a collapsed lung.

This condition is known an a pneumothorax and can be caused by external trauma, an existing lung disease, or by a naturally occurring weakness in the lining of the lungs. It is a serious and possibly life-threatening condition that requires immediate medical attention. For a detailed description of this condition and the events that can trigger it, see What Pulmonary Problems Cause a Pneumothorax?.

The lungs are not muscles. They are sacks of air that respond to constantly shifting pressure variations. Any breach of their surface integrity, regardless of how minute, can result in excruciating pain and possibly death. They seem so fragile and yet a vast majority will live their entire lives without experiencing any breakdown in their delicate routine.

What's the Least Painful Pneumothorax Treatment? - describes the most common treatments available for a collapsed lung, including insertion of a chest tube, and a surgical procedure called a pleurodesis where part of the lung is removed and the remainder is stuck to the inside of the chest wall
What Does Spontaneous Pneumothorax Really Mean? - explains why a collapsed lung can sometimes occur with no apparent cause, and lists common risk factors for this type of pneumothorax

Disclaimer: The information contained within this article is for educational purposes only, and is not intended to be a substitute in any way for care and treatment by a qualified health professional.

The copyright of the article How Do the Lungs and Breathing Actually Work? in General Medicine is owned by Matthew Miller. Permission to republish How Do the Lungs and Breathing Actually Work? in print or online must be granted by the author in writing.

The Lungs and Pleura Work Together in Respiration	The Pleural Cavity Is Important to Lung Function	A Pneumothorax Caused by a Chest Wall Puncture
Chest Tube Used to Fix a Pneumothorax	Pleura Seen in Chest XRay of Pneumothorax