Lungs – Tissue and Systems

Lungs – Tissue and Systems

This article describes the basic form and function of the human lungs. By understanding the normal anatomy and physiology of the lungs, symptoms from diseases of the lung are better understood. There are literally hundreds of possible diseases of the lung. The anatomy and functions of the lungs is quite complex and can easily fill an entire textbook. A complete review of these is beyond the scope of this article however these functions will be described in basic terms.

The lungs perform many functions for the body but the most basic and essential functions they perform are to take up oxygen and to help release carbon dioxide. A partial list of lung functions includes:

  • Gas exchange (oxygen and carbon dioxide)
  • Air filtering
  • Removal of inhaled particulate matter
  • Warming of inhaled air
  • Humidification of inhaled air
  • Acid-base balance of the body

The lungs are often depicted in a cartoon fashion as two balloons. While they do inflate, their structure is far more complex. The entire airway system includes the connecting areas such as mouth, throat and the large bridging airways such as the trachea. The airways that connect the lungs to the central tube (trachea) undergo branches like a tree. There are usually 23 divisions or generations of airways before the air is able to reach the “business end” of the lung where the action occurs. The functional unit of the lungs is the alveoli which are microscopic sacs or divisions in the lung with thin membranes that allow diffusion of gas across it. This is where gases such as oxygen and carbon dioxide diffuse down their gradient.

Diffusion involves movement of a concentrated substance into an area of less concentration. This happens with solutes and naturally in most situations. For example, if you bring 100 people into a big room – they will tend to disperse evenly throughout the room rather than staying in one area clustered. The same mechanism allows movement of gases via a gradient.

The growth and development of the lungs is very complex and there are many steps along the way where errors or different types of lung structuring can occur.

The lungs are divided among lobes; three on the right and two on the left. Each lobe has segments and each segment has clustered areas of bronchi, and so on. The many divisions of the lung account for its large surface area – about the size of a tennis court.

The connecting airways of the lungs are not simply passive tubes but dynamic structures that help regulate the movement of air, clearance of mucus and filtration of inhaled air. The larger airways are lined with small cilia which are microscopic hair-like projections that move in a rhythmic fashion to move mucus and inhaled particles outward. The smaller connecting airways have muscles that help regulate the amount of air moving through. Contraction of small airway muscles allows less air through and relaxation makes the airways larger thus decreasing resistance of air movement. Some diseases such as asthma affect these small airways and cause increased resistance to air movement making breathing harder. Having to inhale deeper or with more effort to move the same amount of air creates fatigue of the breathing muscles and creates a feeling of shortness of breath (also known as dyspnea).

Irritation of the airways or difficulty with clearance of debris in the air can cause cough. Cough can also be produced from excessive production of mucus. Cough can also arise from excessive nasal secretions that can drip down stimulating the cough reflex. Asthma tends to cause cough also because of irritation of the airways.

The movement of air in and out of the lungs has been compared to movements of the ocean tides. The amount of air moving in and out with a single calm breath is known as the tidal volume. There are several other ways to measure and classify volumes of air that we breathe. With calm normal breathing most people breathe in about half a liter of air. With exercise or a forced deep breath, most people can easily inhale 3 or 4 times this amount. The total volume of air contained in a normal adult lung is about 5 or 6 liters. It is impossible to empty all of this air because the connecting airways are stiff and will naturally retain some air even with a very hard forced expiration.

Understanding the normal lung function allows insight into the broad range of symptoms that might occur with lung disease. There are literally hundreds of possible lung diseases, but many of these diseases have symptoms in common.

There are several different cell types in the lung. This helps explain the different types of lung cancers that can arise from each of these cell types. The commonest type of cancer is called non-small cell lung cancer and is related to smoking. Small cell cancer is more rare. Lung cancer is a very important public health concern because it is the most common cause of death from cancer for men and women in the United States. The most important risk factor for development of lung cancer is cigarette smoking. Smoking increases the risk of lung cancer by 22-fold. This highlights the importance of avoiding smoking and quitting.


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  5. This article was originally published on September 3, 2012 and last revision and update was 9/4/2015.