Human Respiratory System

The human respiratory system is an intricate, complex connection of structures through which air, including oxygen (O2), moves from outside the body to the lower recesses of the lung. There, gas exchange occurs through the alveolocapillary membrane, which sits between the lung air sacs (alveoli) and the lung capillaries. This part of the act of breathing is known as inspiration.

Expiration during the breathing cycle is responsible for carbon dioxide (CO2) removal. CO2 moves from the pulmonary capillaries to the alveoli, up the airways to leave the body. Any problems with this system can result in a catastrophe. Fortunately, this system has great resiliency and can often absorb even severe insults.

Respiration, therefore, represents the intake of O2 and its consumption together with the production and removal of CO2.

Human Respiratory System


The structures in this system include the nose and mouth (pharynx), which connect with the larynx and lead to the trachea. From there, the trachea divides (bifurcates) in the lung to form the right and left mainstem bronchi, which divide and subdivide about 23 times to reach the gas exchange area of the lung.

This area begins with the respiratory bronchioles which divide into alveolar ducts, alveolar sacs and eventually into alveoli and all of these structures are lined by alveoli. The alveoli lining this area are adjacent to the pulmonary capillaries and separated from them by a very thin membrane for ease of transfer of oxygen into the blood in these capillaries.

Continue reading Structure of the Respiratory System

See a color-coded respiratory system diagram


Asthma, pneumonia, Chronic obstructive pulmonary disease (COPD), pulmonary emboli, lung cancer, and occupational lung disease are among the most common human respiratory system diseases. They vary greatly, however, in their causes and treatments.

Continue reading Respiratory System Diseases

Blood Gas Levels

This entire human respiratory system is under the control of the central nervous system. The levels of CO2 and O2 in the blood can change a person’s breathing. These blood gas levels affect both the respiratory center in the medulla oblongata and the carotid bodies in the carotid arteries which also affect respiration.


Breathing, the basis of the functions of the respiratory system, is actually a complex physiologic act with an inspiratory component as well as an expiratory component.

Spontaneous (involuntary) inspiration starts when the respiratory center in the medulla oblongata emits neural signals, which travel along the central and peripheral nervous system to the inspiratory muscles (in particular, the diaphragm) causing them to contract.

Continue Reading Functions of the Respiratory System

Irregular Respiratory Rates

Abnormal respiratory rates, such as paroxysmal nocturnal dyspnea, are among the first things a physician assesses during examination of the lungs. A pulmonary exam requires comparison of the findings on both sides of the chest. The examiner first inspects movements of chest to confirm the estimate of the patient’s respiratory rate when the vital signs’ examination was performed, and carefully observes the pattern of breathing (the rate, the rhythm and the depth).

Continue reading Irregular Respiratory Rates

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Image credit:
Patrick J. Lynch; illustrator; C. Carl Jaffe; MD; cardiologist Yale University Center for Advanced Instructional Media Medical Illustrations by Patrick Lynch, generated for multimedia teaching projects by the Yale University School of Medicine, Center for Advanced Instructional Media, 1987-2000. Patrick J. Lynch, Creative Commons Attribution 2.5 License 2006; no usage restrictions except please preserve our creative credits: Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist.

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