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The human body is an interesting thing. There are so many functions which occur simultaneously to keep it healthy and operating correctly. One of the vital functions is the exchange of oxygen and the removal of carbon dioxide for cellular metabolic processes, which takes place through the respiratory system. But have you ever asked yourself how the respiratory system actually works?
The complex anatomy of the respiratory system actually works on a pretty basic physics premise that will be discussed later. First, we need to talk about what happens in the respiratory system.
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Inside the chest, a lining called the pleura forms an airtight seal around the lungs. It is actually one piece that folds back over itself to form two portions; the visceral pleura wraps around the lungs, and the parietal pleura lines the ribcage down to the diaphragm—a dome-shaped muscle at the lower edge of the ribs. Pleural fluid allows the two layers to rub together without causing irritation. As air comes in, the lungs and pleural cavity expand.
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Inspiration—breathing in—is an active process that requires contraction of skeletal muscles. Perhaps the most important of these muscles is the diaphragm. During inspiration, the diaphragm contracts; due to its shape, contraction pulls it down. At the same time, the external intercostals—muscles between the ribs—contract. This pulls the ribcage up and out. Through this action the pleural cavity increases in size, thus increasing in volume. When the muscles relax, the volume of the pleural cavity decreases, and expiration occurs.
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Atmospheric air pressure varies on Earth based on elevation from sea level. When air pressure decreases to below atmospheric pressure, air rushes towards that space in order to equalize. When you first open a can of soup or vegetables, there is a slight whoosh of air as the pressure inside the can rises to equalize with the outside pressure. Conversely, if a closed system has a pressure higher than that of atmospheric pressure, air will try to get out in order to equalize. Tires on cars are closed systems; so if you allow the air out, the pressure in the tire decreases.
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This is where the difference between the parietal pleura and the visceral pleura really comes into play. Boyle's Law states that "under conditions of constant temperature and quantity, there is an inverse relationship between the volume and pressure for an ideal gas." The constant temperature is body temperature. As the pleural cavity's volume increases during inspiration, the pressure within the visceral pleura—and what they are attached to, the lungs—decreases to below the outside atmospheric pressure. The difference in pressure forces air to move towards the lower pressure in order to equalize to atmospheric pressure. The lungs fill with air and gas exchange occurs.
During expiration, the inverse occurs. The thoracic cavity shrinks in volume. This raises the pressure of the lungs and visceral pleura above that of the outside atmosphere. Air rushes out in order to equalize with atmospheric pressure.
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Within a closed system, a mix of gases exert pressure on the boundaries of the system based upon the percentage of the specific gas; this concept is known as partial pressure and plays an important role in how gas exchange occurs in the lungs.
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Surrounding each of the lung's 300 to 400 million alveoli is a complex network of pulmonary capillaries. These tiny blood vessels carry blood low in oxygen from the right ventricle of the heart. The capillaries then carry the oxygen-rich blood to the left atrium.
During inspiration, the air that enters the alveoli has a high partial pressure of oxygen relative to the blood in the capillaries. Oxygen crosses the barrier of the alveoli and capillary walls—a distance of 0.5 μ—attempting to correct this disparity in a process called diffusion. During diffusion, molecules at a high concentration will cross a membrane in order to equalize the concentration of that molecule on the opposite side of the barrier. Within the capillaries, molecules called hemoglobin in the red blood cells bind the oxygen molecules, increasing the concentration and partial pressure of oxygen in the bloodstream.
During expiration, the air in the alveoli has a low partial pressure of carbon dioxide relative to the blood. Carbon dioxide also crosses the capillary wall/alveoli barrier through diffusion. This lowers the partial pressure and concentration of carbon dioxide in the blood that is on its way to the left atrium, and later onto the rest of the body.
It is a misconception that blood coming into the lungs doesn't have oxygen, and blood leaving the lungs doesn't have carbon dioxide. In actuality, the two molecules are ever-present in the bloodstream. The concentrations merely vary before and after a trip to the lungs.
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Eastern Kentucky University: Human Physiology-Respiration
The University of Liverpool: Lecture Notes on Human Respiratory System Physiology
Florida International University: The Respiratory System
University of Leicester: Anatomy and Physiology of Respiratory System Tutorial
University of Nevada, Las Vegas: Human Anatomy and Physiology: The Respiratory System
California State University, Dominguez Hills: Boyle's Law