MCAT Content / Respiratory System / Breathing Mechanisms

Breathing mechanisms

Topic: Respiratory System

The breathing mechanisms of most mammals include two parts: inhalation and exhalation. These mechanisms depend on pressure gradients as well as the muscles in the thoracic cavity.

The mechanism of breathing obeys Boyle’s law which states that that in a closed space, pressure and volume is inversely related as the volume decreases, pressure increases and vice versa. The thoracic cavity always has a slight, negative pressure which aids in keeping the airways of the lungs open. During the process of inhalation, the lung volume expands as a result of the contraction of the diaphragm and intercostal muscles, expanding the thoracic cavity. Due to this increase in volume, the pressure is decreased, based on the principles of Boyle’s Law. This decrease of pressure in the thoracic cavity relative to the environment makes the cavity pressure less than the atmospheric pressure. This pressure gradient between the atmosphere and the thoracic cavity allows air to rush into the lungs; inhalation occurs. The resulting increase in volume is primarily attributed to an increase in alveolar space because the bronchioles and bronchi are stiff structures that do not change in size.

During inhalation, the chest wall expands out and away from the lungs. The lungs are elastic; therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs. The intercostal muscles relax, returning the chest wall to its original position. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity—the increases in pressure within the thoracic cavity relative to the environment. Air rushes out of the lungs due to the pressure gradient between the thoracic cavity and the atmosphere.

Surface tension is the force exerted by water molecules on the surface of the lung tissue as those water molecules pull together. Water (H2O) is a highly polar molecule, so it forms intermolecular bonds with other water molecules. The force of these bonds effectively creates an inward force on surfaces in the lungs lowering the surface area as the tissue is pulled together. As the air inside the lungs is moist, there is considerable surface tension within the tissue of the lungs. Because the alveoli of the lungs are highly elastic, they do not resist surface tension on their own, which allows the force of that surface tension to deflate the alveoli as air is forced out during exhalation by the contraction of the pleural cavity.

The force of surface tension in the lungs is so great that without something to reduce the surface tension, the airways would collapse after exhalation, making re-inflation during inhalation much more difficult and less effective. Cells of the alveoli continually secrete a molecule called surfactant that solves this problem. Surfactant is a lipoprotein molecule that reduces the force of surface tension from water molecules on the lung tissue. As a result, the surface tension of the lungs from water is reduced so that the lungs can still inflate and deflate appropriately without the possibility of collapse from surface tension alone.


Practice Questions


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MCAT Official Prep (AAMC)

Official Flashcards Biology Question 17

Biology Question Pack, Vol. 1 Question 20

Biology Question Pack, Vol 2. Question 42

Biology Question Pack, Vol. 2 Passage 18 Question 113


Key Points

• The mechanics of breathing follow Boyle’s Law which states that pressure and volume have an inverse relationship.

• The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction and chest wall expansion) which results in a decrease in lung pressure in comparison to the atmosphere; thus, air rushes in the airway.

• The process of exhalation occurs due to elastic recoil of the lung tissue, which causes a decrease in volume, resulting in increased pressure in comparison to the atmosphere; thus, air rushes out of the airway.

• There is no contraction of muscles during exhalation; it is considered a passive process.

• Surfactant is a phospholipid and lipoprotein substance produced in the lungs that function similarly to a detergent. In essence, it reduces the surface tension between alveoli tissue and air within the alveoli, thereby reducing the work needed for airway inflation.


Key Terms

diaphragm: a dome-shaped muscle present between the thoracic cavity and the abdominal cavity which assist in the breathing of air.

intercostal: between the ribs

surfactant: Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli.

resiliency: a property to return to its original shape and size

Boyle’s law: states that in a closed space, pressure and volume are inversely related

thoracic cavity: the chamber of the body of vertebrates that contains the lungs protected by the ribs

inhalation: breathing in

bronchiole: branch of bronchi that are 1 mm or less in diameter and terminate at alveolar sacs

elastic recoil: the rebound of the lungs after having been stretched by inhalation

exhalation: breathing out

surface tension: the force exerted by water molecules on the surface of the lung tissue as those water molecules pull together



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