Summarizing Breathing


The structure of the lungs and thoracic cavity control the mechanics of breathing. Upon inspiration, the diaphragm contracts and lowers. The intercostal muscles contract and expand the chest wall outward. The intrapleural pressure drops, the lungs expand, and air is drawn into the airways. When exhaling, the intercostal muscles and diaphragm relax, returning the intrapleural pressure back to the resting state. The lungs recoil and airways close. The air passively exits the lung. There is high surface tension at the air-airway interface in the lung. Surfactant, a mixture of phospholipids and lipoproteins, acts like a detergent in the airways to reduce surface tension and allow for opening of the alveoli.

Breathing and gas exchange are both altered by changes in the compliance and resistance of the lung. If the compliance of the lung decreases, as occurs in restrictive diseases like fibrosis, the airways stiffen and collapse upon exhalation. Air becomes trapped in the lungs, making breathing more difficult. If resistance increases, as happens with asthma or emphysema, the airways become obstructed, trapping air in the lungs and causing breathing to become difficult. Alterations in the ventilation of the airways or perfusion of the arteries can affect gas exchange. These changes in ventilation and perfusion, called V/Q mismatch, can arise from anatomical or physiological changes.


alveolar ventilation

how much air is in the alveoli

anatomical dead space

(also, anatomical shunt) region of the lung that lacks proper ventilation/perfusion due to an anatomical block


measurement of the elasticity of the lung

dead space

area in the lung that lacks proper ventilation or perfusion

elastic recoil

property of the lung that drives the lung tissue inward

elastic work

work conducted by the intercostal muscles, chest wall, and diaphragm


work of breathing performed by the alveoli and tissues in the lung

functional vital capacity (FVC)

amount of air that can be forcibly exhaled after taking the deepest breath possible

intercostal muscle

muscle connected to the rib cage that contracts upon inspiration

intrapleural space

space between the layers of pleura

obstructive disease

disease (such as emphysema and asthma) that arises from obstruction of the airways; compliance increases in these diseases

physiological dead space

(also, physiological shunt) region of the lung that lacks proper ventilation/perfusion due to a physiological change in the lung (like inflammation or edema)


tissue layer that surrounds the lungs and lines the interior of the thoracic cavity


painful inflammation of the pleural tissue layers


process of opening airways that normally remain closed when the cardiac output increases


measurement of lung obstruction

respiratory distress syndrome

disease that arises from a deficient amount of surfactant

respiratory rate

number of breaths per minute

restrictive disease

disease that results from a restriction and decreased compliance of the alveoli; respiratory distress syndrome and pulmonary fibrosis are examples


detergent-like liquid in the airways that lowers the surface tension of the alveoli to allow for expansion

ventilation/perfusion (V/Q) mismatch

region of the lung that lacks proper alveolar ventilation (V) and/or arterial perfusion (Q)

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