Mechanism of Breathing: Definition, Diagram, Functions

What Is Breathing?

Respiration is the physiological exercise through which gases, mainly oxygen and carbon dioxide are exhaled through the lungs of an organism. This is important for cellular respiration as this moves the oxygen required in aerobic respiration to reach the cells and at the same time takes out carbon dioxide, a waste product. Proper functioning of the lungs guarantees a constant supply of oxygen to the cells for their metabolism and regulates the proper distribution of gases, which are significant for the life process.

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The Process Of Breathing

The process of breathing involves the following steps:

Inhalation (Inspiration)

• Inhalation is mainly characterised by the contraction of the diaphragm and the intercostal muscles to raise the ribs and expand the thoracic cavity.

• This contraction generates a vacuum, or negative pressure within the thoracic cavity and air rushes into the lungs.

• During the process of inhalation, contraction of the muscles surrounding the thoracic cavity increases the volume of this cavity and thus forces the air through the respiratory tract and into the alveoli to participate in the exchange of gases.

Exhalation (Expiration)

• Exhalation takes place when the diaphragm and intercostals decrease their size decreasing the volume of the thoracic cavity.

• This relaxation leads to positive pressure in the thoracic cavity which in turn forces air out of the lungs and out of the respiratory tract.

• The reduction of volume ejects the air laden with carbon dioxide ending the breathing cycle.

Mechanics Of Breathing

The mechanism of breathing involves the following:

Diaphragm And Intercostal Muscles

• The diaphragm functions as a muscle which is in the shape of a dome and is situated at the bottom part of the thoracic cavity and the intercostal muscles are located in between the ribs of an individual’s body.

• Inhalation is done by the contraction of the diaphragm, which becomes flattened and the intercostal muscles that lift the ribs, expanding the thoracic cavity. By doing so, the pressure in the thoracic cavity is reduced enabling air to enter the lungs.

• These muscles contract during inhalation and, at the same time, they relax during exhalation to narrow the thoracic cavity hence forcing air out of the lungs.

Lung Compliance And Elasticity

• Lung compliance on the other hand deals with the ease with which the lungs can expand and dilate given some pressure.

• High lung compliance means that the lungs can be expanded easily and can can therefore fill up easily while low compliance means that the lungs are stiff and cannot be filled easily.

• Accessibility of the lungs can be affected by the degree of elasticity of the lung tissues, the quantity and nature of the surface tension-reducing agents (surfactants), and the presence of diseases such as pulmonary fibrosis.

• Lung compliance is an index of work that the respiratory muscles need to do to achieve a certain level of ventilation.

Airway Resistance

• Airway resistance is the opposition force within the respiratory tracts to the flow through them.

• This depends on the dimensions like the diameter of the airway, mucus production, and bronchial constriction.

• There is always the case of high resistance, this is because of the constriction of the tubes through which air has to pass, or the presence of an obstructing factor that makes it difficult to breathe.

• It is important to deal with airway resistance if breathing properly and if fresh and expired air is going to get to the lungs.

Gas Exchange In The Lungs

The gas exchange in the lungs involves the following:

Alveolar Structure

• Alveoli are small, sacs, like structures or bubbles at the end of the bronchioli in the lungs.

• The thin walls consist of only one layer of the epithelial cell layer, this makes it easier for gases to be exchanged.

• Through the extended alveoli, there is a large surface area therefore maximising the chances of diffusion of air and blood because of the maximal contact with the substances.

Diffusion Of Gases

• In the alveoli oxygen from the inhaled air moves passively through the walls of the alveoli into the blood in capillaries, and at the same time, carbon dioxide diffuses active from the blood into the alveoli and thus is exhaled.

• This exchange is driven by partial pressure gradients: oxygen diffuses from the place that has high partial pressure in the alveoli relative to that found in blood and similarly for the diffusion of carbon dioxide.

Hemoglobin And Oxygen Transport

• Oxygen on reaching the lungs gets attached to the haemoglobin molecules and forms oxyhemoglobin which is easily transported to the tissues.

• The oxygen dissociation curve shows how the affinity of haemoglobin for oxygen varies with different concentrations of oxygen partial pressure.

• At high partial pressures, for example in the lungs, haemoglobin grabs more oxygen than from the lower pressure found in tissues where oxygen is needed, for use by the cells.

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