Breathing is a process that we carry out at all times, either consciously or unconsciously. Everyone knows that stopping breathing involves the death of the subject by asphyxiation.

What not everyone knows are the phases involved in this physiological process, and what its particularities are in addition to the existing criteria when applying them.

In this article we will look at all the phases of breathing and which parts of the body are involved in each of them.

What is breathing?

Before going into more detail about the phases of breathing, it is necessary to give a brief description of this process and introduce what its main phases are.

Breathing is a physical process by which an organism performs an exchange of gases with the environment . The aim of this is to introduce oxygen into the organism so that it can reach the cells, which will carry out their metabolic functions. Once these functions have been carried out, these cells will produce another gas, carbon dioxide, as a waste product, which will be expelled to the outside environment.

In the case of human breathing, it is divided into two main phases: inspiration or inhalation, which involves the entry of oxygen into the lungs, and the other, expiration or exhalation, which involves the expulsion of carbon dioxide. These two phases are made possible by the action of multiple muscles which, by contracting and relaxing, allow the volumetric capacity of the ribcage and lungs to be modified.

The human respiratory rate varies according to the age and physical characteristics of the person. On average, an adult in a normal state of health breathes in and out between 10 and 16 times per minute, while in the case of children their breathing rate is higher, between 15 and 20 times per minute. Newborns and people who play sports often can breathe almost 60 times per minute.

Phases of Breathing

Next we will see the phases of breathing, but under two criteria. The first, more of a traditional type, refers to the anatomical characteristics of the process, that is, which muscles are involved during this phase, what is the degree of physical effort of the organism and also which muscles are involved.

The second case is a classification of the phases of respiration from a more organic perspective, that is, taking into account the biochemical processes involved in the entry and exit of oxygen and carbon dioxide on the organism, respectively.

Traditional phases

These phases are defined based on whether or not the thoracic muscles are exerting any force to bring air in or out of the lungs.

1. Inspiration

During inspiration, or inhalation, the air involved, especially oxygen, is introduced into the body , reaching the lungs.

This process is active, and occurs by contracting the intercostal muscles and lowering the diaphragm. The lungs expand, and the pressure inside these organs is negative compared to that of the atmosphere.

The lungs expand, guided by the pleura and pleural fluid . The intrapulmonary pressure drops, below the level of atmospheric pressure. To balance both pressures, the lungs are filled with air from the outside. The volumetric capacity of the ribcage increases. Thus, air enters the lungs and oxygen enrichment of the blood occurs.

During the inspiration phase, the main muscles involved in this process are: diaphragm, anterior serratus, external intercostals, dorsal, scalene, supracostal, sternocleidomastoid and pectoral.

2. Exhalation

Exhalation, also called exhalation, is the phase of breathing in which carbon dioxide is removed from the body . It occurs when the diaphragm rises and the intercostal muscles relax, causing an increase in pressure in the lungs.

The volume in the rib cage and lungs is reduced , resulting in an increase in intrapulmonary pressure, which becomes higher than atmospheric pressure. Thus, gases are released from the lungs to the outside.

This process is passive, as it involves the muscles involved in the previous phase, i.e. inhalation, relaxing, releasing the air, a waste product, which is inside the lungs. During this phase, the following muscles are involved: internal intercostals, oblique, abdominal, transverse and pectoralis major.

Exhalation can be controlled and voluntary or passive and involuntary . We speak of voluntary expiration when the gases in the lungs are retained in a consciously controlled manner. This can be due to different reasons, such as talking, singing, doing sports or simply because you want to control your breathing like when you are diving.

In the case of involuntary expiration, which is totally passive, it obeys the metabolic functions of the organism . It is the one that occurs during sleep or while doing any activity in which it is not necessary to have direct control over the entrance and exit of air from the organism.

Organic phases

First of all, it is necessary to make a distinction between external and internal breathing.

External breathing occurs outside the cells but inside the body , consisting mainly of the exchange of gases in the lungs and their transport through the bloodstream.

Internal breathing is the process of introducing oxygen into the cells of the body.

1. External breathing

External breathing is understood as the exchange of gases between the body and the external environment , particularly the obtaining of external oxygen and the elimination of internal carbon dioxide.

This type of breathing is also called ventilation, and is the process by which the alveoli of the lungs transfer oxygen to the red blood cells.

Within external breathing, three subphases can be mentioned:

First there is the pulmonary ventilation , which is when there is fluid of gases in and out of the pulmonary alveoli, allowing the exchange of gases.

Then comes the pulmonary diffusion or gas exchange , which occurs when the alveoli absorb oxygen from the outside environment that has managed to reach the lungs and carbon dioxide is expelled to the outside.

And finally occurs the transport of gases , which consists of the blood with oxygen traveling to the cells that require this gas to function and collect the residual carbon dioxide product of their metabolic activities.

2. Internal breathing

Internal breathing is understood as the process in which red blood cells, which have obtained oxygen from the alveoli of the lungs, transfer this same gas to the cells so that they can carry out their metabolic processes.

At the same time, the cells give to the red blood cells the residual carbon dioxide , which has been obtained after the physicochemical reactions inside the cells.

This process is made possible by the processes of osmosis and diffusion. The pressure of oxygen is higher in the blood than in the tissues, causing this gas to end up being absorbed by the cells that make up these tissues through capillaries.

In turn, carbon dioxide, which has a higher pressure in the tissue than in the blood , goes into the bloodstream, traveling through the hemoglobin .

The blood with carbon dioxide travels to the heart, being pumped back to the lungs to make the exchange again, obtaining oxygen and repeating the cycle again.

Bibliographic references:

  • Hall, J. (2011). Guyton and Hall textbook of medical physiology (12th ed.). Philadelphia, Pa.: Saunders/Elsevier.

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  • Koen, C- L.; Koeslag, J. H. (1995). “On the stability of subatmospheric intrapleural and intracranial pressures”. News in Physiological Sciences. 10: 176–178.
  • West, J. B. (1985). Respiratory physiology: the essentials. Baltimore: Williams & Wilkins. pp. 21-30, 84-84, 98-101.