The phases of respiration, and how they occur in the human body

Breathing is a process that we carry out at all times, whether consciously or unconsciously. Everyone knows that to stop breathing implies the death of the subject by suffocation.

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

In this article we are going to see all the phases of breathing and what parts of the body are involved in each of them.

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What is the breath?

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 exchanges gases with the environment. The objective of this is to introduce oxygen into the body so that it can reach the cells, which will carry out their metabolic functions. Once they have been carried out, these cells will produce as a waste another gas, carbon dioxide, which will be expelled to the outside environment.

In the case of human respiration, 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. These two phases are possible thanks to the action of multiple muscles which, by contracting and relaxing, allow modifying the volumetric capacity of the rib cage and lungs.

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

Phases of respiration

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, which is the degree of physical exertion 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 some kind of force to bring air in or out of the lungs.

1. Inspiration

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

This process is active, and occurs through the contraction of the intercostal muscles and the descent of the diaphragm. The lungs expand, and the pressure inside these organs is negative compared to that in the atmosphere.

The lungs distend, guided by the pleura and pleural fluid. Intrapulmonary pressure falls below the level of atmospheric pressure. To balance both pressures, the lungs fill with air from outside. Increases the volumetric capacity of the rib cage. Thus, the air enters the lungs and the enrichment of oxygen in the blood occurs.

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

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2. Expiration

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 increased pressure in the lungs.

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

This process is passive, since it implies that the muscles involved in the previous phase, that is, the inhalation, relax, releasing the air, the waste product, which is found within the lungs. During this phase, the following muscles are involved: internal intercostals, obliques, abdominal, transverse, and pectoralis major.

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

In the case of involuntary expiration, which is totally passive, obeys the metabolic functions of the body. It is the one that occurs during sleep or while doing any activity that does not require direct control over the entry and exit of air from the body.

Organic phases

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

External respiration occurs outside the cells but within the body, consisting mainly of gas exchange in the lungs and their transport through the bloodstream.

Internal respiration is the process of introducing oxygen into cells of the organism.

1. External breathing

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

This type of respiration is also called ventilation, and it is the process in which the pulmonary alveoli transfer oxygen to the red blood cells.

Within external respiration, three sub-phases can be mentioned:

Pulmonary ventilation occurs first, which is when there is a flow of gases into and out of the pulmonary alveoli, allowing gas exchange.

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

Y finally gas transport occurs, which consists in that the blood with oxygen travels to the cells that require this gas to function and collect the residual carbon dioxide product of its metabolic activities.

2. Internal breathing

Internal respiration is understood as the process in which the red blood cells, which have obtained oxygen given up through the pulmonary alveoli, give this same gas to the cells so that they can carry out their processes metabolic.

At the same time, cells give red blood cells residual carbon dioxide, which has been obtained after having carried out the physicochemical reactions within the cells.

This process occurs thanks to 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 the 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 perform 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.
• 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.