Breathing, inspiration and expiration

In humans, and other living things, respiration is the process by which oxygen is absorbed from the environment and gases, particularly carbon dioxide , are removed . The main characteristic of respiration is this gas exchange .

Respiration can occur as external respiration (gas exchange between the atmosphere and the body) or as internal respiration (gas exchange within the body).

The inspiration , is one of the phases of respiration, specifically in which air inhaled from the outside environment into the body, in order to obtain the oxygen necessary to carry out metabolic processes.

On the other hand, during expiration, the body releases residual gases , specifically carbon dioxide .

Both, inspiration and expiration, are phases that represent complementary movements of respiration and that allow gas exchange to take place in a living being.

DefinitionIt is the action by which an organism acquires oxygen from the environment and releases carbon dioxide, as a result of a metabolic and mechanical process.It occurs when the air from the external environment of an organism is inhaled into the body, allowing the absorption of oxygen.Also known as exhalation, it is the process by which the body expels carbon dioxide and other waste gases during breathing.
  • It is divided into two phases: inspiration and expiration.
  • Both the diaphragm and the intercostal muscles play an important role in this process.
  • External respiration consists of the exchange of gases from the exterior and interior of an organism.
  • Internal respiration consists of the exchange of gases within an organism.
  • The volume and pressure of gases are determining factors in the respiratory process.
  • It is an active process.
  • The diaphragm contracts.
  • Oxygen is extracted from the air.
  • Follow the opposite route to expiration.
  • The intrapulmonary pressure is lower than atmospheric.
  • The lungs expand.
  • It is a passive process.
  • The diaphragm is relaxed.
  • The release of carbon dioxide occurs.
  • Follow the opposite route of inspiration.
  • Intrapulmonary pressure is higher than atmospheric.
  • It can be given voluntarily or involuntarily.
  • The lungs contract.
Main muscles involvedThe diaphragm and intercostal muscles (with participation of other secondary muscle groups).
  • Diaphragm.
  • External intercostals.
  • Pectorals.
  • Dorsals.
  • Internal intercostals.
  • Oblique
  • Pectoral major.
  • ABS.
  • Pulmonary ventilation.
  • Lung diffusion or gas exchange.
  • Gas transport.
  • Peripheral gas exchange or internal respiration.
  • Contraction of the diaphragm and expansion of intercostal muscles.
  • Increase in the volume of the thorax.
  • Lung distention
  • Drop in intrapulmonary pressure.
  • Absorption of oxygen from outside.
  • Relaxation of the diaphragm and intercostal muscles.
  • Reduction of the volume of the thorax.
  • Lungs contract.
  • Increased intrapulmonary pressure.
  • Expulsion of carbon dioxide present in the body.

What is the breath?

The breath is the name given to the physical process by which an organism takes exchange of different gases , carrying oxygen from the air to the lungs, and these cells of the organism, and disposing of carbon dioxide from the tissues.

In particular, it is about transferring oxygen to the cells that use it to produce energy and get rid of waste gases when carrying out metabolic processes.

In humans and other living beings, respiration involves the exchange of gases, with the intention of providing the body with the oxygen (O 2 ) necessary to survive, as well as the release of carbon dioxide (CO 2 ), which is a product of this process.

Respiration is divided into two parts or phases, these are inspiration and expiration, constituting a respiratory cycle. This cycle is possible thanks to respiratory mechanics, or the process carried out by different muscles.

Two main types of muscles are involved in the breathing process, the diaphragm muscles and the intercostal muscles. These allow air to enter the body.

The respiratory rate of an adult person is between 10-16 breaths per minute, while in children it is about 15-20 breaths per minute. Athletes and newborn babies have a higher respiratory rate, which can reach 60 breaths per minute.

Respiration characteristics

  • It consists of two phases: inspiration and expiration.
  • Different muscle groups are involved, particularly the diaphragm and intercostal muscles.
  • External respiration involves pulmonary ventilation and the exchange of gases from the external environment to the interior of an organism and vice versa.
  • Internal respiration consists of the exchange of gases within the body, from the blood to the tissues and vice versa.
  • Atmospheric, intrapulmonary and intrapleural pressure play an important role during the respiratory process.

External breathing

In humans, external respiration consists of the exchange of gases between the body and the external environment , particularly oxygen and carbon dioxide. This type of breathing is also known as ventilation. During this process, the pulmonary alveoli transfer oxygen to the red blood cells.

External respiration includes other processes such as pulmonary ventilation, pulmonary gas diffusion, and gas transport. Different muscle groups participate in it and it occurs thanks to the compliance of the lungs.

Intercostal muscles

In the region of the rib cage, the external intercostal muscles make a movement that expands the rib cage to allow inspiration. During expiration, the internal intercostals are responsible for allowing the rib cage to close.


The diaphragm is the muscle that lies horizontally under the rib cage and performs an opening movement during inspiration.

Lung compliance

Also known as compliance , it is the ability of the lungs (in addition to the rib cage) to stretch and narrow during pulmonary ventilation. Thanks to the fact that the lungs are able to modify their capacity, increasing their volume, the respiratory process can be carried out.

Pulmonary ventilation

The pulmonary ventilation is the process in which there is a flow of gas into and out of pulmonary alveoli, allowing the exchange between oxygen and carbon dioxide.

During pulmonary ventilation, the diaphragm and intercostal muscles are the main mechanisms that actively participate in the process that allows inspiration and expiration.

In the case of the lungs, these do not participate actively, but they are able to adapt to the movement caused by the diaphragm and the intercostal muscles, thanks to the pleural fluid since they can be distended.

Lung ventilation, gas volume and pressure

Pulmonary ventilation works thanks to the pressure of gases and the volume of a space.

According to Boyle’s law , developed by the natural chemist and philosopher Rober Boyle (1627-1691), there is a relationship between the volume of space that a gas occupies and the pressure exerted by it. Basically, a gas has less pressure when it is in a higher volume space. In the same way, the pressure increases with the reduction of the volume of the space.

This happens because when there is a greater volume in a space, the molecules of a gas have greater freedom of movement. When the volume of the space is reduced, the gas molecules have less room to maneuver, being “tight” and without the same ability to move.

When the pressure of a gas increases inside a container, and the pressure outside it is lower, when this gas is released, an equilibrium point is reached between the exterior and interior space.

An example of the above can be seen when a soccer ball is fully “inflated”. The internal pressure can be felt, as the air inside it occupies a small volume of space, creating more pressure, so the ball feels solid.

If this soccer ball were to suffer a puncture, the internal pressure, which is higher than atmospheric, would cause the air to leave the ball, until reaching a point of equilibrium. When the pressure is reduced, the balloon deflates, becoming “softer” and more malleable.

Atmospheric pressure

Atmospheric pressure is the pressure found in the atmosphere and represents the force that gases (air) exert on the earth’s surface and objects that are in it. This pressure decreases with altitude, so the highest atmospheric pressure is at sea level.

Atmospheric pressure has a value of 760 mm Hg (millimeters of mercury), equivalent to one atmosphere, above sea level. We speak of a negative atmospheric pressure when it is below 760 mm Hg, and a positive atmospheric pressure, when it is above this figure.

Intrapulmonary pressure

Also known as intra-alveolar , intrapulmonary pressure is the pressure of air in the alveoli . This reaches an equilibrium level with atmospheric pressure during the lung ventilation process. Thanks to the muscles of the body, intrapulmonary pressure varies, being negative or positive. This change in pressure allows gases to flow from the outside into the body and vice versa.

This pressure has the same value as atmospheric pressure.

Intrapleural pressure

Intrapleural pressure refers to the air pressure in the pleural cavity . The pleurae are sacs or membranes that surround the lungs and protect them from contact with other organs, maintaining their proper functioning. This pressure is negative, relative to intrapulmonary and atmospheric pressure, since the lungs contract when the rib cage expands.

Its value is variable, staying at about -4 mm Hg with respect to intrapulmonary and atmospheric pressure.

Transpulmonary pressure

Transpulmonary pressure is the difference between intrapulmonary and intrapleural pressure, being that the higher the transpulmonary pressure, the greater the lung volume.

Lung diffusion or gas exchange

Gas diffusion or exchange occurs when the alveoli absorb oxygen that has reached the lungs. The capillaries release carbon dioxide found in the blood and receive oxygen.

This exchange occurs through the alveolo-capillary membrane or respiratory membrane.

Importance of pulmonary diffusion

  • When pulmonary diffusion occurs, the blood becomes enriched with oxygen. This allows the proper functioning of the aerobic metabolism.
  • Eliminates carbon dioxide found in the blood.

Gas transport

Gas transport occurs when gas diffusion or exchange has occurred within the lungs. When the blood gets oxygen, it must be transported in order to allow it to be absorbed by the cells.

The one in charge of carrying out this task is the cardiovascular system , since it is the one that pumps the blood through the body. The blood that has received the oxygen goes to the heart and it pumps it to the rest of the body.

The oxygen is transported mostly along with hemoglobin (Hb) by blood. The latter determines the oxygen transport capacity of the blood.

The carbon dioxide , however, follows the reverse path of the oxygen, and there are several ways is eliminated. It can be transported together with hemoglobin (20% of the carbon dioxide present); dissolved with plasma (7%); or it is transformed into bicarbonate ions (70%).

Internal respiration or peripheral gas exchange

External respiration occurs at the moment when the red blood cells transfer the inspired oxygen to the cells, and the cells give up the carbon dioxide to the red blood cells. On the other hand, internal respiration is the process in which there is an exchange of gases within the body , specifically in the cells of the tissues , after external respiration has taken place.

During internal breathing, pressure allows a diffusion or osmosis process to be triggered. Since the pressure of oxygen varies depending on the environment, it is higher in the blood than in the tissue. That is why it is transferred from one medium to another, from the blood to the tissues.

Stages of internal breathing

  1. After external respiration, oxygen travels with hemoglobin to the body’s tissues.
  2. Hemoglobin loses oxygen due to the higher pressure it has in the blood, and this is absorbed by the tissues through the capillaries.
  3. The carbon dioxide present in the tissue has a higher pressure than in the blood, so it escapes from it and is absorbed by the blood (it travels with the hemoglobin, or as bicarbonate or dissolved).
  4. The blood reaches the heart and is pumped back to acquire more oxygen and get rid of carbon dioxide, repeating the process again.

You may also be interested in seeing Types of breathing .

What is inspiration?

During inspiration , the air found in the atmosphere enters the body, to carry oxygen to the body, through the lungs. Along with expiration, it is one of the two steps of respiration.

Inspiration is an active process and occurs when there is a contraction of the intercostal (external) muscles and a descent of the diaphragm . At that moment, the air enters the lungs, allowing the blood to be enriched with oxygen.

Inspiration characteristics

  • Inspiration is an active process.
  • During inspiration, the diaphragm contracts and the rib cage expands.
  • Air is inhaled to obtain oxygen.
  • The intrapulmonary pressure is negative with respect to the atmospheric one.
  • The lungs expand or distend.

Stages of inspiration

  1. The diaphragm contracts and moves down to the lower abdomen.
  2. The external intercostal muscles expand, opening the space between the ribs (intercostal).
  3. Both movements cause an increase in the volume of the rib cage.
  4. The lungs are distended, guided by the pleurae and the pleural fluid that supports them to cover the volume of the rib cage.
  5. As a result of the opening of the rib cage and the expansion of the lungs, the intrapulmonary pressure drops below the level of atmospheric pressure.
  6. Thus, to balance both pressures, the lungs are filled with air from outside.

Main muscles involved in inspiration

  • Diaphragm.
  • Serratus anterior.
  • External intercostals.
  • Scalenes
  • Dorsals.
  • Supracostal.
  • Esternocleidomastoid.
  • Pectorals.

What is expiration?

The exhalation is the process in which a respiratory current which arises is removed carbon dioxide from the body. It is one of the two phases of breathing, along with inspiration.

It occurs when the diaphragm rises and the internal intercostal muscles relax , increasing internal pressure. Expiration, which is also known as exhalation, is a passive process . Once the diaphragm rises, and the rib cage contracts, carbon dioxide is removed from the body.

Characteristics of expiration

  • It is a passive process.
  • During this process, the diaphragm relaxes and the chest cavity shrinks.
  • Waste gases are removed, mainly carbon dioxide.
  • It basically goes the opposite way of inspiration.
  • It can be voluntary or involuntary
  • Intrapulmonary pressure is higher than atmospheric.
  • The lungs contract.

Stages of expiration

  1. After inspiration, the diaphragm and intercostal muscles relax, contracting inward.
  2. The rib cage reduces its volume, as well as the lungs.
  3. Intrapulmonary pressure rises in such a way that it exceeds atmospheric pressure.
  4. This results in the release of gases found in the lungs to the outside.

Types of expiration

Expiration can be controlled / voluntary, or it can be involuntary / passive.

When expiration is controlled , the gases in the lungs are voluntarily retained. This is produced with various objectives, and comes into operation when speaking activities are carried out, when singing or doing some specific type of sport, among others.

In the case of involuntary expiration , this is usually totally passive. It obeys the metabolic functions of the body. This type of expiration can occur, for example, during sleep.

Main muscles involved in expiration

  • Internal intercostals.
  • Oblique
  • Transverse.
  • ABS.
  • Pectoral major.

Add a Comment

Your email address will not be published. Required fields are marked *