Mechanism Of Breathing

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Some things need not be taught and that includes the very first instinct of survival, our first breath. Our lungs were never taught how to breathe.

It’s astounding that one doesn’t need to learn how to smell, how to see or how to breathe, to be able to perform any of these functions. Regardless of whether we know the mechanism of these involuntary functions or not, it is imperative that we comprehend their functioning since these are mechanisms that facilitate our survival.

Here we shall take a look at the mechanism of breathing and the steps involved in the mechanism of respiration in humans in detail.

What is Breathing?

The process in which air moves in and out of the lungs is known as breathing. This is carried out through various respiratory organs. In other words, breathing is a simple give and take process.

When we breathe, we take in air rich in oxygen from the atmosphere, in return of which, we give out carbon dioxide-rich in the atmosphere which is utilized by the plants for photosynthesis.

This is a continuous process and goes on throughout the life of an organism.

The process of taking in oxygen-rich air is called inhalation. On the contrary, the process of giving out air that is rich in carbon dioxide is known as exhalation.

In a day, a person breathes several times. One breath comprises one inhalation and one exhalation. In a minute, the number of times a person breathes is termed as his/her breathing rate. By calculating the breathing rate, we can know the number of times we breathed in a day.

However, the breathing rate varies which is dependent upon a person’s activity. It raises when a person is brisk walking, running or after a heavy exercise; similarly, decreases when a person is calm.

The breathing rate of an adult is 15-18 times per minute. However, during heavy exercise, the breathing rate exceeds 25 times per minute.

Also Read: Difference Between Breathing and Respiration

Mechanism of Breathing

The air that we breathe in and out of the lungs varies in its pressure. So basically when there is a fall in air pressure the alveolar spaces fall and the air enters the lungs (inspiration) and as the pressure of the alveoli within exceeds the atmospheric pressure, the air is blown from the lungs (expiration). The flow rate of air is in proportion to the magnitude of the pressure difference.

The breathing mechanism involves two processes:

  • Inspiration
  • Expiration


In the process of inspiration, there would be a contraction of muscles attached to the ribs on the outer side which pulls out the ribs and results in the expansion of the chest cavity.

Later, the diaphragm, contracts, moves downwards and expands the chest cavity resulting in the contraction of the abdominal muscles.

The expansion of the chest cavity produces a partial vacuum which sucks air into the lungs and fills the expanded alveoli.

Mechanism Of Inspiration

  • The process of intake of atmospheric air is known as inspiration. It is an active process.
  • When the volume of the thoracic cavity increases and the air pressure decreases, inspiration takes place.
  • Contraction of external intercostal muscles increases the volume of the thoracic cavity.
  • Contraction of the diaphragm further increases the size of the thoracic activity. Simultaneously, the lungs expand.
  • With the expansion of the lungs, the air pressure inside the lungs decreases.
  • The pressure equalizes and the atmospheric air rushes inside the lungs.


The expiration process is considered once after the gaseous exchange occurs in the lungs and the air is expelled. This expulsion of air is called expiration.

During this process, muscles attached to the ribs contract, the muscles of the diaphragm and the abdomen relax which leads to a decrease in the volume of the chest cavity and increases the pressure of the lungs, causing the air in the lungs to be pushed out through the nose.

Mechanism Of Expiration

  • The process of exhaling carbon dioxide is called expiration. It is a passive process.
  • It occurs when the size of the thoracic activity decreases and the air pressure outside increases.
  • Now the external intercostal muscles relax and the internal intercostal muscles contract.
  • As a result, the ribs are pulled inwards and the size of the thoracic cavity is reduced.
  • The diaphragm is relaxed and the lungs get compressed.
  • Consequently, the pressure increases and the air is forced outside.


Mechanism of Respiration

Mechanism of respiration involves the breathing mechanism and exchange of gases.

The gaseous exchange occurs by diffusion in the alveoli. It depends upon the pressure differences between blood and tissues, or atmospheric air and blood. The exchange of gases takes place at the surface of the alveolus.

The mechanism of breathing has already been explained above. Let us have a look at the steps involved in the exchange of gases.

Exchange of Gases

The exchange of gases takes place in the following manner:

Transport Of Oxygen

  • Oxygen in the blood is carried to the tissue in two forms- Oxyhaemoglobin- chemical composition of oxygen with haemoglobin, and solution of oxygen in the blood plasma.
  • The oxygen in the blood combines with haemoglobin when the concentration of oxygen is high in the blood.
  • Oxyhemoglobin, being unstable, dissociates to release oxygen. Low oxygen, low pH and high temperatures stimulate the dissociation process.

Internal Respiration

The gaseous exchange taking place in the tissues is called internal respiration. Here, the oxygen carried in the form of oxyhemoglobin gets dissociated to release oxygen.

This oxygen breaks down glucose to release carbon dioxide, water, and energy. The energy is utilized by the body, while the carbon dioxide is diffused from the tissues.

Also Read: Respiration

Transport Of Carbon dioxide From Tissues To Lungs

Carbon dioxide is transported by three mechanisms:

  • Some carbon dioxide dissolves in the water of plasma to form carbonic acid.
  • Carbonic acid ionizes to form bicarbonate ions. The hydrogen ions are catalyzed by the enzyme carbonic anhydrase. Bicarbonate ions combine with sodium and potassium to form sodium bicarbonate and potassium bicarbonate.
  • Some carbon dioxide combines with haemoglobin for the formation of carbaminohemoglobin.
  • It is finally carried to the lungs and released out of the body through expiration.

Intrapleural Breathing

Intrapleural breathing is used to refer to the pressure that is present in the space between the pleura and the lungs. This space is referred to as the pleural cavity. The pressure in this region is normally less than the atmospheric pressure. This is the reason why pleural pressure is termed as negative pressure.

The lung movement is governed by the pressure gradient, the transpulmonary pressure, which exists between the pleura and the lungs. The difference in the pressures between intrapulmonary and intrapleural pressures is known as transpulmonary pressure.

The pressure in the pleural cavity while breathing turns negative while there is an increase in the transpulmonary pressure causing the lungs to expand. While expiration, the lungs recoil as a result of an increase in the pleural pressure.

The competing forces inside the thorax results in the formation of negative intrapleural pressure, one of these forces is associated with the lung’s elasticity. The lungs have elastic tissues which cause it to be pulled inwards off the thoracic wall. An inward pull of the lung tissue is also generated by the surface tension of the alveolar fluid. The inward tension generated from the lungs is opposed by forces from the thoracic wall and the pleural fluid.

Respiratory Gas Transport

After the gases have scattered in the lungs, causing the blood to become oxygenated, leaving carbon dioxide, the next phase of transportation of oxygen-rich blood to the tissues takes place. Meanwhile, the next round of deoxygenated blood needs to be brought to the lungs for the cycle to continue.

In the bloodstream, the transportation of gases occurs all through the body which is contributed to the cardiovascular system comprising the blood vessels and the heart. The blood carrying oxygen leaves the lungs to flow into the heart through the pulmonary veins, which are pumped to the rest of the body from the left ventricle through the aorta and its corresponding branches.

This was an overview of the mechanism of breathing and the mechanism of respiration in the human body.

Also Read: Respiratory System

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Key Points on Mechanism of Breathing

  • Breathing is the physical process of inhaling oxygen and exhaling carbon dioxide.
  • The mechanism of breathing involves two main processes: inspiration and expiration.
  • Inspiration occurs when the diaphragm and the external intercostal muscles contract.
  • Expiration occurs when the diaphragm and the intercostal muscles relax.
  • The contraction or relaxation of muscles around the lungs changes the entire volume of air inside the lungs, and so does the pressure.
  • If the pressure inside the lungs is more than the outside, the air rushes out. If the opposite happens, the air rushes in.
  • Due to the high elasticity of the lung tissue and low surface tension of moisture in the lungs, the lungs have higher compliance.

To know more about the mechanism of breathing and mechanism or respiration, visit BYJU’S website or download the BYJU’S app for further reference.

Frequently Asked Questions


What is breathing?

Breathing is the process of inhaling oxygen and exhaling carbon dioxide by the lungs. There is a system of organs involved in the process.


What is inspiration?

Inspiration is the process of inhaling oxygen. When we breathe in, the diaphragm contracts and moves downward. This increases the space in the chest cavity into which our lungs expand. Thus, the intercostal muscles between the ribs enlarge the chest cavity.


What is the mechanism of expiration?

Expiration is the process of releasing carbon dioxide from the body. During expiration, the diaphragm relaxes and the volume of the thoracic cavity decreases, and the pressure inside it increases. Consequently, the lungs contract and the air is forced out.


How is breathing differently from respiration?

Breathing is a simple process of inhaling oxygen and exhaling carbon dioxide. On the contrary, respiration is the process of producing energy by the oxidation of complex organic substances to carry out different life processes.


What is the main function of the lungs?

The lungs are involved in the important process of breathing. The oxygen inhaled from the outside environment enters the blood and the carbon dioxide leaves the blood. This inhaling and exhaling are facilitated by the lungs.


What triggers breathing?

Breathing is triggered by carbon dioxide. Our body needs to get rid of the carbon dioxide present in our body. Hence, carbon dioxide is the main trigger of breathing. However, low oxygen levels are also a trigger to breathe, but a much weaker trigger than carbon dioxide.


What is better- breathing through the chest or the stomach?

We inhale through our nose. The air moves to our stomach with the contraction of the diaphragm. Our belly expands and our lungs are filled with air. This way of breathing pulls down on the lungs and is the most efficient way to breathe.


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