Mechanism of Breathing- Definition, Muscles and Mechanism Involved

Have you ever wondered how an organism breathes? What is the mechanism involved in this process? Breathing is one of the most important characteristics of all living organisms. It is one of the essential functions that begins from the time of birth of the organism. We inhale oxygen and exhale carbon dioxide in the air; this process is called breathing. Though breathing involves the movement of gases in and out the body, it could be performed in different ways in different organisms based on organs involved, habitat, species, etc. In this article, we will learn what breathing is, what are the different muscles involved in this process and what is the exact Mechanism of Breathing.

What is Breathing?

The simple definition of breathing is the process of inhalation of air (oxygen) from the nose or mouth into the lungs due to muscle contraction, and exhaling it out due to muscle relaxation is known as breathing. Breathing is also known as pulmonary ventilation since pulmonary muscles are involved in the process. Pulmonary ventilation comprises two phases, inspiration also known as inhalation and expiration, also known as exhalation.

Animals breathe in oxygen released by plants at the end of the photosynthesis process and release carbon dioxide which is used by plants. How do you think all the organisms breathe? Different organisms have different methods of breathing. Plants breathe through the small pores present in their leaves known as stomata which are guarded by guard cells whereas animals breathe through their nose or sometimes mouth. Not all animals breathe through their noses, there are few exceptions. Certain animals like amphibians or reptiles respire from their skin. Fishes have a specialized organ known as gills that perform the function of respiration. Now let us study the mechanism of breathing in animals, particularly mammals.

Fig: Process of Breathing

Study The Concept Of Breathing Here

Differences Between the Respiration and Breathing

Respiration and breathing are two processes that are often confused with being the same, but which is not at all the truth. Cellular respiration and breathing are two completely different processes with significant differences between them. These differences are mentioned in brief for a better understanding.

Muscles Involved in the Breathing

The muscles that perform the function of respiration are known as breathing pump muscles.
The inherent potential that causes breathing action is caused by all the muscles that are attached to the rib cage.
The muscles that perform the function of expanding the thoracic cavity are called inspiratory muscles as they help in inhalation, whereas the muscles that help to contract or compress the thoracic cavity are called expiratory muscles as they induce exhalation. The speciality of these muscles is that they are made up of fatigue-resistant muscle fibres. The muscles involved in inspiration elevate the ribs and sternum, and the muscles involved in expiration depress the ribs and sternum. Certain accessory muscles are recruited during exercise due to increased metabolic activity and also during the dysfunction in the respiratory system. There are mainly three groups of muscles involved in respiration.

1. Diaphragm
2. Intercostal muscles (Ribcage muscles)
3. Abdominal muscles

Fig: Muscles Involved in the Breathing

Diaphragm: It is a thin internal double doomed sheet of skeletal or striated muscle that is located in the inferior most aspect of the rib cage and separates the abdomen from the thoracic region. It is a special parachute-shaped fibrous muscle. It expands and contracts rhythmically during respiration. It flattens and contracts during inspiration which creates a vacuum effect that pulls air into the lungs. During expiration, the diaphragm relaxes, and the air is pushed out of the lungs.

Intercostal muscles: The intercostal muscles lie in between the ribs in the chest cavity. They help in expanding and shrinking or compressing the chest cavity while breathing. There are two kinds of intercostals that help in respiration, internal intercostals and external intercostals. The external intercostal is the one that helps in breathing. Some fibres are angled obliquely from downward to the upward layer of ribs that help in contracting and relaxing the rib cage while breathing. On the other hand, internal intercostal muscles are angled obliquely downward and backward from ribs to ribs, thereby helping it during exhalation.

Abdominal muscles: These are the accessory muscles that help to raise the diaphragm during inspiration and give power to the diaphragm to inhale air, and also helps to relax the diaphragm during exhalation. Once the air inhaled crosses the diaphragm bearing limits and the pleural pressure is more than the atmospheric pressure, the abdominal muscles facilitate the diaphragm for easy exhalation of air.

Mechanism of Breathing

The space between the outer surface of the lungs and the inner thoracic wall is called the pleural space. It is usually packed with pleural fluid, which forms a seal to hold the lungs against the thoracic wall by the force of surface tension. This seal assures that when the thoracic cavity enlarges or decreases, the lungs undergo expansion or reduction in size accordingly.

During breathing, the contraction and relaxation of muscles lead to a change in the volume of the thoracic cavity. As the thoracic cavity and lungs move together, the change in the volume of the lungs changes the pressure inside the lungs. The major mechanisms that drive pulmonary ventilation are the three types of pressures.

1. Atmospheric pressure \(\left( {{{\rm{P}}_{{\rm{atm}}}}} \right)\) is the amount of force exerted by the gas in the air.
2. Alveolar pressure \(\left( {{{\rm{P}}_{{\rm{alv}}}}} \right)\) is the pressure within the alveolar cavity.
3. Intrapleural pressure \(\left( {{{\rm{P}}_{{\rm{ip}}}}} \right)\) is the pressure within the pleural cavities.

The mechanism of breathing follows Boyle’s law states that the volume of gas is inversely proportional to pressure (at constant temperature). Hence,
When the volume of the thoracic cavity raises: The volume of the lungs increases, and the pressure within the lungs decreases.
When the volume of the thoracic cavity falls: The volume of the lungs decreases, and the pressure within the lungs increases.

Fig: Simple Diagram to Show Breathing Process

The intra-alveolar pressure is always equal to the atmospheric pressure since it is connected to the atmosphere via tubings of the airways, whereas inter pleural pressures are always lower due to certain characteristics of the lungs. The pressure difference drives the pulmonary ventilation as the air flows down the pressure gradient (the air flows from the region of higher pressure to the region of lower pressure). Air moves into the lungs based on this principle.

The atmospheric pressure is higher than the intra-alveolar pressure, which is higher than the intrapleural pressure. The same principle is followed during expiration; that is, the pressure in the intrapleural cavity is greater than the intra-alveolar cavity, which is greater than the atmospheric pressure.
Breathing involves two major steps: Inspiration (the process that allows air into the lungs) and Expiration (the process that allows air to leave the lungs). One sequence of inspiration and expiration comprises a respiratory cycle. Let us learn these steps in more detail.

Fig: Mechanism of Breathing

Inspiration

Inspiration is the process through which air enters the nostrils and reaches the lungs. It occurs due to the increase in the lung volume due to the diaphragm contraction and chest wall expansion, which results in a decrease in lung pressure compared to the atmospheric pressure; thereby, air rushes into the airway. In general, two kinds of muscles are used during normal inspiration, diaphragm and external intercostal muscles. Certain accessory muscles are also used during a deep breath.

1. During the contraction of the diaphragm, the diaphragm moves inferiorly towards the abdominal cavity, creating a larger thoracic cavity and a larger space for the lungs.

2. Since the external intercostal muscles contract, the ribs move upwards and outwards, causing the expansion of the rib cage, thus, increasing the volume of the thoracic cavity.
3. The pleural cavity has pleural fluid in it, which has the adhesive force and also facilitates the expansion of the thoracic cavity. Thus, it forces the lungs to stretch and expand.
4. As there is an increase in the volume of the lungs, it leads to a decrease in the intra-alveolar pressure, which creates a pressure lower than the atmospheric pressure.
5. As a result, the pressure gradient is created, and hence the air is driven into the lungs.

Fig: Simple Flowchart of Inhalation Process

Expiration

Expiration is the process through which the air present in the lungs is exhaled out. It occurs due to the decrease in the lung volume, which happens because of the elastic recoil of the lung tissue, which increases the lung pressure in comparison to the atmospheric pressure; thereby, air moves out of the airway. There is no muscle contraction during exhalation. Therefore, it is considered as a passive process which means that there is no utilization of energy for the outward movement of air from the lungs.

1. The elasticity of the lung tissue helps to recoil the lungs since the diaphragm and external intercostal muscles relax the following inspiration.
2. Followed by the decrease in the thoracic cavity and lung cavity, there is an increase in intrapulmonary pressure.
3. The increase in the intrapulmonary pressure increases the atmospheric pressure, which creates a pressure gradient, allowing the air to flow out of the lungs.

Fig: Simple Flowchart of Exhalation Process

Respiratory Volumes and Capacities

Air present in the lungs is measured in terms of lung volumes and lung capacities. Lung volumes estimate the amount of air for a particular function, whereas lung capacities are the sum of two or more volumes. Lung volumes are measured by a technique called spirometry.
Various animals show different lung capacities depending on their activities. For instance, cheetahs have developed a much higher lung capacity than us to provide enough oxygen to all the muscles of the body and allow them to run pretty fast. Similarly, Elephants also have a large lung capacity due to their heavy body and their requirement to take up oxygen by their body size. In simple terms:

Volume: Volume measures the amount of air for one function, such as inhalation or exhalation. Capacity: Capacity is any two or more volumes, for example, how much can be inhaled from the end of a maximal exhalation.
The volume in the lung can be divided into four units:
1. Tidal Volume (TV): It measures the amount of air that is inspired and expired during a normal breath.
2. Expiratory Reserve Volume (ERV): It is the additional or reserve amount of air that can be exhaled after a normal exhalation.
3. Inspiratory Reserve Volume (IRV): It is the additional amount of air that can be inhaled after a normal inhalation.
4. Residual Volume (RV): It is the amount of air left after expiratory reserve volume is exhaled.

The lung capacities can be explained by the terms mentioned below:
1. Total Lung Capacity (TLC): It is the total volume of air-filled in the lungs after a forced inspiration. Hence, we can say that, \({\rm{TLC = TV + ERV + IRV + RV}}\)
2. Vital Capacity (VC): It is the total volume of air that can be expired after maximum inhalation, or in simple words, it is the maximum air that a person can breathe in after forced expiration.
\({\rm{VC = TV + ERV + IRV}}\)
3. Inspiratory Capacity (IC): It is the total volume of air that can be inspired.
\({\rm{IC = TV + IRV}}\)
4. Functional Residual Capacity (FRC): It is the total volume of air residing within the lungs after an exhalation process.
\({\rm{FRC = ERV + RV}}\)

Fig: Lung Volumes and Capacity

Summary

Breathing is a natural process that involves inhaling oxygen and exhaling carbon dioxide. Inspiration and expiration are two main processes involved in the mechanism of breathing. Contraction of the diaphragm and the external intercostal causes inspiration, and relaxation of these muscles causes expiration. Change in atmospheric pressure, alveolar pressure and intrapleural pressure causes contraction and relaxation of muscles leading to gaseous movement from the air to the body and reverse. The diaphragm, intercostal muscles (Rib cage muscles), and abdominal muscles are the main muscles involved in breathing.

Frequently Asked Questions (FAQs) on Mechanism of Breathing

The most frequently asked questions about Mechanism of Breathing are answered here:

Learn The Mechanism of Respiration

We hope this detailed article on the Mechanism of Breathing helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.