Introduction 

Organisms use oxygen (O2) to chemically break down nutrient molecules like glucose and derive energy in the form of ATP (Adenosine triphosphate) to perform various tasks. 

During these catabolic events, harmful carbon dioxide (CO2) is emitted. Therefore, oxygen (O2) must be constantly supplied to the cells and the carbon dioxide (CO2) produced by the cells must be eliminated. Breathing is the mechanism of exchanging oxygen from the environment with carbon dioxide produced by the cells. In order to provide oxygen to the tissues and remove carbon dioxide from the body, the processes of inspiration (breathing in) and expiration (breathing out) are of importance.

Inhaling oxygen (O2) and expelling carbon dioxide (CO2) is the mechanical process of breathing. 

Inhalation or inspiration and exhalation or expiration are the two phases of the breathing mechanism. The process of inhaling air into the lungs is known as inspiration. During inspiration, the diaphragm muscles contract and the diaphragm moves downwards. As a result, the volume of the thoracic cavity expands while the pressure within the thoracic cavity decreases. This allows oxygen (O2) to rush into the lungs and fill the alveoli of the lungs. After that, the oxygen is transferred through the alveolar walls into the blood capillaries and is subsequently delivered to all the body’s tissues. At the same time, carbon dioxide (CO2), a waste product of tissues, is absorbed by the blood and transported to the alveoli of the lungs for expiration. 

The process of exhaling air from the lungs is known as expiration. In this process, the diaphragm muscles relax and the diaphragm moves upwards. As a result, the volume of the thoracic cavity decreases, whereas the pressure within the thoracic cavity increases. This causes carbon dioxide (CO2) to be expelled from the body.

Breathing

Breathing is the process by which air moves in and out of the lungs and is carried out by numerous respiratory organs such as the nostrils, pharynx, larynx, trachea, bronchi, bronchioles, lungs and alveoli. When we breathe, we take in oxygen-rich air from the environment and expel carbon dioxide-rich air into the atmosphere, which is used by plants for the process of photosynthesis.

Breathing Rate

An individual breathes multiple times during the day. One inspiration and one expiration constitute one breath. The number of times a person breathes in a minute is referred to as the breathing rate.

The normal breathing rate of an adult is nearly 15-18 times per minute. It may exceed 25 times per minute during vigorous activity.

The Mechanism of Breathing

• The pressure of air plays a significant role in understanding the mechanism of breathing.
• The pleural space is the area between the outer surface of the lungs and the inner thoracic wall. 
• This is normally filled with pleural fluid, forming a barrier that uses surface tension to retain the lungs against the thoracic wall. 
• This barrier assures that as the thoracic cavity expands or contracts, the lungs expand or contract in proportion.
• The contraction and relaxation of muscles during breathing causes the volume of the thoracic cavity to change. The volume of the lungs varies as the thoracic cavity and lungs move together, causing the pressure inside the lungs to change.

Boyle’s Law

When the temperature remains constant, Boyle’s law states that the volume of a gas is inversely proportional to pressure. Therefore:

• When the thoracic cavity’s volume increases, the lungs’ volume increases as well and the pressure within the lungs decreases.
• When the thoracic cavity’s volume decreases, the lungs’ volume reduces as well,and the pressure within the lungs increases.

The mechanism of breathing has two phases: inspiration and expiration.

1. Inspiration or inhalation

• Inspiration is an active process that requires energy and usually takes around two seconds.
• During inspiration, the diaphragm and the external intercostal muscles contract, causing the diaphragm to flatten and move towards the abdominal cavity.
• The sternum moves towards the ventral and anterior sides, whereas the ribs move towards the outer and ventral sides.
• As a result, the volume of the thoracic cavity expands while the pressure within the thoracic cavity decreases as compared to the atmospheric pressure.
• Thus, the air rushes into the lungs and fills the expanded alveoli.

2. Expiration or exhalation

• Expiration is a passive process that does not require energy and usually takes around three seconds.
• During expiration, the diaphragm and external intercostal muscles relax.
• The diaphragm, sternum and ribs all return to their original positions as a result of this relaxation.
• Due to this, the volume of the thoracic cavity decreases while the pressure within the thoracic cavity increases.
• As a result, the air in the lungs is forced out through the respiratory system.

Amount of Air Breathed During Respiration

The only difference between inhaled and exhaled air is the concentrations of oxygen (O2), carbon dioxide (CO2) and water vapour in each.

• In the case of oxygen (O2), the air we inhale has a higher proportion of oxygen than the air we exhale, which has a lower proportion of oxygen.
• In the case of carbon dioxide (CO2), the air we inhale has a lower carbon dioxide content, whereas the air we exhale has a higher carbon dioxide content.
• When it comes to water vapour, the air we inhale includes only a small amount of water vapour; however, the air we exhale has a larger amount of water vapour.

Conclusion

Breathing is the physiological mechanism through which terrestrial animals transport air in and out of their lungs. The cells of an organism require oxygen from the air in order to obtain energy from glucose through respiration which generates carbon dioxide and water as waste. As a result, oxygen is required for every aspect of proper cellular activity and oxygen shortage can result in serious health problems. Inhalation and exhalation are two steps in the breathing process. In this process, the oxygen (O2) is being taken in and the carbon dioxide (CO2) and water are being eliminated.