Mechanism of Breathing

Introduction

Breathing is a process carried out by an organism throughout life. During breathing, we inhale oxygen from our environment and exhale carbon dioxide. Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation. Lower invertebrates like sponges, coelenterates, flatworms, etc, exchange O2 with CO2 by simple diffusion over their entire body surface. Earthworms use their moist cuticle and insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body. Among vertebrates, fishes use gills whereas amphibians, reptiles, birds and mammals respire through lungs. 

Inspiration

At the time of inspiration, contraction in diaphragm and external intercostal muscles takes place.

Diaphragm becomes flat and is pushed towards the abdominal cavity.

Sternum moves towards ventral and anterior direction.

Ribs move towards the outside and ventral side. As a result of all these reactions, the volume of the thoracic cavity is increased. 

So the pressure of coelomic fluid over lungs is decreased. Due to the spongy nature, lungs expand. 

As a result of this, air pressure in the lungs is decreased by 1-3 mm mercury columns as compared to atmospheric pressure. Now air rushes from outside, through the respiratory track and fills the lungs. Inspiration is an active process. Normally it takes around 2 seconds.Inhalation is also known as inspiration. 

Expiration

Normal expiration is a passive activity. It takes around 3 seconds. During expiration, contraction(in any muscle) does not take place. 

During expiration, relaxation in diaphragm and external intercostal muscles takes place. As a result of this relaxation, diaphragm, sternum and ribs attain their actual (normal) position.

 Due to which the volume of thoracic cavity is decreased, and pressure of thoracic cage on lung is increased.Thus air which was filled in lungs goes outside through respiratory tract. Expiration during sneezing, coughing and yogabhyas is called forceful expiration. 

Exhalation, also known as expiration.

Mechanism of Forced Breathing

A cell in the digestive system requires glucose and oxygen to perform its functions, leading to the formation of ATP molecules, which act as energy resources for cells. Any activity requiring ATP is called an active process.

Forced breathing, also known as hyperpnea, is an active mode of breathing that occurs during exercises or actions that require manipulation of breathing such as singing. Both inspiration and expiration occur during forced breathing. In addition to diaphragm and intercostal muscles, accessory muscles, such as internal intercostal muscles and abdominal muscles also contract. 

Active Inspiration

Along with the diaphragm and external intercostal muscles, active inspiration includes the contraction of the accessory muscles required for breathing. 

The following muscles work together and expand the thoracic cavity: 

• Serratus anterior elevates the ribs and pulls the scapula forward around the thorax

• Scalenes elevate the upper ribs

• Latissimus dorsi elevates the lower ribs

• Sternocleidomastoid elevates the sternum

• Pectoralis major and minor pull the ribs outwards

Active Expiration

Several thoracic and abdominal muscles contract during active expiration. The following muscles reduce the thoracic cavity volume. 

• Internal intercostal and innermost intercostal muscles lower the ribs

• The anterolateral abdominal wall helps in increasing the intra-abdominal pressure and pushes the diaphragm upwards to the thoracic cavity

Composition of Air

Rate of Breathing 

The number of breaths taken by a person per minute is called the respiration rate. It is measured by counting the chest rises per minute when at rest. The rate of breathing can vary based on a person’s health. It may increase in case of a medical condition. 

The rate of breathing depends on a person’s activity. For an adult, it is around 12 to 16 breaths per minute. It increases by 20 to 25 times per minute during a vigorous workout. The spirometer measures the volume of air involved in breathing movements.

Respiratory Volumes and Capacities

• Tidal volume (TV) is the volume of air inhaled or exhaled during respiration. Around 6000-8000 ml of air per minute is observed in a healthy man

• Residual volume (RV) is the volume of air present in the lungs even after a forced exhalation, averaging 1100-1200 ml

• Inspiratory reserve volume (IRV) is the volume of air inhaled forcibly, averaging 1900-3300 ml

• Expiratory reserve volume (ERV) is the volume of air that can be exhaled forcibly, averaging 1000-1100 ml 

Some pulmonary capacities are derived from the above respiratory volumes for clinical diagnosis:

• Inspiratory capacity (IC = TV + IRV): Inclusive of the tidal volume and inspiratory reserve, inspiratory capacity is the volume of air one can inhale after completing exhalation

• Expiratory capacity (EC = TV + ERV): Inclusive of the tidal volume and expiratory reserve, expiratory capacity is the volume of air one can exhale after completing inhalation

• Functional residual capacity (FRC = ERV + RV): The volume of air present in the lungs after expiration

• Vital capacity (VC): The maximum volume of air one can inhale after a forced exhalation and includes ERV, TV, and IRV or the peak volume of air that is exhaled after a forced inhalation

• Total lung capacity: The total volume of air present after a forced inspiration. It includes RV, ERV, TV, IRV, or residual volume (RV) + vital capacity (VC)

Conclusion 

Breathing is a process where we inhale oxygen into our lungs and exhale carbon dioxide out of our bodies. A similar breathing process as that of humans is observed in animals. This process gets reversed in plants. The rate of breathing varies across species. The diaphragm is essential in the entire breathing process. It relaxes during expiration and contracts during inspiration.