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Introduction

Organisms breathe in oxygen in order to break down food molecules into glucose, amino acids, etc. This process results in breathing out the carbon dioxide produced by the cells of our body. Therefore, the process of breathing in oxygen (O2) and breathing out carbon dioxide (CO2) is called respiration. 

The continuous exchange of gases is a catabolic process. The breathing and exchange of gases allow the body to function smoothly and produce energy. In this process, the gases are inhaled and exhaled in the lungs through the nostrils.

The Respiratory System

The human respiratory organs comprise a pair of nostrils. It continues to the nasal chamber till the pharynx and opens through the sound-box, i.e., the larynx, into a straight tube called the trachea. The glottis is covered by a thin elastic layer called the epiglottis that prevents food from entering the larynx. 

The trachea divides the mid-thoracic cavity into two parts called bronchi. These bronchi are further divided into secondary and tertiary bronchi and terminal bronchioles. These bronchioles produce innumerable thin, undulating, walled and vascularised bag-like structures termed alveoli. 

Human Respiratory System

Both the lungs are double-layered by a pleural membrane. There is a pleural fluid between the lungs. The respiratory organs from the nostrils to the terminal bronchioles form the conducting part. On the other hand, the alveoli and their ducts comprise the respiratory or exchange part.

The functioning of the conducting part includes:

  • Transporting the air to alveoli
  • Clearing all the unwanted particles
  • Bringing the air to a normal temperature

The lungs are arranged in an anatomically airtight chamber called a thoracic chamber. The arrangement is such that any change in the thoracic cavity volume will indirectly be visible in the pulmonary cavity. As a result, this arrangement plays a crucial role in the breathing and exchange of gases.

The process of respiration takes place through the following steps:

  • The first step is pulmonary ventilation (also known as breathing). Here, the oxygen-rich air is drawn in, and the carbon dioxide-rich air is drawn out
  • Across the alveolar membrane, diffusion of gases (oxygen and carbon dioxide) takes place
  • Transport of the gases
  •  Diffusion of gases takes place between tissues and blood
  • The process of cellular respiration takes place

Mechanism Of Breathing

There are two stages of breathing, namely inspiration and expiration.

  • Inspiration: In this process, the oxygen from the atmospheric air is drawn into the nostrils. Here, the intrapulmonary pressure is lower than the atmospheric pressure. The contraction of the diaphragm takes place, leading to an increase in the volume of the thoracic chamber

As a result, the pulmonary volume increases, causing the intrapulmonary pressure to decrease than the atmospheric pressure. Because of this, the oxygen-rich air from the atmosphere can reach the lungs quickly.

  • Expiration: In this process, the carbon dioxide from the alveoli is released out into the air. Here, the intrapulmonary pressure is comparatively higher than the atmospheric pressure

After inspiration, the diaphragm relaxes, leading to an increase in the intrapulmonary pressure than the atmospheric process. This causes the carbon dioxide-rich air from the lungs. This process continues in all human beings.

Respiratory Volumes

  • Tidal volume means the volume of air inspired and expired during respiration. Typically, a healthy human inspires or expires approximately 6000 mL to 8000 mL of air every minute
  • Inspiratory reserve volume is the additional air volume that a human can inspire forcibly. Generally, it ranges somewhere around 2500 mL to 3000 mL
  • Expiratory reserve volume is the additional volume of air that a human can forcibly expire. It ranges between 1000 mL to 1500 mL
  • Residual volume is the volume of air left in the lungs after forcible expiration. It is generally 1100 mL to 1200 mL

Respiratory Capacities

  • Inspiratory capacity is the addition of tidal volume and inspiratory reserve volume
  • Expiratory capacity is the addition of tidal volume and expiratory reserve volume
  • Functional residual capacity is the addition of expiratory reserve volume and residual volume
  • Vital capacity is the addition of inspiratory reserve volume, expiratory reserve volume, and tidal volume
  • Total lung capacity is the addition of residual value and vital capacity

Exchange of Gases

The exchange of gases occurs in the alveoli by simple diffusion based on the pressure or the concentration gradient. The diffusion rate can also be affected by factors such as the solubility of gases and the thickness of membranes.

The pressure added by a single mixture of various gases is known as partial pressure (pO2 for oxygen and pCO2 for carbon dioxide). As shown in the above-given diagram, the pO2 in the alveoli is 104 mmHg and in the blood is 40 mmHg. On the other hand, the pCO2 in the alveoli is 40 mmHg and in the blood is 45 mmHg. The concentration gradient is created between the blood and the alveoli.

Conclusion

Breathing involves two stages- inspiration and expiration. In these two stages, the human body experiences an intake of oxygen-rich air and releases the waste gases such as carbon dioxide. This process helps the human body to produce energy. The respiratory tract is the path of respiratory organs that allows the transport of gases in and out (from the nose to the lungs). 

The breathing and exchange of gases help the human body to function smoothly. The exchange of gases (oxygen and carbon dioxide) is undertaken in the alveoli. Hence, alveoli are known as the functional units of the lungs.