The respiratory system, in simple words, is responsible for the exchange of carbon dioxide and oxygen between an organism and its surrounding environment.

The respiratory system: An overview

The respiratory system is involved in the gaseous exchange between an organism and its surroundings. The processes of inhalation and exhalation are regulated by this system. The organization, anatomy, and physiology of the respiratory system vary from organism to organism. Some of the factors affecting the functioning and organization of the respiratory system include-

  • Environment; habitat of the organism under consideration.
  • Evolutionary history of the organism.
  • Size of the organism.

It is important to note that in most mammals and reptiles, air-sacs which are rich in blood supply called alveoli, come in contact with air via hollow tubes or cylinders (commonly known as the trachea) and enable the exchange of gases. In birds, these air-sacs are called atria. In aquatic organisms, the respiratory system is composed of gills. Gills allow the entry and exit of water into the body and carry out a gaseous exchange with the help of filaments called lamellae. These filaments provide a large surface area of vascularized tissues for gaseous exchange.

Fish gills

The respiratory system of amphibians is considerably different from both reptiles and mammals. Amphibians have primitive lungs made up of large alveoli that have a very slow rate of diffusion, hence making gaseous exchange through the lungs inefficient. To compensate for the lack of well-developed lungs, amphibians have developed the ability to respire through their skin which comes in contact with both air and water. Respiration is predominantly controlled and regulated by the process of buccal pumping. Frogs and toads

Insects and other invertebrates have very primitive respiratory structures, with respiration usually taking place via the skin.

Respiration in humans

Humans have a well-developed respiratory system for gaseous exchange. The respiratory system is made up of the following components-

  • Nasal cavity
  • Trachea, also known as the windpipe.
  • Sinuses 
  • Pharynx and larynx.
  • Bronchi 
  • Alveoli or air-sacs
  • Capillaries; are carriers of oxygenated blood. Anatomy and physiology of the respiratory system.

The breathing process

The breathing process in humans and mammals is an intricate one, evolved to ensure maximum efficiency and minimum energy consumption. The first step of the breathing process is inhalation followed by-

  • Movement of air into the lungs via the nasal passage and subsequently the trachea.
  • After air reaches the lungs, it is exposed to several bronchi. Bronchi are further differentiated into bronchioles which possess numerous tiny air-sacs called alveoli. The human body has over 600 million alveoli.
  • The alveoli are surrounded by small tubes with large surface areas, called capillaries. These capillaries are the site of interaction between the inhaled oxygen and blood in the body.
  • The oxygenated blood is then carried to the heart where it is pumped and delivered to the rest of the body through arteries.
  • Veins deliver deoxygenated blood to the lungs, where carbon dioxide is expelled and the entire aforementioned process is repeated all over again.

Inhalation and exhalation

The processes of inhalation and exhalation, simply put, mean the entry of air and exit of air respectively. The entry of air is enabled by the downward movement of the diaphragm which creates a vacuum that results in the rapid movement of air into the body. Similarly, when the diaphragm relaxes or deflates, the air is pushed out of the body. This is also coupled with the deflation of the lungs.

Diaphragm- anatomy

The diaphragm is located below the lungs and is a large dome-shaped muscle. It is characterized by its rhythmic contraction and relaxation which supports the entry and exit of air into the lungs for respiration.

Maintenance of respiratory hygiene

The human body is highly evolved so it’s no wonder that it has developed certain mechanisms to ensure that the respiratory system remains clean and uncontaminated. These defenses include-

  • Hair in the nostrils traps dust particles and other contaminants.
  • Cilia, or tiny hair-like structures present along the air passages, sweep away dust and dirt that makes its way into the body.
  • The bronchi and bronchioles have a mucosal lining that keeps the air passages clean by trapping dust, allergens, and pathogens. The mucus is usually coughed up or brought out through the nose to expel the contaminants.

SEM image of cilia

Respiratory diseases in humans

Respiratory diseases in humans can be caused due to both external and internal factors. Lifestyle has a major impact on respiratory health. For instance, habits such as tobacco smoking can seriously compromise the proper functioning of the respiratory system and in critical cases lead to cancer of the lungs. Some other major respiratory diseases and disorders include-

  • Asthma- is caused due to overproduction of mucus in the airways.
  • Pneumonia- is caused due to inflammation of the alveoli.
  • Tuberculosis- is caused due to infection by the pathogen Mycobacterium tuberculosis.
  • Emphysema- is caused due to inflation of the alveoli.
  • Idiopathic pulmonary fibrosis- is defined as the scarring of the lung tissue.
  • Cystic fibrosis- is a genetic condition that majorly affects respiration.


The organization and functioning of the respiratory system vary across organisms. The process of respiration is crucial for the life of any organism and hence each organism has a respiratory system developed for the optimal exchange of gases. In humans, the respiratory system is complex and efficient. It is self-regulating and does not require conscious control. The functioning of the respiratory system may be compromised by the entry of pathogens, mutagens, or the presence of genetic abnormalities.


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