Gases - Exchange and Regulation

Gases exchange is a process where the oxygen enters into the bloodstream from the lungs and carbon dioxide is eliminated to the lungs from the bloodstream. Apart from oxygen and carbon dioxide certain organisms also exchange methane, nitrogen, and hydrogen across their cell membrane with the environment. Oxygen is an important component for the survival of most of the living beings on Earth. Gas exchange is considered a physical process that takes place through diffusion. In most living beings, Gases are taken in by metabolic reactions or cellular activity. Hence for the proper exchange of gases, the interior of the cell and the external environment are two major criteria that play an important role. Organisms are divided into unicellular and multicellular. In both, the process of exchange, transportation, and regulation of gases vary. In unicellular organisms like Protozoa and Bacteria, the exchange of gases takes place through the cell membrane. However, this mechanism is advanced when it comes to multicellular organisms wherein, the exchange of gases takes place between internal organs (Lungs) and the external environment. 


When it comes to organisms, the gas exchange is the major principle for the process of Respiration. This involves inhaling oxygen and exhaling carbon dioxide since our body requires oxygen to carry out many functions in the body. This is reversed when it comes to plants since the process of photosynthesis in plants requires carbon dioxide to convert light energy into chemical energy. 

Diffusion being a passive process requires no energy when it comes to transportation. It follows Fick’s law. Generally, Gas molecules travel from the region of high concentration to a region of low concentration. In living beings, the gas exchange requires a moist environment. For faster diffusion, the inclination in the concentration should be higher across the gas exchanging surface. For example, when it comes to unicellular organisms the amount of gas they produce at a given time is less when compared to the surface area of their cell membrane. Hence the process of gas exchange does not need any modification. The more the organism increases in size, it’s surface area and volume change. However, in multicellular organisms, the respiratory organs like lungs, gills, and skin tend to provide extra surface area for gas exchange with respect to the external environment.



To understand the process of the exchange of gases, the anatomy of the organ responsible for the process of respiration is necessary. When it comes to Mammals, the air first enters into the body through the Nose and moves into the Pharynx. It then passes through the Larynx and enters the Trachea. The Trachea branches into the right and left Bronchus to enter into the Lungs. Within the Lungs, these further divide into branches known as Bronchioles. The Bronchioles end into Alveoli. Alveoli are tiny air sacs in which the process of exchange of oxygen and carbon dioxide molecules between the air in the environment and the bloodstream takes place. These Alveoli inflate during inhalation of air and deflate at the time of exhalation much like blowing air in a balloon to look puffed up and releasing the air to look collapsed. In Mammals, the exchange of gases is meant to be the absorption of oxygen into the bloodstream from the Lungs. In exchange, carbon dioxide is eliminated from the bloodstream to the Lungs. The process begins with inhalation of air and then exhalation of the same. The process of exchange within the lungs will be between the Alveoli and Capillaries that are tiny blood vessels located on the walls of the Alveoli. There is a membrane that lies between the Alveoli and the Capillaries which they share and in which the oxygen and Carbon dioxide tend to move freely between the bloodstream and the Respiratory system. The Red blood cells collect the oxygen molecules from these Capillaries and these molecules travel to the Heart. In return, the carbon dioxide molecules are released from the body with each exhalation. 

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Apart from mammals in which the process of respiration takes place through the Lungs, there are other animals in which the organ of gaseous exchange in the body will be through Gills and Skin like aquatic animals and some worms and amphibians respectively. 

There is a network of capillaries that lie beneath the skin. Through this, the exchange of gases takes place between the external habitat and the circulatory system of the organism. Since the diffusion of gases takes place easily in moist surfaces, hence the respiratory surface must remain moist for the gases to diffuse easily across the cell membrane.

The animals that live underwater also need oxygen for survival. They respire through Gills. The animals living underwater use dissolved oxygen in the water for the process of respiration. The dissolved oxygen in the water is lower in concentration when compared to the external habitat. 

Gills are folded tissue filaments that can be seen over aquatic animals. When the water enters the Gills, the oxygen that is dissolved in water easily diffuses into the Gills reaching the bloodstream of the aquatic animals. Then it is the duty of the circulatory system to carry the oxygenated blood to the rest of the body. This is how the process of gaseous exchange takes place in the water. Few animals have coelomic fluid rather than blood. The coelomic fluid is secreted by organisms like Earthworms to retain moisture which enables them to carry out their physiological activities like burrowing and respiration. This fluid comprises of the plasma, a watery matrix, and a vast number of coelomocytes. When it comes to exchanging gases in the water medium, the concentration of oxygen molecules in water is higher than that in the Gills. Since in the process of diffusion, the molecules travel from higher concentration levels to lower concentration, the oxygen then easily diffuses from the water into the Gills and the carbon dioxide is released back from the blood back to the Gills and enters the water. 

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In Insects, respiration takes place through the Tracheal system. The tracheal system is nothing but the distribution of small tubes in the entire body. These tubes are responsible for carrying oxygen throughout the body. The tubes of the Tracheal system are made up of material known as Chitin. Chitin is a fibrous material comprising of polysaccharides. This is a primary and important component in the exoskeleton of Arthropods. And also they form cell walls in fungi. 

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The body of the insect opens into openings known as Spiracles along Thorax and Abdomen. These openings are responsible for passing oxygen into the body and regulating the diffusion of carbon dioxide that is let out back into the environment.


The birds also respire through the Lungs but unlike other animals, their Lungs are smaller. The 9 air sacs play an important role in respiration along with Lungs. The exchange of gases takes place between blood capillaries and air capillaries rather than Alveoli. The birds require a lot of oxygen for respiration during their flights since flying consumes a lot of energy.

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In the higher altitudes, the oxygen level is low and during the flight sessions of birds, the flow of air is in the opposite direction from the blood flow. The air enters the posterior air sacs and reaches the lungs and then comes out of the anterior air sacs Thus the exchange of gases takes place more easily. This type of breathing helps birds to obtain maximum oxygen supply even at higher altitudes. While other animals require one cycle of inhalation and exhalation to complete the breathing process, the birds require two cycles of inhalation and exhalation to complete the breathing and get the air out of the lungs completely due to the direction of airflow. 


Plants require carbon dioxide for the process of photosynthesis. They need oxygen for respiration and carbon dioxide for photosynthesis. Leaves are responsible to carry out this process. The gases diffuse into the intercellular spaces of the pores of the leaves that lie on the underside of the leaf called Stomata. Plants photosynthesize during day time by the intake of carbon dioxide, in turn, releases oxygen and water vapor into the environment. This process, however, reverses during night time when the plants do not photosynthesize but respire. They release the water vapor but now they intake oxygen and release carbon dioxide. 


When we respire air, there are several dust particles that enter into our respiratory system. These particles if reach to the Lungs might cause damage to the Lungs. Hence our respiratory system has certain protective mechanisms to avoid this damage. The protective layer starts right in the nose that has hair to trap any dust particles that enter into the nasal cavity and prevent them from entering further. Our lungs produce mucus that traps the particles if they enter further. There are also cilia that are hair-like projections present on the walls of bronchi and bronchioles. These cilia trap the unwanted particles and along with the mucus eliminate it out of the respiratory system by moving the mucus particle back to the throat in the form of reflex cough and can spit out from the mouth or else it is swallowed and eliminated through the Esophagus by entering into the digestive system. 

FAQ (Frequently Asked Questions)

1. How is the Gas exchange regulated at the body system level?

Ans: The gases like oxygen, carbon dioxide, and the PH level serve as important factors to adjust blood flow in the capillary network with regard to Alveoli. The diameter of the airways is responsible for ventilation. However, the diameter of the blood vessels is responsible for perfusion.

2. What effects the Gaseous exchange?

Ans: Conditions like Asthma and COPD affect the Gaseous exchange. Activities like cycling, exercising also tend to increase the lung capacity and respiration rate. Also, the Heart rate gets affected. Smoking also has an effect on Gaseous exchange since it depletes the quality of Lungs, damages the Alveoli, and also decreases the surface area for the gas exchange.

3. What is the main site for gas exchange in humans?

Ans: Alveoli is the main site of gas exchange in humans. These are the anatomical structures located inside the lungs that appear like small balloons that inflate while inhalation and deflate on exhalation. Inside the Lungs, these are attached to the end of the bronchioles.

4. What are the 4 phases of respiration?

Ans: The respiration cycle comprises of 4 phases: 

  • The inspiratory flow phase

  • Pause phase during inspiration

  • Expiratory flow phase

  • Pause phase during expiration.

5. What is the importance of Gas exchange?

Ans: Gas exchange is very crucial for the survival of an organism or the animals. Since it provides oxygen to the cells of the living organisms so that they can acquire energy from organic molecules without which the organism can be considered dead.