Exchange of Gases: Do you know how the exchange of gases takes place in our body? Breathing is a process in which oxygen is taken in, and carbon dioxide is released out of the body. During normal breathing, two processes take place, i.e., inspiration and expiration. Inspiration is an active process, whereas expiration is a passive process. Inspiration means intake of atmospheric oxygen, and expiration means the release of carbon dioxide out of the body. Read this article to know more about how the exchange of gases takes place in our bodies.

Respiration

It is a process that involves the exchange of gas in our bodies. The process of respiration involves the following steps:

1. Inflow of air between the atmosphere and the alveoli of the lungs
2. Diffusion of gases that is oxygen and carbon dioxide across the alveolar membrane
3. Transport of gases by the blood
4. Diffusion of oxygen and carbon dioxide between the blood and tissues
5. Utilization of oxygen by the cells for catabolic reactions and resultant release of carbon dioxide

This entire above-mentioned process is known as breathing.

Respiration is of two types: Aerobic respiration and anaerobic respiration. Aerobic respiration is the type of respiration that occurs in the presence of oxygen, which means food is oxidized in the presence of oxygen. It involves the complete breakdown of the substrates into carbon dioxide, water, and energy is released. On the other hand, when food is oxidized in the absence of oxygen, it is known as anaerobic respiration. Here, organic food is incompletely oxidized to liberate energy, and the end products formed are lactic acids, carbon dioxide, and ethyl alcohol.

Difference between Breathing and Respiration

Breathing	Respiration
It is the intake of fresh air and exhale of foul air	It is the oxidation of food to form carbon dioxide, water, and energy
It occurs outside the cell, hence called as extracellular process	It occurs inside the cell, hence called as intracellular process
It is a physical process	It is a biochemical process
Energy is not released in this process	Energy is released in this process
Enzymes are not involved in this process	Enzymes are involved in this process

Human Respiratory System

Humans have a pair of external nostrils opening out above the upper lips that open into two nasal cavities. Nasal cavities are separated from each other by a cartilaginous septum called the nasal septum. Air then passes to the pharynx, larynx and then enters into the trachea. The trachea further divides into the right and left bronchi, which enter into the right and left lung, respectively. In the lungs, bronchi continue to divide further into small branches called bronchioles. At the end of terminal bronchioles, alveoli are present. Alveoli are tiny air sacs where the exchange of gases takes place.

Exchange of Gases in Humans

Blood acts as a transport medium for the exchange of oxygen and carbon dioxide between the lungs and the tissues. Tissue cells take up oxygen from the blood and eliminate carbon dioxide into it. The exchange of gases occurs at the alveolar surface and also between blood and tissues in humans.

Exchange of Gases at the Alveolar Surface

The gaseous exchange takes place between alveoli present in the lungs and the blood capillaries which are present around them. This is a process in which passive diffusion occurs. Oxygen is moved from the alveoli into the blood, and carbon dioxide is moved from the blood capillaries into the alveoli which is exhaled out of the body.

Process of Gas Exchange in Lungs

When blood passes through the lungs, oxygen is picked up, and carbon dioxide is released by the diffusion process. This exchange of gases occurs between alveoli and blood. Blood entering the pulmonary capillaries is venous blood which has a low partial pressure of oxygen, that is pO₂ is 40 mmHg, whereas partial pressure of carbon dioxide is relatively high, that is pCO₂  is 46 mm Hg. Alveolar blood has pO₂ at 104 mmHg. Diffusion of gases takes place from higher partial pressure to low partial pressure. Thus, oxygen diffuses from the alveoli into the blood. Now, the partial pressure of oxygen in pulmonary capillaries is the same as that of the alveoli, which is 104 mmHg. On the other hand, pCO₂ in pulmonary capillaries is 46 mmHg, and pCO₂ in alveoli is 40 mmHg, so carbon dioxide diffuses from the blood into the alveoli.

Fig: Section of Alveolus during Gaseous Exchange

Exchange of Gases Between Blood and Tissues

The exchange of gases occurs between the blood and the tissue cells through tissue fluids that are present around the tissue cells.  The arterial partial pressure of oxygen is 104 mmHg, and partial pressure in the adjacent cell is 40 mmHg. So, oxygen diffuses from high partial pressure to low partial pressure until an equilibrium is achieved. Similarly, partial pressure of carbon dioxide is more in tissue cells as compared with that in the blood. The partial pressure of carbon dioxide in the cells is 45 mmHg whereas in the blood it is 40 mmHg. So, carbon dioxide diffuses from cells of tissues into the blood. Diffusion of carbon dioxide occurs till equilibrium is attained.

Due to these differences in partial pressure of gases, oxygen from blood diffuses into the tissues, and carbon dioxide from tissues diffuses into the blood.  Blood present in the veins has low pO₂ and a high pCO₂. This type of blood in which the amount of oxygen is less is known as deoxygenated blood. This deoxygenated blood returns to the heart and then to the lungs for getting oxygenated.  This cycle repeats itself in the body.

Fig: Gas Exchange Showing the Difference in Partial Pressure of Oxygen and Carbon Dioxide

Factors Affecting Gaseous Exchange are:

1. The surface area should be large for gaseous exchange. It is provided by the presence of numerous alveoli in the lungs.
2. The diffusion path should be short between alveolar air and blood for faster gaseous exchange. If the diffusion pathway is longer, then an exchange of gas occurs at a slower rate.
3. The difference in the partial pressure of oxygen and carbon dioxide gas affects the gaseous exchange. Greater is the difference in the partial pressure of gases; faster is the rate of diffusion.
4. The respiratory membrane should be thin for the effective exchange of gases.

Exchange of Gases in Plants

Diffusion is very important in plants since it is the only means for gaseous movement within the plant body. Movement by diffusion is slow, passive, and occurs through a membrane from high to low concentration area. Energy is not utilized in the process of diffusion. In plants, the exchange of gases in plants takes place with the help of stomata, lenticels, etc.

Stomatal Apparatus: Stomata are the tiny pore-like structures that are present in the epidermal surface of leaves. Stomata are surrounded by kidney-shaped or bean-shaped epidermal cells called guard cells. The walls of guard cells are thick on the inner side and thin on the outer side. These guard cells are surrounded by some specialized epidermal cells called subsidiary cells or accessory cells. When turgor is developed within the guard cells, the thin outer wall gets bulged out, and the inner wall becomes crescent in shape. This causes the opening of the stomata. When guard cells lose turgor, the thin walls regain their original shape resulting in the closure of stomata.

Factors affecting opening and closing of stomata are:

1. Carbon dioxide concentration affects stomatal movements. A high concentration of carbon dioxide in intercellular spaces of leaves causes closure of stomata while low concentration induces its opening.
2. Light is required for the opening of the stomata, and darkness causes its closure. The maximum opening occurs in red and blue light.
3. Temperature causes the opening of stomata only to a certain extent. High temperatures above 30 degrees celsius lead to the closure of stomata.
4. Water deficiency causes closure of stomata, whereas its excess causes opening of stomata.
5. The presence of glucose lowers the water potential, causing an influx of water into the guard cell, resulting in the opening of stomata.
6. The uptake of potassium ions by the guard cells causes the opening of stomata. Potassium ions cause an increase in the osmotic pressure in the guard cells, thus causing the opening of stomata.
7. An increase in the presence of organic acid in the plants causes the opening of the stomata.
8. Abscisic acid is the hormone that is present in plants. This hormone gets accumulated in the plants during water scarcity, thus responsible for the closing of stomata in water insufficient conditions.
9. Auxins and cytokinin also help in the opening and closing of the stomata. The presence of auxins in the optimum range stimulates the opening of the stomata because its high concentration inhibits the opening of stomata. The presence of cytokinin in high concentrations causes the opening of the stomata.

Summary

Breathing is a process in which oxygen is taken in, and carbon dioxide is released out of the body. Respiration is a process that involves the exchange of gas in our bodies. The exchange of gases in the human body occurs at the level of the alveolar membrane and between the blood and tissues. Diffusion of gas takes place from an area of high concentration to an area of low concentration. Various factors affect the flow of gas across a membrane. The exchange of gases in plants takes place with the help of stomata.

Frequently asked questions (FAQs) on Exchange of Gases

Q.1. Define respiration.
Ans: Respiration is a process that involves the exchange of gas in our bodies.

Q.2. Where does the exchange of gases take place in humans?
Ans: The exchange of gases occurs at the alveolar surface and also between blood and tissues in humans.

Q.3. What are the factors affecting gaseous exchange in humans?
Ans: Factors affecting gaseous exchange are surface area, diffusion path between alveolar air and blood, the concentration gradient of oxygen and carbon dioxide gas, and thickness of the respiratory membrane.

Q.4. How does the exchange of gases take place in plants?
Ans: The exchange of gases in plants takes place with the help of stomata and the structures associated with it. The opening and closing of stomata occur during the exchange of gases.

Q.5. What are the factors responsible for the opening and closing of stomata?
Ans: Various factors responsible for the opening and closing of stomata are light, temperature, carbon dioxide concentration, water, glucose, hormone-like abscisic acid, etc.

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