Mechanism of Breathing- Definition, Diagram and Functions

Mechanism of Breathing: Definition, Diagram, Functions

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:40 PM IST

Breathing, or respiration, is the process of taking oxygen in and carbon dioxide. The mechanism of breathing contains two phases which are inhalation (inspiration), whereby the diaphragm contracts and the chest cavity expands, admitting air to the lungs and exhalation (expiration), whereby the diaphragm relaxes, forcing air out. The respiratory system coordinates this process for gas exchange. The understanding of the mechanism of respiration, especially in classes 10 and 11, helps explain how inspiration and expiration maintain the body's oxygen-carbon dioxide balance. This important topic from the Breathing Exchange of gases Chapter from Biology.

This Story also Contains

What is Breathing?
Process of Respiration
Mechanism of Breathing
Mechanism of Respiration

Mechanism of Breathing: Definition, Diagram, Functions

What is Breathing?

Respiration is the physiological exercise through which gases, mainly oxygen and carbon dioxide are exhaled through the lungs of an organism. This is important for cellular respiration as this moves the oxygen required in aerobic respiration to reach the cells and at the same time takes out carbon dioxide, a waste product. Proper functioning of the lungs guarantees a constant supply of oxygen to the cells for their metabolism and regulates the proper distribution of gases, which are significant for the life process.

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Process of Respiration

The process of breathing involves the following steps:

Inhalation (Inspiration)

Inhalation is mainly characterised by the contraction of the diaphragm and the intercostal muscles to raise the ribs and expand the thoracic cavity.
This contraction generates a vacuum, or negative pressure within the thoracic cavity and air rushes into the lungs.
During the process of inhalation, contraction of the muscles surrounding the thoracic cavity increases the volume of this cavity and thus forces the air through the respiratory tract and into the alveoli to participate in the exchange of gases.

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Exhalation (Expiration)

Exhalation takes place when the diaphragm and intercostals decrease their size decreasing the volume of the thoracic cavity.
This relaxation leads to positive pressure in the thoracic cavity which in turn forces air out of the lungs and out of the respiratory tract.
The reduction of volume ejects the air laden with carbon dioxide ending the breathing cycle.

Process of Respiration

Mechanism of Breathing

The mechanism of breathing involves the following:

Diaphragm and Intercostal Muscles

The diaphragm functions as a muscle which is in the shape of a dome and is situated at the bottom part of the thoracic cavity and the intercostal muscles are located in between the ribs of an individual’s body.
Inhalation is done by the contraction of the diaphragm, which becomes flattened and the intercostal muscles that lift the ribs, expanding the thoracic cavity. By doing so, the pressure in the thoracic cavity is reduced enabling air to enter the lungs.
These muscles contract during inhalation and, at the same time, they relax during exhalation to narrow the thoracic cavity hence forcing air out of the lungs.

Lung Compliance and Elasticity

Lung compliance on the other hand deals with the ease with which the lungs can expand and dilate given some pressure.
High lung compliance means that the lungs can be expanded easily and can therefore fill up easily while low compliance means that the lungs are stiff and cannot be filled easily.
Accessibility of the lungs can be affected by the degree of elasticity of the lung tissues, and the quantity and nature of the surface tension-reducing agents. This leads to disorders in respiration.
Lung compliance is an index of work that the respiratory muscles need to do to achieve a certain level of ventilation.

Airway Resistance

Airway resistance is the opposition force within the respiratory tracts to the flow through them.
This depends on the dimensions like the diameter of the airway, mucus production, and bronchial constriction.
There is always the case of high resistance, this is because of the constriction of the tubes through which air has to pass, or the presence of an obstructing factor that makes it difficult to breathe.
It is important to deal with airway resistance if breathing properly and if fresh and expired air is going to get to the lungs.

Mechanism of Respiration

Some important mechanisms of respiration are discussed below:

Intrapleural Breathing

The pressure in the pleural cavity, the space between the lungs and pleura, is referred to as intrapleural breathing. This pressure is less than that of atmospheric pressure, known as negative pressure, which is very vital in the mechanics of respiration. The transpulmonary pressure or the pressure difference is accountable for the lung movement during breathing. The intrapleural pressure becomes more negative during inhalation and causes the lungs to expand. During expiration, the pressure increases, causing the lungs to recoil.

The lungs' elasticity and the surface tension of alveolar fluid pull the lungs inward, while the thoracic wall and pleural fluid create an opposing force. This balance results in negative intrapleural pressure, a key concept in the mechanism of breathing.

Respiratory Gas Transport

Respiratory gas transport refers to the transport of oxygen and carbon dioxide by the blood. Oxygen-rich blood from the lungs flows to the heart through pulmonary veins. The heart pumps this oxygenated blood to the rest of the body through the aorta.At the same time, deoxygenated blood with carbon dioxide is returned to the lungs by the pulmonary arteries for gas exchange. The whole process is repeated continuously so that oxygen, carbon dioxide, and carbon dioxide balance are maintained in the body.

This explanation of the mechanism of breathing, how to inspire and expire, helps to describe more precisely how the respiratory system works in a human being.

The gas exchange in the lungs involves the following:

Diffusion of Gases

In the alveoli oxygen from the inhaled air moves passively through the walls of the alveoli into the blood in capillaries, and at the same time, carbon dioxide diffuses active from the blood into the alveoli and thus is exhaled.
This exchange is driven by partial pressure gradients. Oxygen diffuses from the place that has high partial pressure in the alveoli relative to that found in blood and similarly for the diffusion of carbon dioxide.

Hemoglobin and Oxygen Transport

Oxygen on reaching the lungs gets attached to the haemoglobin molecules and forms oxyhemoglobin which is easily transported to the tissues.
The oxygen dissociation curve shows how the affinity of haemoglobin for oxygen varies with different concentrations of oxygen partial pressure.
At high partial pressures, for example in the lungs, haemoglobin grabs more oxygen than from the lower pressure found in tissues where oxygen is needed, for use by the cells.

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Frequently Asked Questions (FAQs)

1. What is the mechanism of breathing?

Respiration is the process by which gases are taken into the lungs and expelled out of the lungs through the act of breathing. When the diaphragm and other such as intercostal muscles contract and expand the thoracic cavity the pressure drops and air rushes into the lungs. Expiration takes place when these muscles relax reducing the volume of the thoracic cavity and applying pressure on the lungs to force out the air.

2. What is the mechanism of breathing?

The mechanism of breathing is the process by which air moves in and out of the lungs. It involves two main phases: inhalation (breathing in) and exhalation (breathing out). These phases are controlled by changes in the volume of the thoracic cavity, which are caused by the contraction and relaxation of respiratory muscles, primarily the diaphragm and intercostal muscles.

3. How does the diaphragm help in breathing?

The diaphragm is a conical muscle which splits the thoracic cavity from the abdominal cavity. When the person breathes in, it relaxes and becomes thin to expand the volume of the thoracic cavity and produce a negative pressure that sucks in air into the lungs. When breathing out, the diaphragm becomes flattens, hence decreasing the volume of the thoracic cavity and forcing an expulsion of air from the lungs.

4. What is the difference between inhalation and exhalation?

Inspiration is the process of taking in air into the lungs and is accompanied by contraction of the diaphragm and the intercostal muscles which causes a negative pressure in the thoracic cavity.

Expiration is the process of the removal of air from the lungs and is characterized by relaxation of the diaphragm and intercostal muscles, hence leading to the reduction of the thoracic cavity volume and the pressure increases leading to expulsion of air.

5. How is breathing regulated in the body?

This is through the respiratory centre in the brainstem, especially the medulla oblongata and pons. These centres check the levels of the concentration of carbon dioxide, amount of oxygen and blood pH and modify the depth and frequency of breath. Other specialists also called sense organs help to feedback to various organs particularly the respiratory centre to ensure proper gas exchange and stability.

6. What are common disorders of the respiratory system?

Some of the diseases affecting the respiratory system are asthma, which involves inflammation and constriction of the airways; COPD embracing emphysema and chronic bronchitis cause obstruction of the airways; pneumonia, which is an infection affecting the lungs; and pulmonary fibrosis, which results from the formation of scar tissues in the lungs hence restricting breathing.

7. How does the composition of inhaled and exhaled air differ?

Inhaled air contains approximately 21% oxygen, 78% nitrogen, 0.04% carbon dioxide, and small amounts of other gases. Exhaled air contains about 16% oxygen, 78% nitrogen, 4% carbon dioxide, and is saturated with water vapor. The main differences are the decrease in oxygen content and the increase in carbon dioxide and water vapor in exhaled air, reflecting the gas exchange that occurs in the lungs.

8. What is the function of surfactant in the lungs?

Surfactant is a mixture of lipids and proteins produced by specialized cells in the alveoli. Its primary function is to reduce surface tension within the alveoli, preventing them from collapsing during exhalation. This allows the alveoli to remain open and functional, facilitating efficient gas exchange. Surfactant also helps in lung compliance, making it easier for the lungs to expand during inhalation.

9. What is the significance of residual volume in lung function?

Residual volume is the amount of air that remains in the lungs after a maximal exhalation. It's important because it prevents the lungs from completely collapsing, maintains airway patency, and provides a reservoir of oxygen between breaths. Residual volume also helps maintain alveolar gas exchange during the breathing cycle and contributes to the lung's overall elasticity and compliance.

10. What is the difference between anatomical and physiological dead space in the respiratory system?

Anatomical dead space refers to the volume of air in the conducting airways (like the trachea and bronchi) that doesn't participate in gas exchange. Physiological dead space includes the anatomical dead space plus alveolar dead space - areas of the alveoli that are ventilated but not perfused with blood. Understanding these concepts is important for assessing the efficiency of breathing and gas exchange in both healthy and diseased states.

11. How does the concept of partial pressures apply to gas exchange in the lungs?

Gas exchange in the lungs relies on the principle of partial pressures. Oxygen moves from the alveoli into the blood because its partial pressure is higher in the alveolar air than in the deoxygenated blood. Conversely, carbon dioxide moves from the blood into the alveoli due to its higher partial pressure in the blood. This pressure gradient-driven diffusion ensures efficient gas exchange without requiring energy expenditure.

12. How does the diaphragm contribute to breathing?

The diaphragm is a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. During inhalation, it contracts and flattens, increasing the volume of the thoracic cavity. This creates negative pressure in the lungs, causing air to rush in. During exhalation, the diaphragm relaxes and returns to its dome shape, decreasing thoracic volume and pushing air out of the lungs.

13. What role do intercostal muscles play in breathing?

Intercostal muscles are located between the ribs. During inhalation, the external intercostal muscles contract, pulling the ribcage upward and outward, which increases the thoracic volume. During exhalation, the internal intercostal muscles contract, pulling the ribcage downward and inward, decreasing thoracic volume. This coordinated action helps in the expansion and contraction of the lungs during breathing.

14. What is Boyle's Law, and how does it relate to breathing?

Boyle's Law states that the pressure of a gas is inversely proportional to its volume at constant temperature. In breathing, as the thoracic cavity expands during inhalation, the volume increases, causing a decrease in pressure within the lungs. This lower pressure draws air into the lungs. Conversely, during exhalation, the thoracic cavity decreases in volume, increasing pressure and forcing air out of the lungs.

15. How does pressure gradient affect air movement during breathing?

Breathing relies on pressure gradients. During inhalation, the expansion of the thoracic cavity creates a negative pressure (lower than atmospheric pressure) in the lungs, causing air to flow in from the higher-pressure atmosphere. During exhalation, the contraction of the thoracic cavity creates a positive pressure (higher than atmospheric pressure) in the lungs, pushing air out into the lower-pressure atmosphere.

16. What is the difference between eupnea and dyspnea?

Eupnea refers to normal, relaxed breathing that occurs without conscious effort. It's the regular breathing pattern observed in healthy individuals at rest. Dyspnea, on the other hand, is the medical term for difficulty breathing or shortness of breath. It's a subjective feeling of breathing discomfort and can be caused by various factors such as physical exertion, anxiety, or underlying health conditions.

17. How does the nervous system control breathing?

Breathing is controlled by the respiratory center in the brainstem, specifically in the medulla oblongata and pons. This center receives input from various sources, including chemoreceptors that detect changes in blood CO2, O2, and pH levels. It then sends signals to the respiratory muscles to adjust breathing rate and depth. The nervous system can also modify breathing in response to emotional states, physical activity, and conscious control.

18. What is the role of pleural membranes in breathing?

Pleural membranes consist of two layers: the parietal pleura lining the chest wall and the visceral pleura covering the lungs. Between these layers is a thin layer of pleural fluid. This arrangement allows the lungs to adhere to the chest wall and move smoothly during breathing. The negative pressure in the pleural space helps keep the lungs inflated and facilitates their expansion during inhalation.

19. How does the elasticity of lung tissue affect breathing?

Lung tissue is naturally elastic, meaning it tends to recoil and return to its resting position. During inhalation, the lungs stretch as they fill with air. During exhalation, the elastic recoil of the lung tissue helps push air out of the lungs. This elasticity contributes to the passive nature of normal exhalation and helps maintain the lungs' shape and function.

20. What is tidal volume, and how does it relate to breathing mechanics?

Tidal volume is the amount of air that moves in or out of the lungs during a normal breath at rest. It's typically about 500 mL in adults. Tidal volume is directly related to the extent of thoracic cavity expansion and contraction during breathing. Factors that affect thoracic volume changes, such as the strength of respiratory muscles and lung compliance, can influence tidal volume.

21. What is the role of the vagus nerve in breathing regulation?

The vagus nerve plays a crucial role in breathing regulation. It carries sensory information from the lungs and airways to the respiratory center in the brain. This includes data from stretch receptors that detect lung inflation and chemoreceptors that monitor blood gas levels. The vagus nerve is involved in the Hering-Breuer reflex and helps modulate breathing rate and depth in response to various physiological conditions.

22. What is the difference between external and internal respiration?

External respiration refers to the exchange of gases between the air in the alveoli and the blood in the pulmonary capillaries. It involves the movement of oxygen from the alveoli into the blood and carbon dioxide from the blood into the alveoli. Internal respiration, on the other hand, is the exchange of gases between the blood and body tissues. Here, oxygen moves from the blood into the cells, and carbon dioxide moves from the cells into the blood.

23. What is the role of the medulla oblongata in breathing control?

The medulla oblongata, located in the brainstem, contains the main respiratory control centers:

24. How does the negative pressure in the pleural cavity contribute to breathing?

The negative pressure in the pleural cavity, also known as intrapleural pressure, is crucial for breathing. It helps keep the lungs inflated and adhered to the chest wall. During inhalation, as the thoracic cavity expands, the negative pressure becomes more negative, helping to pull the lungs outward and facilitating their expansion. This pressure difference is essential for the efficient inflation and deflation of the lungs during the breathing cycle.

25. How does the process of breathing differ between rest and exercise?

During rest, breathing is primarily controlled by the autonomic nervous system and is relatively shallow and slow. During exercise, breathing rate and depth increase to meet the body's increased oxygen demand. This involves greater use of accessory muscles, increased tidal volume, and higher respiratory rate. The transition from nose to mouth breathing often occurs to reduce airway resistance and increase air intake.

26. What is the role of accessory muscles in breathing?

Accessory muscles of respiration, such as the sternocleidomastoid, scalene, and pectoralis minor muscles, are not typically used during quiet breathing. However, during deep or labored breathing, these muscles contract to further expand the chest cavity. They assist in lifting the upper ribs and sternum, increasing thoracic volume and enhancing air intake. This is often seen during exercise or in individuals with respiratory difficulties.

27. How does the Hering-Breuer reflex influence breathing?

The Hering-Breuer reflex is a protective mechanism that prevents over-inflation of the lungs. Stretch receptors in the lungs detect excessive expansion and send signals to the respiratory center in the brain. This triggers the switch from inhalation to exhalation, preventing lung damage from over-inflation. The reflex helps regulate breathing depth and rate, particularly during deep breathing or in infants.

28. How does the concept of compliance relate to breathing mechanics?

Lung compliance refers to the lungs' ability to expand and contract in response to pressure changes. It's a measure of the lungs' stretchiness or elasticity. High compliance means the lungs expand easily with small pressure changes, while low compliance requires more effort to inflate the lungs. Factors affecting compliance include lung elasticity, surfactant function, and chest wall flexibility. Proper compliance is crucial for efficient breathing and gas exchange.

29. What is the significance of the respiratory quotient in understanding breathing efficiency?

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during respiration. It provides information about the type of fuel being metabolized (carbohydrates, fats, or proteins) and the efficiency of cellular respiration. An RQ of 1.0 indicates pure carbohydrate metabolism, while lower values suggest fat or protein metabolism. Understanding RQ helps in assessing metabolic state and respiratory efficiency.

30. How does the structure of the alveoli contribute to efficient gas exchange?

Alveoli are tiny, thin-walled air sacs at the end of the bronchioles. Their structure is optimized for gas exchange in several ways:

31. What is the role of hemoglobin in the breathing process?

While hemoglobin is not directly involved in the mechanical process of breathing, it plays a crucial role in the overall respiratory function:

32. How does the concept of ventilation/perfusion ratio affect breathing efficiency?

The ventilation/perfusion (V/Q) ratio is the ratio of air reaching the alveoli to blood flow in the pulmonary capillaries. An ideal V/Q ratio ensures optimal gas exchange. Mismatches can occur due to various factors:

33. How does the pH of blood affect breathing rate?

Blood pH is closely regulated by the respiratory system. When blood becomes too acidic (low pH), chemoreceptors in the brain and major blood vessels detect this change and signal the respiratory center to increase the breathing rate. This increased ventilation helps expel more CO2, which is acidic, thus raising blood pH. Conversely, if blood becomes too alkaline (high pH), the breathing rate decreases to retain more CO2 and lower the pH.

34. How does positive pressure ventilation differ from normal breathing mechanics?

Normal breathing relies on negative pressure created by the expansion of the thoracic cavity. In contrast, positive pressure ventilation, used in mechanical ventilation, pushes air into the lungs under pressure. This reverses the normal pressure gradients in the chest and can have several effects:

35. What is the significance of the respiratory exchange ratio in exercise physiology?

The respiratory exchange ratio (RER) is the ratio of CO2 produced to O2 consumed during respiration. In exercise physiology, RER provides insights into:

36. How does the closing capacity of the lungs affect breathing, especially in older adults?

Closing capacity is the lung volume at which small airways begin to close during exhalation. As people age, closing capacity increases due to loss of lung elasticity. This can lead to:

37. What is the role of chemoreceptors in breathing regulation?

Chemoreceptors are specialized cells that detect changes in blood chemistry and play a vital role in breathing regulation:

38. How does the concept of dead space ventilation impact breathing efficiency?

Dead space ventilation refers to the portion of each breath that doesn't participate in gas exchange. It includes:

39. What is the significance of functional residual capacity in lung function?

Functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal exhalation. Its significance includes: