Breathing and Exchange of gases


The process of breathing and exchange of gases is a fundamental and intricate physiological mechanism that sustains life in all living organisms. Through this remarkable process, the respiratory system efficiently ensures the supply of oxygen to cells and the elimination of carbon dioxide, enabling essential metabolic activities to occur. From the inhalation of atmospheric air to the diffusion of gases across specialized membranes within the body, this intricate process exemplifies the harmonious collaboration between various organs and systems. Respiratory organs, which play a pivotal role in facilitating the exchange of life-sustaining gases, are known as respiratory structures, while the interconnected network of these organs forms the respiratory system.

Respiratory organs

All organisms have different respiratory organs for the exchange of gases.


Respiratory organ

1.    Human and terrestrial vertebrates


2.    Fishes, Mollusks (aquatic sp.), Crustaceans (Crabs and Lobsters), Hemichordates

Gills (Lung fishes have lungs to supplement respiration)

3.    Echinoderms

Water vascular system

4.    Molluscs (terrestrial sp.)

Mantle Cavity

5.    Insects


6.    Amphibians

Lungs and Moist skin

7.    Earthworms

Moist skin

8.    Spiders and Scorpions:

Book Lungs

9.    Sponges, Cnidarians

Diffusion through cell membrane/body tissue

10. Platyhelminthes (flatworms)

Diffusion through moist skin

Human respiratory system

  • The human respiratory system facilitates the essential processes of gas exchange, ensuring oxygen supply to cells and removal of carbon dioxide.
  • It involves intricate structures like nostrils, nasal passages, pharynx, larynx, trachea, bronchi, and alveoli. The lungs, positioned within the thoracic chamber, form a complex network of airways.
  • The structures associated with the respiratory system and their functions are:
  1. Nostrils and Nasal Passage: Pair of external nostrils located above upper lips, lead to the nasal chamber via the nasal passage.
  2. Pharynx and Larynx: Nasal chamber opens into the pharynx which functions as a common passage for food and air. Pharynx connects to the larynx, a cartilaginous box for sound production. Epiglottis is a thin, elastic cartilaginous flap located just above the larynx. It plays a crucial role in preventing food or liquids from entering the airway (trachea) during swallowing.

The nose, mouth, larynx, and pharynx are known as the upper respiratory tract.

  1. Trachea and Bronchi: Larynx opens into the trachea, a straight tube extending to the mid-thoracic cavity (5th thoracic vertebra). Here, the trachea divides into the right and left primary bronchi. Bronchi further divide into secondary, tertiary, and higher generation bronchi, and bronchioles. Cartilaginous rings support the trachea and initial bronchi. The first airway branches that no longer contain cartilage rings are termed bronchioles.
  2. Alveoli and Lungs: Terminal bronchioles end in vascularized alveoli. Alveoli are thin, irregular-walled structures for gas exchange. Lungs are composed of a branching network of bronchi, bronchioles, and alveoli.
  3. Pleura and Thoracic Chamber: Lungs covered by double-layered pleura with pleural fluid. Pleura reduces friction between the lungs and thoracic lining. The thoracic chamber is formed by the vertebral column, sternum, ribs, and diaphragm.
  • Nostrils to terminal bronchioles form conducting part of the respiratory system which transports, humidifies, and filters air.
  • Alveoli are the exchange part where O2 and CO2 diffusion takes place.

Breathing Process

  1. Pulmonary ventilation: Inhale atmospheric air, exhale CO2-rich alveolar air.
  2. Pulmonary exchange: O2 and CO2 exchange across the alveolar membrane.
  3. Blood Transport: Gases transported in blood.
  4. Tissue Exchange: O2 and CO2 diffusion between blood and tissues.
  5. Cellular Utilization: Cells use O2 for catabolic reactions, and release CO2.
  6. Breathing Regulation: Thoracic cavity volume changes affect the pulmonary cavity for breathing. The anatomical setup allows for breathing regulation.

The Process of Breathing



Contraction of the diaphragm and external intercostal muscles

Relaxation of the diaphragm and intercostal muscles

Diaphragm moves downward, ribs upward and the sternum outward

Diaphragm moves upward, ribs and sternum move to their normal positions

Increase in the volume of lungs

Decrease in the volume of lungs

Decrease in the pulmonary pressure (as compared to the atmospheric pressure)

Increase in the pulmonary pressure (as compared to the atmospheric pressure)

Air from outside flows into the lungs

Air forced to move out

NOTE: During expiration, the diaphragm and the intercostal muscles relax to normal conditions.

Volume and capacities of lungs

  • Lung volumes refer to the specific amounts of air that the lungs can hold at different points during breathing.
  • Lung capacity refers to the combination of lung volumes

Lung volumes

Tidal volume (TV)

The volume of air entering or leaving the lungs during the normal inspiration.

6000-8000 mL/min

Inspiratory Reserve Volume (IRV)

The extra amount of air inspired during the deepest inspiration after a normal inspiration.

2500-3000 mL/min

Expiratory Reserve Volume (ERV)

The amount of air breathed out in a forceful expiration.

1000-1100 mL/min

Residual Volume (RV)

The amount of air remaining in the lungs after a forceful expiration.

1100-1200 mL

Lung Capacities

Inspiratory Capacity (IC)


Expiratory Capacity (EC)


Functional Residual Capacity (FRC)


Vital Capacity (VC)


Total Lung Capacity (TLC)


= VC + RV

Pulmonary and tissue Exchange of gases

The diffusion of gases between the lung alveoli & blood and blood & tissues depends on the following factors:

1. The partial pressure of gases (Pgas): According to Dalton’s law, the total pressure exerted by the mixture is equal to the sum of the pressures exerted by the individual gases. These individual pressures are termed partial pressures. The difference between the partial pressure of each gas determines the rate and direction of diffusion of that particular gas. Factors like temperature and concentration affect Pgas and therefore will also affect the diffusion of gas.


Pgas (atmospheric)

Pgas (pulmonary)

Pgas (veins)

Pgas (artery)

Pgas (tissues)


159 mmHg

104 mmHg

40 mmHg

95 mmHg

40 mmHg


0.3 mmHg

40 mmHg

45 mmHg

40 mmHg

45 mmHg

2. The thickness and surface area of the membrane across which gases diffuse: Fick’s Law of diffusion, states that the rate of diffusion is directly proportional to the surface area (and the concentration gradient of the gases), and inversely proportional to the thickness of the membrane.

3. Solubility of gas in liquid: Henry’s Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. The greater the solubility of a gas in a given medium, the faster it will diffuse.

Carbon dioxide is considerably more soluble than oxygen (O2), with CO2 being 20-25 times more soluble than O2. Due to this difference in solubility, even a slight disparity in partial pressures can lead to a larger amount of CO2 dissolving in the liquid compared to O2, given the same difference in their partial pressures.

4. Distance between diffusing surfaces: The greater the distance between two surfaces the lower will be the rate of diffusion (Point no. 2).

In the lungs, the gases diffuse through three layers. The alveolar squamous epithelium, the basement membrane and the capillary endothelium.

Oxygen and carbon dioxide diffuses through three layers in the lungs

Transport form of Oxygen and Carbon dioxide


In RBC (as HbO2 or HbCO2)

Dissolved in Plasma

As bicarbonate








  • Hb bound to oxygen (HbO2) is called oxyhaemoglobin, and Hb bound to CO2 is called carbamino-haemoglobin.
  • CO2 combines with water (H2O) to create carbonic acid (H2CO3), which promptly breaks down into hydrogen ions (H+) and bicarbonate ions (HCO3). This chemical transformation is facilitated by the enzyme Carbonic Anhydrase (CA). This process predominantly occurs within red blood cells (RBCs), with a smaller portion taking place in the blood plasma.

                                              CO2 + H2O H2CO3 H+ + HCO3

  • In this reversible reaction, the progression of the reaction towards the right is favoured in tissues, where CO2 is acquired. Conversely, in the lungs, where CO2 is discharged, the reverse reaction gains favour.

Transport of Oxygen and Carbon dioxide


In Lungs

In Tissues




In alveoli 104 mmHg

In blood capillaries 40 mmHg


In capillaries 95 mmHg

In cells 40 mmHg



In alveoli 40 mmHg

In blood capillaries 45 mmHg


In tissue 45 mmHg

In blood capillaries 40 mmHg

H+ concentration




Favour O2 pickup from alveoli to blood and release of CO2 from blood to alveoli

Favour CO2 pickup from tissues to blood and release of O2 from blood to tissue

O2 or CO2 delivered by each dL (100 ml) blood

5 mL O2 delivered to tissues

4 mL CO2 delivered to lung alveoli

Oxygen dissociation curve

It is a graphical representation of the relationship between the partial pressure of oxygen (PO2) and the saturation of haemoglobin with oxygen in the blood. This curve helps us understand how haemoglobin’s affinity for oxygen changes under different oxygen concentrations.

The curve typically exhibits a sigmoidal shape. At lower PO2 levels (as seen in tissues), haemoglobin has a lower affinity for oxygen, allowing it to release oxygen to the surrounding tissues for their metabolic needs. As PO2 increases (as seen in the lungs), haemoglobin’s affinity for oxygen rises, enabling it to bind oxygen more effectively.

Oxygen dissociation curve showing % saturation of Hb at different partial pressure of Oxygen

Regulation of Respiration

  • Breathing is all about the rhythmic activation of muscles. The diaphragm and intercostal muscles are the main players. During breath in, the muscles contract. This starts the process of inhaling. Once that burst of signals stops, the muscles relax, and we naturally breathe out as the lungs spring back to their original shape.
  • This process is regulated mainly by the centres located in the medulla oblongata (medulla) and pons. In the medulla, the respiratory rhythm generator (centre). along with other nuclei in the medulla provides the rhythmic input to the motor neurons innervating the inspiratory muscles.
  • An area in the upper pons called the pneumotaxic centre modulates the activity of the respiratory rhythm centre.
  • The peripheral chemoreceptors, located high in the common carotid arteries and the arch of the aorta are called the carotid bodies and aortic bodies sense the concentration of CO2, O2, and H+. They sense the change in concentration of CO2, O2, and H+ and send the necessary signals for the adjustment of respiratory rhythm.
  • By controlling the pace of breathing, the body manages its need for more or less oxygen and carbon dioxide.
A flow chart showing regulation of respiration in human
A box and pipe diagram to represent blood flow, oxygen and carbon dioxide diffusion. Box represent heart chambers and pipes represent the blood vessels. Arrows represent the direction of blood flow and oxygen and carbon dioxide diffusion.

Disorders of the respiratory system


  • Asthma is a chronic respiratory condition that causes airways to become inflamed, narrowed, and sensitive.
  • Symptoms: Wheezing, shortness of breath, chest tightness, and coughing, especially at night or early in the morning.
  • Causes: Various factors can trigger asthma symptoms, such as allergens (pollen, pet dander), respiratory infections, cold air, exercise, smoke, and irritants.


  • Emphysema is a progressive lung disease characterized by the gradual destruction of the air sacs (alveoli) in the lungs, leading to reduced lung function and difficulty in breathing.
  • Symptoms: Shortness of breath, especially during physical activity, chronic cough, wheezing, chest tightness, and a limited ability to exhale fully.
  • Causes: Long-term exposure to irritants, especially cigarette smoke. Other causes can include exposure to air pollution, workplace dust, fumes, or genetic factors, such as a deficiency of alpha-1 antitrypsin, a protein that helps protect the lungs.

Occupational Respiratory Disorders

  • These are lung conditions that result from exposure to harmful substances in the workplace. These disorders can range from temporary irritation to chronic and irreversible damage. A few of these are:


  • Silicosis is a lung disease caused by inhaling fine silica dust, common in industries like mining, construction, and sandblasting.
  • Symptoms: Shortness of breath, cough, fatigue, and severe cases can lead to respiratory failure.


  • Asbestosis is caused by inhaling asbestos fibres, leading to lung scarring and reduced lung function.
  • Symptoms: Shortness of breath, persistent cough, chest discomfort.


  • Nelson, D. L., & Cox, M. M. (2012). Lehninger principles of biochemistry (6th ed.). New York, NY: W.H. Freeman.
  • Hall, J. E., PhD. (2015). Guyton and Hall Textbook of Medical Physiology. Elsevier Health Sciences.
  • Widmaier, E. P., Raff, H., & Strang, K. T. (2003). Vander et Al’s Human Physiology: With OLC Bind-In Card. McGraw-Hill Science, Engineering & Mathematics.
  • Biology: Text book for class xi. (2006)

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