The Respiratory System Study Guide

Lesson objectives:

  • Understand the structure of the human respiratory passage.
  • Understand the process of the exchange of gases between our body and the environment.

1. Understanding human respiratory passage.

  • As we breathe the air enters our body via the external nostrils opening out above the upper lips.

  • It leads to a nasal chamber through the nasal passage. The pharynx opens through the larynx region into the trachea (Larynx: a cartilaginous box which helps in the production of sound and hence is also known as the sound box)

  • Next, the air inhaled is led into the trachea which is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and left primary bronchi.

  • Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles, ending up in very thin terminal bronchioles.

  • Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.

2. Explain what happens if we try to swallow while breathing.

  • During swallowing, the glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx. This makes it really difficult for us to (consciously) breathe while eating our food. Sub-consciously, it almost never happens!



3. Explain the role of the double membraned structure around the lungs.

  • We have two lungs which are covered by a double layered pleura, with pleural fluid between them. The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface.The function of this pleura membrane is to reduce friction on the lung surface.

🤯 Lungs comprise of the branching network of bronchi, bronchioles and alveoli.

4. Define the processes of breathing and respiration.

  • Breathing is just the the process of taking air into and expelling it from the lungs.

  • Respiration is the process in living organisms involving the production of energy, typically with the intake of oxygen and the release of carbon dioxide from the oxidation of complex organic substances.

🤯 The oxygen metabolises in the mitochondria to prduce ATP- the energy currency of our cells. Meanwhile, the opposite is true for plants. While performing photosynthesis plants release oxygen to produce food for themselves.

5. Explain why the lungs are said to be anatomically in an ‘air-tight’ chamber.

  • The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm. The anatomical setup of the lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity.

👆 Important point to note: Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

6. Explain inspiration, expiration and the set of muscles involved in these two activities.

  • Inspiration: process by which atmospheric air is drawn into the lungs.

    • Set of muscles involved: Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs.
  • Expiration: process by which air in the lungs is expelled into the atmosphere.

    • Set of muscles involved: Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduces the thoracic volume and thereby the pulmonary volume. This leads to an increase in the intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs.

7. How does the exchange of gases occur in the lungs?

  • Primary site for exchange of gases are the alveoli. Exchange of gases takes place by simple diffusion based on the concentration gradient of the gases.

  • Two important factors which affect the rate of diffusion of gases are:

    • thickness of the membrane
    • solubility of gases
  • The actual exchange of gases takes place based on the difference in the partial pressure of gases, leading to the formation of a concentration gradient i.e. gases diffuse from regions of higher concentration to regions of lower concentrations.

  • From lungs to blood: The concentration of oxygen in the blood is lower than that of the alveoli. Due to the concentration gradient, oxygen diffuses into the bloodstream from the alveoli.

  • From tissues to blood: Similar to the above case, a reverse concentration gradient is present for carbon dioxide letting the gas diffuse from tissues to blood and from the blood to the alveoli.

🤯 Solubility of carbon dioxide is much higher than that of oxygen (almost 20 times more!). As a result, the amount of carbon dioxide that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of oxygen.

  • Diffusion Membrane: The diffusion membrane is made up of three major layers:
    • the thin squamous epithelium of the alveoli
    • the endothelium of the alveolar capillaries
    • the basement substance (composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them.

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8. How are oxygen and carbon dioxide transported in the bloodstream?

  • Oxygen is carried by haemoglobin, a red coloured iron-containing pigment. Binding of oxygen with haemoglobin is affected, primarily, by partial pressure of oxygen. When partial pressure of oxygen is high (eg: in alveoli) binding of oxygen with haemoglobin occurs. When partial pressure of oxygen is low (eg: in tissues) oxygen disassociates.

  • Carbon dioxide is also carried by haemoglobin in the form of carbamino-haemoglobin. Binding of carbon dioxide is also affected by its partial pressure in a region.

🤯 Only 20-25% carbon dioxide is carried as carbamino-haemoglobin. 70% of carbon dioxide in the blood is carried as bicarbonate.

9. What is carbonic anhydrase, and what is its role in respiration?

  • Carbonic anyhydrase is an enzyme that is abundantly present in red blood cells.
  • It facilitates the following reaction which helps in transport of oxygen and carbon dioxide:

Carb Anhyd equation

Source: Kleinman et al. N Engl J Med. 1967.

10. How is respiration regulated?

  • A specialised centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for this regulation.

  • Another centre present in the pons region of the brain called pneumotaxic centre can moderate the functions of the respiratory rhythm centre. Neural signal from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.

  • A chemosensitive area is situated adjacent to the rhythm centre which is highly sensitive to CO2 and hydrogen ions. Increase in these substances can activate this centre, which in turn can signal the rhythm centre to make necessary adjustments in the respiratory process by which these substances can be eliminated.

  • Receptors associated with the aortic arch and the carotid artery also can recognise changes in CO2 and H+ concentration and send necessary signals to the rhythm centre for remedial actions. The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

11. What are some common respiratory disorders?

  • Asthma: A difficulty in breathing causing wheezing due to inflammation of the bronchi and bronchioles.

  • Emphysema: It is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. One of the major causes of this is cigarette smoking.

  • Occupational Respiratory Disorders: In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the defense mechanism of the body cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage.

We hope you enjoyed studying this lesson and learned something cool about the Respiratory System! Join our Discord community to get any questions you may have answered and to engage with other students just like you! Don’t forget to download our App and check out our awesome VR room for this guide – we promise, it makes studying much more fun 😎


  1. The respiratory system, breathing and respiration. Accessed Nov 17, 2021.
  2. Lung Diseases Overview. Accessed Nov 17, 2021.

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