CBSE Class 11 Biology Chapter 17 Revision Notes

Chapter 17: Breathing and Exchange of Gases Revision Notes

  • Breathing is the process of taking in oxygen and giving out carbon dioxide.
  • Respiration is a catabolic process of the breakdown of energy-rich molecules to produce the energy needed for the survival of the organism.
  • Different organisms have different types of respiratory organs depending upon the habitat and level of their organization.
  • The earthworm has moist skin that participates in respiration. This type of respiration is known as Cutaneous Respiration.
  • Insects have tracheal tubes that are respiratory in function. 
  • Simple organisms can exchange gasses directly with the environment by general body surface. 
  • Aquatic animals have gills as respiratory organs.
  • Higher animals, including humans, have lungs for respiration.

Human Respiratory System

  • Humans have a pair of nostrils that lead into the nasal passage.
  • The nasal chamber then leads to the pharynx, which is a common passage for food and air.
  • The pharynx opens through the larynx region into the trachea
  • The larynx is responsible for sound production and is commonly called the sound box.
  • The trachea is a straight tube that divides into the right and left bronchi.
  • Primary bronchi are further divided into secondary and tertiary bronchi and bronchioles.
  • Each terminal bronchiole gives rise to a thin vascularized bag-like structure known as Alveoli.
  • Humans have pair of lungs that are covered by a membrane known as the Pleural Membrane.
  • The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber.

Respiration involves the following steps:

 • Pulmonary ventilation involves taking in atmospheric oxygen and carbon dioxide.

 • Diffusion of oxygen and carbon dioxide across the alveolar membrane.

 • Transport of gasses by the blood.

 • Diffusion of O2 and CO2 occurring in blood and tissues.

 • Utilization of oxygen by the cells for catabolic reactions and release of carbon dioxide.


 There are two processes of Breathing – Inspiration and Expiration.


  • Process of taking atmospheric air in is known as inspiration.

  • It is an active process.

  • Pressure inside the lungs is less than atmospheric pressure.

  • External intercostal muscles contract, which raises the ribs to increase the volume of the thoracic cavity.


 • Process of giving out carbon dioxide is known as expiration.

 • It is a passive process.

 • Pressure inside the lungs is more than the atmospheric pressure.

 • External intercostal muscles relax, which lowers the ribs to decrease the volume of the thoracic cavity.

Inspiration and Expiration

Source: Inspiration and Expiration

Respiratory Volumes and Capacities

  • Tidal Volume (TV): Volume of air inspired or expired during a normal respiration.
  • Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration.
  • Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration.
  • Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration.
  • Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration.
  • Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration.
  • Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV.
  • Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration.
  • Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration.

Exchange of gasses

  • The exchange of gasses occurs in alveoli.
  • Two important parameters that affect the rate of diffusion are –solubility of gases and the thickness of the membrane.
  • Pressure contributed by each gas from a mixture of gas is known as partial pressure.
  • The partial pressure of oxygen and carbon dioxide is represented by pO2 and pCO2, respectively.
  • The partial pressure of oxygen in the alveoli is 104 mmHg, whereas in the blood it is 40 mmHg.
  • Similarly, the partial pressure of carbon dioxide is 40 mmHg in alveoli and 45 mmHg in blood.
  • This creates a concentration gradient between the blood and the alveoli.
  • The diffusion membrane comprises 3 layers – the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance in between them.

Exchange of gases

Source: Exchange of gases

How are oxygen and carbon dioxide transported in blood?

  • The transport of oxygen and carbon dioxide occurs via blood.
  • About 97% of the transport of oxygen occurs by blood.
  • And the remaining 3% is transported by plasma
  • Similarly, about 70% of carbon dioxide is transported in the form of bicarbonate in deoxygenated blood around 25% is transported via red blood cells, and around 7% is transported in a dissolved state via plasma.

Transport of oxygen

  • Oxygen transport within the human body occurs through both convection and diffusion.
  • Oxygen diffuses from the alveoli into the pulmonary capillaries and the systemic capillaries into the tissues.
  • Red blood cells contain an iron-containing red-colored pigment known as Hemoglobin.
  • Hemoglobin reversibly binds oxygen to form oxyhemoglobin.
  • A single hemoglobin molecule can bind 4 oxygen molecules.
  • The partial pressure of oxygen determines the binding of oxygen with hemoglobin. 
  • Oxygen binds to hemoglobin in the lungs and gets dissociated in tissues.

Transport of carbon dioxide

  • CO2 is carried by hemoglobin as carbamino-hemoglobin.
  • This is due to the partial pressure of CO2 .
  • When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs.
  • When the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from takes place.

Regulation of Respiration

  • Our neural system gives us the ability to maintain and moderate our respiratory rhythm based on the needs of the different tissues.
  • The respiratory rhythm center present in the medulla region is responsible for this regulation.
  • The pneumotaxic center present in the pons region moderates the functions of the respiratory rhythm center.

Disorders of the Respiratory System

  • Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
  • Emphysema is a chronic disorder in which alveolar walls are damaged due to which the respiratory surface is decreased.

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