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CBSE Class 11 Biology Chapter 17 Revision Notes

The absorption of O2 from blood and release of CO2 from tissue cells is known as Breathing. The gas exchange allows the body to replenish oxygen while expelling carbon dioxide. Both are essential for existence.

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.