Breathing And Exchange Of Gases Class 11 Notes Biology Chapter 17 - CBSE

Chapter : 17

What Are Breathing And Exchange Of Gases ?


Breathing is the process of exchange of O2 from the atmosphere with CO2 produced by the cell. There are two processes of Breathing - Inspiration and Expiration.


Respiration is a catabolic process of the breakdown of energy-rich molecules to produce the energy needed for the survival of the organism.

Respiratory Organs

Mechanism of breathing varies in different organism according to their body structure and habitat.

Respiratory Organs Organisms
Entire Body surface Sponges, coelenterate, flatworms.
Skin Earthworm.
Tracheal system Insects
Gills Pisces, aquatic arthropods.
Lungs Amphibians, mammals.

Human Respiratory System

The human respiratory system consists of a pair of Nostrils → Nasal cavity → Internal nares → Nasopharynx → Larynx → Trachea → Bronchus → Alveoli → Lungs

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.  

Exchange Of Gases

  • 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 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 of 3 layers - the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance in between them.

Transport Of Oxygen And Carbon Dioxide 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 four 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

Types of centre Location Function
Inspiratory centre Medulla oblongata Inspiratory
Expiratory centre Medulla oblongata Expiration
Pneumotaxic centre Pons Produce normal breathing

Respiratory Volume

  • Tidal volume is the amount of air that moves in or out of the lungs with each respiratory cycle.
  • It measures around 500 mL in an average healthy adult male and approximately 400 mL in a healthy female.

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.
  • Occupational Respiratory Disorders occurs due to occupation of individual. This is caused by inhalation of gas, fumes or dust present in surrounding of work place. This includes Silicosis, Asbestoses due to exposer of silica and asbestos. The symptom includes proliferation of fibrous connective tissue of upper part of lung causing inflammation.
  • Pneumonia is acute infection or inflammation of the alveoli of the lungs due to bacterium streptococcus
    pneumoniae. Alveoli become acutely inflamed and most of air space of the alveoli is filled with fluid and dead
    white blood corpuscles limiting gaseous exchange.