NCERT Notes Class 11 Biology Chapter 14 – Breathing and Exchange of Gases

Notes for NCERT Class 11 Biology Chapter 14 Breathing and Exchange of Gases: Breathing (ventilation) is a process of moving air into and from the lungs so that the exchange of gases can be taken place with the internal environment. The main purpose is to expel carbon dioxide and take in oxygen. Respiration is a process in which the cells of an organism get energy by the oxidation or burning of the glucose molecules when we inhale oxygen from the air. The liberated energy is used up by our body to perform some functions like muscle contraction, nerve impulse transmission, protein synthesis, etc. There are some organs involved in these processes which perform specific functions.

Class 11 Biology Chapter 14 Breathing and Exchange of Gases: Explains the respiratory system is the network of many organs which help in breathing. It comprises the airways, lungs and blood vessels. These work together to transport oxygen throughout the body and clean out the waste gases like carbon dioxide. The respiratory system performs many functions. It helps to inhale (breathe in) and exhale (breathe out), allows us to talk and to smell. It also helps to deliver oxygen to the cells, warms up the air according to the body’s temperature and moisturises it to the humidity level according to the need. Removal of gases waste, including carbon dioxide, is another job, it performs. Some conditions may affect the organs and tissues of the respiratory system. Some may be developed due to the irritants we inhale from the air, including viruses or bacteria which cause infection.

Breathing

Breathing is a physical process that involves the exchange of gases, i.e. oxygen and carbon dioxide. It takes place outside the cells (intercellular) without enzymes. The energy is not liberated during the process.

Respiration

Respiration is a biochemical process that involves the oxidation or burning of glucose molecules. It takes place within a cell (intracellular) where some respiratory enzymes are required. Therefore, also known as cellular respiration. The energy is liberated during the process.

Respiratory Organs

In some unicellular organisms, gaseous exchange can be done by diffusion, in which their whole body or the cell comes in direct contact with the air, e.g., the unicellular aquatic organisms, consume oxygen dissolved in the surrounding water medium. In multicellular organisms, well-differentiated or elaborated organs are involved to carry out the same process.

Some Examples of Respiratory Organs

Respiratory Organs	Examples
General Body Surface	Sponges, Coelenterates, Flatworms
Moist Skin	Earthworms, Frogs
Tracheal Tubes	Insects
Lungs	Reptiles, Birds, Mammals
Gills	Aquatic Arthropods, Molluscs, Fishes

Human Respiratory System

The human respiratory system is divided into the upper respiratory tract and the lower respiratory tract which, comprise some organs which are listed below along with their description and functions:

The Upper Respiratory Tract

Structure	Description	Function
Nares/Nostrils	Openings into the nasal cavities	Passage of air into nasal cavities
Nasal Cavities	Hollow spaces in the nose	Filter, warm and moisten the air
Pharynx	Lies between the nasal cavity and the larynx. It forms the common passage for food and air.	Makes a connection to the surrounding regions
Glottis	Opening into the larynx. It can be covered by a cartilaginous flap called epiglottis to prevent the entry of food into the larynx	Passage of air into the larynx
Larynx	It is cartilaginous and has vocal cords (voice or sound box)	Production of sound

The Lower Respiratory Tract

Structure	Description	Function
Trachea	A flexible tube connects the larynx with the bronchi. They have cartilaginous rings which prevent them from being collapsed	Passage of air into bronchi
Bronchi	Paired tubes that enter the lungs	Passage of air to lungs
Bronchioles	Branched tubes originated from the bronchi to the alveoli (air sacs). They are divided into primary, secondary and tertiary bronchioles, according to the fine main branch or their fine divisions	Passage of air to each alveolus
Lungs	Soft and cone-shaped organs	Contain alveoli and provide a rich supply of blood as they contain blood vessels
Alveoli	Grapes-like or vascularised bag-like structures.	Ultimate site of respiration or exchange of gases

Human has two lungs, enveloped by a double-layered membrane, known as pleura. In between the pleura, pleural fluid is present, which reduces friction on the surface of the lungs. The outer pleural membrane lies close to the thoracic lining, while the inner pleural membrane lies close to the surface of the lungs.

Steps Involved in Respiration

• Breathing or pulmonary ventilation, in which oxygen from the atmosphere is drawn in and CO₂-rich air from the alveoli releases.
• Diffusion of gases (O₂ and CO₂ ) occurs at the alveolar membrane.
• Transportation of gases by the blood.
• Diffusion of O₂ and CO₂ occurs between blood and tissues.
• The utilisation of O₂ by the cells and release of CO₂ (cellular respiration).

Mechanism of Breathing

The mechanism of breathing involves the events that occur during two processes, i.e. inhalation or inspiration and exhalation or expiration. The events involved are given below:

Events During Inspiration	Events During Expiration
The respiratory centre initiates the stimulus/stimuli.	The respiratory centre terminates the stimulus/stimuli.
Diaphragm and expiratory muscles contract.	Diaphragm relaxes and internal intercostal muscles contract.
Thoracic volume increases because the chest wall expands.	Thoracic volume decreases because the chest wall contracts.
Intra-pulmonary pressure (inside the lungs) is reduced.	Intra-pulmonary pressure (inside the lungs) is increased.
Alveolar pressure decreases more than atmospheric pressure.	Alveolar pressure increases more than atmospheric pressure.
Air is taken inside due to the expansion of the alveoli.	Air is given out due to the contraction of the alveoli.
Air rushes into the alveoli until the alveolar pressure equalizes to the atmospheric pressure and the alveoli become inflated.	Air rushes out of the alveoli until the alveolar pressure equalizes to the atmospheric pressure and the alveoli become deflated.

Respiratory Volumes and Capacities

Respiratory Volume	Definition	Capacities
Tidal Volume (TV)	It is the volume of the air which is inspired or expired during a normal respiration	It is approx. 500 mL. A healthy person can inspire or expire approx. 6000-8000 mL of air per minute
Inspiratory Reserve Volume (IRV)	It is the additional volume of the air which, a person can inspire by a forcible inspiration	It is approx. 2500 mL to 3000 mL
Expiratory Reserve Volume (ERV)	It is the additional volume of the air which, a person can expire by a forcible expiration	It is approx. 1000 mL to 1100 mL
Residual Volume (RV)	It is the volume of the air which remains in the lungs even after a forcible expiration	It is approx. 1100 mL to 1200 mL
Inspiratory Capacity (IC)	It is the total volume of the air a person can inspire after a normal expiration	This includes tidal volume and inspiratory reserve volume ( TV+IRV), i.e. 9700 mL
Expiratory Capacity (EC)	It is the total volume of the air a person can expire after a normal inspiration	This includes tidal volume and expiratory reserve volume (TV+ERV), i.e. 8000 mL
Functional Residual Capacity (FRC)	It is the volume of the air that remains in the lungs after a normal expiration	This includes ERV+RV, i.e. 2100 mL
Vital Capacity (VC)	It is the maximum volume of the air which, a person can breathe in after a forced expiration. This includes ERV, TV and IRV or the maximum volume of the air a person can breathe out after a forcible inspiration	It is approx. 10000 mL
Total Lung Capacity	It is the total volume of the air which is present in the lungs at the end of a forcible inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume	It is approx. 11000 mL

Exchange of Gases

During the exchange of gases, oxygen is transported from the lungs to the bloodstream. At the same time, carbon dioxide is passed from the blood to the lungs. This happens in the lungs between the alveoli and a network of blood vessels (capillaries), which are located in the walls of the alveoli. The diagrammatic explanation of the exchange of gases is given below:

Transport of Gases

Blood is the medium for the transportation of O₂ and CO₂. The transport of both gases is discussed below:

Transport of Oxygen

About 97 per cent of the O₂ is transported by RBCs in the blood, and the remaining 3 per cent is transported in the dissolved state through plasma.

• O₂ can be bound with haemoglobin to form oxyhemoglobin.
• The partial pressure of CO₂, hydrogen ion concentration and temperature are the factors which interfere with this binding.
• A sigmoid curve is formed when the percentage of the saturation of haemoglobin with O₂ is plotted against the pO₂. This formed curve is known as the Oxygen dissociation curve.

• Factors Favourable for the Formation of Oxyhemoglobin: In the alveoli, pO₂ is high, pCO₂ is low, H⁺ concentration is less and temperature is lower.
• Factors Favourable for the Dissociation of Oxyhemoglobin: In the tissues, pO₂ is low, pCO₂ is high, H⁺ concentration is high and temperature is high.
• This shows that O₂ is bound to haemoglobin in the lung (surface) and becomes dissociated in the tissues.

Transport of Carbon Dioxide

Approx. 20-25 per cent of the CO₂ is transported by RBCs by binding with it, to form carbamino-haemoglobin, 70 per cent is carried as bicarbonates and around 8-9 per cent of CO₂ is transported in a dissolved state through plasma.

• The formation of carbamino-hemoglobin is related to pCO₂ which is a major factor that affects the binding.
• Factors Favourable for the Binding of Carbon Dioxide: In the tissues, high pCO₂ and low pO2 are required.
• Factors Favourable for the Dissociation of Carbon Dioxide: In the alveoli, low pCO₂ and high pO₂ are required.
• CO₂ bound to haemoglobin from the tissues is transported to the alveoli.
• RBCs have a high concentration of carbonic anhydrase enzyme and low quantities of it are present in the plasma.

• In the tissues with high pCO₂ due to catabolism, CO₂ diffuses into the blood (RBCs and plasma) and forms HCO^3– and H⁺.
• In the alveoli, with low pCO₂, the reaction is followed in the reverse direction which leads to the formation of CO₂ and H₂O.
• CO₂ is formed as the bicarbonates in the tissues and transported to the alveoli where it is released as CO₂.

Regulation of Respiration

Respiration is regulated by two mechanisms, i.e. nervous mechanism and the chemical mechanism. In the nervous regulation, respiratory centres are present in the medulla oblongata which senses the level of oxygen and carbon dioxide in the blood and sends impulses by the respiratory muscles. The pneumatic centre can change the function of the respiratory rhythm centre by signalling in order to reduce the rate of inspiration. The diagrammatic representation of the regulation of respiration is given below:

Disorders of the Respiratory System

Respiratory Disorder	Description
Asthma	It is a chronic lung disorder which is caused by the inflammation and tightness of the muscles around the airways, because of which breathing can be difficult.
Emphysema	It is a lung disorder which causes shortness of breath because the air sacs in the lungs (alveoli) are damaged
Occupational Respiratory Disorders	These are the lung disorders that have been caused by long-term exposure to some irritants in the workplace

FAQs on Breathing and Exchange of Gases

Q1: What is Residual Volume?

Answer:

It is the volume of the air which remains in the lungs even after a forcible expiration. It is approx. 1100 mL to 1200 mL

Q2: Write the Steps Involved in Respiration.

Answer:

The steps involved in respiration are given below:

• Breathing or pulmonary ventilation in which oxygen from the atmosphere is drawn in and CO₂ rich air from the alveoli releases out.
• Diffusion of gases (O₂ and CO₂ ) occur at the alveolar membrane.
• Transportation of gases by the blood.
• Diffusion of O₂ and CO₂ occur between blood and tissues.
• Utilisation of O₂ by the cells and release of CO₂ (cellular respiration).

Q3: What is the Name of the Respiratory Organs Present in Molluscus?

Answer:

Gills are the rrespiratory organs present in Molluscus.