NCERT Class 11 Biology Breathing and Exchange of Gases Important Notes

Respiration:

The process of exchange of O₂ (from the atmosphere) with CO₂ (produced by cells) is called breathing. This process is commonly known as respiration.

Respiratory Organs:

• Mechanisms of breathing vary among different animal groups. It usually depends on the habitat and level of organization.

• In case of lower invertebrates, exchange of gases takes place by simple diffusion over the entire body surface, e.g. sponges, coelenterates, flatworms, etc.

• The moist cuticle of earthworms facilitates exchange of gases.

• Insects have a network of tubes through which air is transported within the body. These tubes are called tracheae.

• Gills are special vascularised structures which are present in most of the aquatic arthropods and mollusks.

• In terrestrial animals, vascularised bags; called lungs; are present for the exchange of gases. 

HUMAN RESPIRATORY SYSTEM

The human respiratory system is composed of following organs:

Pharynx: There is a pair of external nostrils which open out above the upper lips. The nostrils lead to a nasal chamber through the nasal passage. The nasal chamber opens into nasopharynx. Nasopharynx is a part of pharynx. Pharynx is the common passage for food and air.

Larynx: Nasopharynx opens through glottis of the larynx into the trachea. Larynx is a cartilaginous box which helps in sound production. Due to this, larynx is also called the sound box. There is a thin elastic cartilaginous flap; called epiglottis. The epiglottis covers the glottis during swallowing. This prevents the entry of food into the larynx.

Trachea: Trachea is a straight tube which extends up to the mid-thoracic cavity. The trachea divides at the level of 5th thoracic vertebra into right and left primary bronchi.

Bronchi: Each bronchus undergoes repeated divisions to form secondary and tertiary bronchi and bronchioles. They finally end up in very thin terminal bronchioles. The tracheae, bronchi and the initial bronchioles are supported by incomplete cartilaginous rings. Each terminal bronchiole gives rise to a number of very thin alveoli. An alveolus is an irregular-walled and vascularised bag-like structure.

Lungs: The lungs are composed of the branching network of bronchi, bronchioles and alveoli. Each lung is covered by a double-layered pleura. The pleura is filled with pleural fluid. The pleural fluid reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining. The inner pleural membrane is in contact with the lung surface.

Conducting Part of Respiratory System: The conducting part is constituted by the external nostrils, pharynx, larynx, bronchi and the terminal bronchioles. The conducting part transports the atmospheric air to the alveoli. This part also clears the air from foreign particles, humidifies and brings the air to body temperature.

Exchange Part of Respiratory System: The exchange part of the respiratory system is composed of the alveoli and their ducts. Actual diffusion of O₂ and CO₂ (between blood and atmospheric air) takes place in the exchange part of the respiratory system.

Thoracic Chamber: The lungs are situated in the thoracic chamber. This is anatomically 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. Any change in the volume of the thoracic cavity would be reflected in the lung cavity. This is possible because of the typical anatomical setup of lungs in thorax. Such an arrangement is essential for breathing, because we cannot directly alter the pulmonary volume.

Steps of Respiration:

• Breathing or pulmonary ventilation facilitates intake of atmospheric air and expulsion of alveolar air.

• Diffusion of gases takes place across alveolar membrane.

• Oxygen is transported to the tissue by blood.

• Diffusion of O₂ and CO₂ takes place between blood and tissues.

• Catabolism leads to utilization of O₂ by the cells and release of CO₂. 

MECHANISM OF BREATHING

Breathing involves two stages, viz. inspiration and expiration. The atmospheric air is drawn in during inspiration and the alveolar air is released out during expiration. The movement of air into and out of the lungs is facilitated by creating a pressure gradient between the lungs and the atmosphere. When the pressure within the lungs is less than the atmospheric pressure, inspiration takes place. When the intra-pulmonary pressure is higher than the atmospheric pressure, expiration takes place.

The pressure gradient is generated by the diaphragm and a specialized set of muscles. These special muscles are external and internal intercostals between the ribs.

Mechanism of Inspiration: Inspiration is initiated by the contraction of diaphragm. The contraction of diaphragm increases the volume of thoracic chamber in the antero-posterior axis. The external inter-costal muscles contract to lift up the ribs and the sternum. This causes an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume results in a similar increase in pulmonary volume. The increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure. This pressure gradient forces the air from outside to move into the lungs and inspiration takes place.

Mechanism of Expiration: Diaphragm and inter-costal muscles relax which results in the diaphragm and the sternum returning to their normal positions. This reduces the thoracic volume and thus the pulmonary volume. The reduction in pulmonary volume results in an increase in intra-pulmonary pressure to slightly above the atmospheric pressure, which causes the expulsion of air from the lungs, and expiration takes place.

RESPIRATORY VOLUMES AND CAPACITIES

Tidal Volume (TV): The volume of air inspired or expired during a normal respiration is called Tidal Volume. It is approximately 500 ml in a healthy man. This means that a healthy adult can inspire or expire about 6 to 8 litre of air per minute.

Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration is called Inspiratory Reserve Volume. This is about 2500 ml to 3000 ml in normal adult.

Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration is called Expiratory Reserve Volume. This is about 1000 ml to 1100 ml in a normal adult.

Residual Volume (RV): The volume of air remaining in the lungs even after a forcible expiration is called Residual Volume. This is about 1100 ml to 1200 ml in a normal adult.

Inspiratory Capacity (IC): The total volume of air a person can inspire after a normal expiration is called Inspiratory Capacity. This includes the tidal volume and inspiratory reserve volume, i.e. IC = TV + IRV.

Expiratory Capacity (EC): The total volume of air a person can expire after a normal inspiration is called Expiratory Capacity. EC = TV + ERV.

Functional Residual Capacity (FRC): The volume of air which remains in the lungs after a normal expiration is called Functional Residual Capacity. FRC = ERV + RV.

Vital Capacity (VC): The maximum volume of air a person can breathe in after a forceful expiration is called Vital Capacity. This is also defined as the maximum volume of air a person can breathe out after a forceful inspiration. VC = ERV + TV + IRV.

Total Lung Capacity: Total volume of air accommodated in the lungs at the end of a forced inspiration is called Total Lung Capacity. Total Lung Capacity = VC + RV = (ERV + TV + IRV) + RV.

EXCHANGE OF GASES

Alveoli are the main sites of exchange of gases. However, exchange of gases also occurs between blood and tissues. The exchange of O₂ and CO₂ at these sites happens by simple diffusion which is mainly based on pressure/concentration gradient. Some important factors which can affect the rate of diffusion are; solubility of gases and thickness of the membranes involved in diffusion. Pressure contributed by an individual gas in a mixture of gases is called partial pressure. It is represented as pO₂ for oxygen and pCO₂ for carbon dioxide.

TRANSPORT OF GASES

Blood is the medium of transport for O₂ and CO₂. About 97% of oxygen is transported by RBCs. The remaining 3% of oxygen is carried in a dissolved state through the plasma. About 20-25% of carbon dioxide is transported by RBCs. About 70% of carbon dioxide is carried as bicarbonate and about 7% is carried in a dissolved state through plasma.

Transport of Oxygen

Oxygen can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of oxygen. Binding of oxygen with haemoglobin is mainly related to the partial pressure of O₂. Partial pressure of CO₂, hydrogen ion concentration and temperature are the other factors which can interfere with this binding. When percentage saturation of haemoglobin with O₂ is plotted against pO₂, we get a sigmoid curve. This curve is called Oxygen Dissociation Curve. This curve is very useful in studying the effect of factors; like pCO₂, H⁺ concentration, etc. on binding of O₂ with haemoglobin.

Transport of Carbon dioxide

The binding of carbon dioxide with haemoglobin is related to partial pressure of CO₂. The partial pressure of O₂ is a major factor which can affect this binding. In the tissues, pCO₂ is higher than pO₂ and hence more binding of carbon dioxide occurs at the tissue level. In the alveoli, pCO₂ is lower than pO₂ and hence dissociation of carbamino-haemoglobin takes place in the alveoli.

RBCs contain a very high concentration of the enzyme; carbonic anhydrase. Minute quantities of the same enzyme are present in plasma as well. This enzyme facilitates the following reaction.

At the tissue site, partial pressure of CO₂ is high due to catabolism. At this level, CO₂diffuses into blood and forms bicarbonate and hydrogen ions.

At the alveolar site, pCO₂ is low. At this level, the reaction proceeds in the opposite direction and thus carbon dioxide and water are formed. Thus, carbon dioxide trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂. Every 100 ml of deoxygenated blood delivers about 4 ml of CO₂ to the alveoli.

REGULATION OF RESPIRATION

The regulation of respiration is done by the neural system. The respiratory rhythm centre is present in the medulla and is mainly responsible for the regulation of respiration. Another region; called pneumotaxic centre is present in the pons. The pneumotaxic centre can moderate the functions of the respiratory rhythm centre.

A chemosensitive area is situated adjacent to the rhythm centre. This is highly sensitive to CO₂ and hydrogen ions. Increase in these substances can activate this chemosensitive area. This; in turn; gives signal to the rhythm centre to make necessary adjustments in the respiratory process so that these substances can be eliminated.

Receptors associated with aortic arch and carotid artery can also recognize changes in CO₂ and H⁺ concentration. These receptors send necessary signals to the rhythm centre for corrective actions.

It is important to remember that the role of oxygen in the regulation of respiratory rhythm is quite insignificant.

DISORDERS OF RESPIRATORY SYSTEM

Asthma: Asthma is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. Constriction of bronchii leads to asthmatic attacks.

Emphysema: Emphysema is a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. Smoking is a major cause of emphysema.

Occupational Respiratory Disorders: In some industries, a huge amount of dust is involved. The dust particles often get inside the lungs of the workers because the body’s defence system is unable to cope with the huge amount of dust. Long term exposure can lead to severe lung damage. Workers usually wear masks to prevent the entry of dust particles inside their lungs.

NCERT Solution for Class 11 Biology Breathing and Exchange of Gases

Question 1. Define vital capacity. What is its significance?

Answer: Vital Capacity (VC): The maximum volume of air a person can breathe in after a forceful expiration is called Vital Capacity. This is also defined as the maximum volume of air a person can breathe out after a forceful inspiration. VC = ERV + TV + IRV.

Vital capacity of a person gives important clues while diagnosing a lung disease. Measurement of this capacity helps the doctor to decide about the possible causes of the diseases and about the line of treatment.

Question 2. State the volume of air remaining in the lungs after a normal breathing.

Answer: Functional Residual Capacity (FRC): The volume of air which remains in the lungs after a normal expiration is called Functional Residual Capacity. FRC = ERV + RV.

ERV=1000 to 1100 ml

RV = 1100 to 1200 ml

So, FRC = 2100 to 2300 ml

Question 3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?

Answer: The exchange part of the respiratory system is composed of the alveoli and their ducts. Actual diffusion of O₂ and CO₂ (between blood and atmospheric air) takes place in the exchange part of the respiratory system. The thin membrane of the alveoli is suited for diffusion of gases, while other parts of the respiratory system are not structured to serve this purpose. Hence, diffusion of gases takes place in the alveolar region only and not in the other parts of the respiratory system.

Question 4. What are the major transport mechanisms for CO₂? Explain.

Answer: The binding of carbon dioxide with haemoglobin is related to partial pressure of CO₂. The partial pressure of O₂ is a major factor which can affect this binding. In the tissues, pCO₂ is higher than pO₂ and hence more binding of carbon dioxide occurs at the tissue level. In the alveoli, pCO₂ is lower than pO₂ and hence dissociation of carbamino-haemoglobin takes place in the alveoli.

Question 5. What will be the pO₂ and pCO₂ in the atmospheric air compared to those in the alveolar air ?

i. pO₂ lesser, pCO₂ higher

ii. pO₂ higher, pCO₂ lesser

iii. pO₂ higher, pCO₂ higher

iv. pO₂ lesser, pCO₂ lesser 

Answer:- (ii) pO₂ higher, pCO₂ lesser

Question 6. Explain the process of inspiration under normal conditions.

Answer: Inspiration is initiated by the contraction of diaphragm. The contraction of diaphragm increases the volume of thoracic chamber in the antero-posterior axis. The external inter-costal muscles contract to lift up the ribs and the sternum. This causes an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume results in a similar increase in pulmonary volume. The increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure. This pressure gradient forces the air from outside to move into the lungs and inspiration takes place.

Question 7. How is respiration regulated?

Answer: The regulation of respiration is done by the neural system. The respiratory rhythm centre is present in the medulla and is mainly responsible for the regulation of respiration. Another region; called pneumotaxic centre is present in the pons. The pneumotaxic centre can moderate the functions of the respiratory rhythm centre.

A chemosensitive area is situated adjacent to the rhythm centre. This is highly sensitive to CO₂ and hydrogen ions. Increase in these substances can activate this chemosensitive area.

This; in turn; gives signal to the rhythm centre to make necessary adjustments in the respiratory process so that these substances can be eliminated.

Receptors associated with aortic arch and carotid artery can also recognize changes in CO₂ and H⁺ concentration. These receptors send necessary signals to the rhythm centre for corrective actions.

It is important to remember that the role of oxygen in the regulation of respiratory rhythm is quite insignificant. 

Question 8. What is the effect of pCO₂ on oxygen transport?

Answer: Binding of oxygen with haemoglobin is mainly related to the partial pressure of O₂. Partial pressure of CO₂, hydrogen ion concentration and temperature are the other factors which can interfere with this binding. Increased partial pressure of CO₂ can increase haemoglobin’s affinity towards oxygen and vice-versa is also true.

Question 9. What happens to the respiratory process in a man going up a hill?

Answer: A man going uphill has to exert more effort to climb. This increases the consumption of oxygen. As a result, the partial pressure of oxygen in haemoglobin decreases which creates more demand for oxygen. This is compensated by an increased breathing rate.

Question 10. What is the site of gaseous exchange in an insect?

Answer: Insects have a network of tubes through which air is transported within the body. These tubes are called tracheae. The tracheae open on the lateral surface of the animal through minute pores; called spiracles.

Question 11. Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?

Answer: When percentage saturation of haemoglobin with O₂ is plotted against pO₂, we get a sigmoid curve. This curve is called Oxygen Dissociation Curve.

Oxygen has a high affinity with haemoglobin. Binding of initial molecules of oxygen is difficult, but binding of subsequent molecules becomes easier. This is evident by the rising trend in the initial phases of the sigmoid curve. Once the oxygen binding reaches its optimum level, haemoglobin cannot take up any more oxygen molecules and hence the graph shows a plateau phase. These are the reasons for S-shape of the graph.

Question 12. Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.

Answer: Lack of adequate oxygen supply to whole body or a part is called hypoxia. Hypoxia generally happens because of a mismatch between oxygen demand and supply.

Question 13. Distinguish between

(a) IRV and ERV

Answer: The additional volume of air which can be forcefully inspired is called IRV, while the additional volume of air forcefully expired is called ERV. In a normal adult, the IRV is about 2500 ml to 3000 ml, while the ERV is about 1000 ml to 1100 ml.

(b) Inspiratory capacity and Expiratory capacity.

Answer: The total volume of air which can be inspired after a normal expiration is called Inspiratory Capacity, while the total volume of air which can be expired after a normal inspiration is called expiratory capacity. IC = TV + IRV, while EC = TV + ERV. 

(c) Vital capacity and Total lung capacity.

Answer: The maximum volume of air which a person can breathe in after a forced expiration is called Vital Capacity, while the total volume of air accommodated in the lungs at the end of a forced inspiration is called Total Lung Capacity.

VC = ERV + TV + IRV

TLC = RV + (ERV + TV + IRV)

Question 14. What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.

Answer: Tidal Volume (TV): The volume of air inspired or expired during a normal respiration is called Tidal Volume. It is approximately 500 ml in a healthy man. This means that a healthy adult can inspire or expire about 6 to 8 litre of air per minute.

Tidal Volume = 500 ml

Respiration rate = 12 per minute 

Hence, Tidal Volume in 1 hour = 500 ml x 12 x 60 minute = 360000 ml = 360 litre