Class 11-science NCERT Solutions Biology Chapter 14 - Breathing And Exchange Of Gases

• The maximum volume of air a person can breathe in after a forced expiration is called vital capacity.
• The vital capacity represents the maximum amount of air which can be renewed in the respiratory system in a single respiration.
• The more amount of air inhaled indicates the maximum amount of oxygen available for the oxidation of glucose, and thus more energy becomes available for the body.

• The volume of air which remains in the lungs after a normal expiration is called functional residual capacity (FRC).
• FRC is the sum of expiratory reserve volume (ERV) + residual volume (RV).
• Expiratory reserve volume (ERV): It is the additional volume of air a person can expire by a forced expiration. It is 1000–1100 ml.
• Residual volume (RV): The volume of air which remains in the lungs after a forced expiration. It is 1100–1200 ml
  

• The structure of alveoli is specially made for the diffusion of gases.
• The total thickness of the alveolar membrane is lesser than a millimetre.
• The outer surface of the membrane is in close contact with the network of blood capillaries.

• The alveolar membrane and the endothelial membrane of blood capillaries are separated by a very thin basement substance.
• Because the barrier between the alveoli and capillaries is very thin, it allows easy diffusion of gases.
• Such a structure is not found in any other part of the respiratory system.
• Hence, the diffusion of gases occurs in the alveolar region and not in the other parts of the respiratory system.
• Also, the alveolar air which comes in close contact with the blood capillaries has high pO₂ and low pCO₂ which facilitates the diffusion of gases. 

Transport of carbon dioxide:

• Carbon dioxide is released in the blood by a more active tissue than a less active tissue.
• Each 100 ml of blood receives on an average 3.7 ml of carbon dioxide from tissues.
• CO₂is carried by the blood in three forms:
• As a simple solution:
  • About 5–10% of CO₂ of the total blood dissolves in plasma and is carried as a simple physical solution.
• As bicarbonate ions:
• At the tissue site, the partial pressure of CO₂ is high because of catabolism.

• CO₂ diffuses into the blood and reacts with water to form carbonic acid. The reaction takes place in the presence of the enzyme carbonic anhydrase.

  

• Carbonic acid dissociates into bicarbonate  and H⁺ ions.

• Some amount of bicarbonate ions is used in maintaining the pH of blood, and hydrogen ions are taken up by proteins.

• The rest of the bicarbonate ions are carried by plasma.

• As carbamino-haemoglobin:
  • When the partial pressure of carbon dioxide is high and the partial pressure of oxygen is low in tissues, CO₂ combines loosely with the globin part of the reduced haemoglobin to form carbamino-haemoglobin.
    
  • When pO₂ is high and pCO₂ is low in alveoli, CO₂ dissociates from carbamino-haemoglobin; hence, CO₂ bound to haemoglobin is released in the alveoli.

(ii) pO₂ higher, pCO₂ lesser

pO₂ will be higher and pCO₂ will be lower in the atmosphere.

Every gas in a mixture of gases exerts a pressure called partial pressure. Gases always diffuse across the pressure gradient. When pO₂ is higher in the atmosphere, oxygen can easily enter the lungs. Similarly, CO₂ can diffuse out of the body only if pCO₂ is lower in the atmosphere.

• Inspiration is the process by which fresh air enters the lungs.

• It occurs when the pressure inside the lungs, i.e. intrapulmonary pressure is lesser than the atmospheric pressure.
• Inspiratory muscles, i.e. muscles of the diaphragm, external intercostal muscles and abdominal muscles make inspiration possible.
• Muscles of the diaphragm contract and pull the diaphragm down towards the abdominal cavity making it flat.
• This results in an increase in the thoracic cavity vertically in the antero-posterior direction.
• The external intercostal muscles contract and lift up the ribs and sternum. This results in the expansion of the thoracic chamber dorso-ventrally.
• The overall increase in the thoracic volume causes an increase in the pulmonary volume.
• The increase in the pulmonary volume decreases the pressure inside the lungs, and the atmospheric pressure forces air from the outside to move into the lungs.
• Abdominal muscles relax to compress the abdominal organs which increase the strength of inspiration.
• The passage of air during inspiration is as follows:
  External nares → Nasal cavities → Internal nares → Pharynx → Glottis →Larynx → Trachea → Bronchi → Bronchioles → Alveolar ducts → Alveoli 

Regulation of Respiration:

• Human beings have the capability to maintain and moderate their respiratory rhythm to suit the demands of the body tissues.
• The respiratory rhythm centre present in the medulla oblongata is responsible for the regulation of respiratory rhythm.
• The inspiration respiratory rhythm centre controls the normal rhythm of respiration.
• One respiratory cycle is of 5 seconds—2 seconds for inspiration and 3 seconds for expiration.
• The expiration respiratory rhythm centre is dormant during normal respiration, but it controls the powerful contraction of the intercostal and abdominal muscles during deep breathing.
• The pneumotaxic centre present in the pons regulates or moderates the functions of the respiratory rhythm centre.
• A neural signal from the pneumotaxic centre can reduce the duration of inspiration and hence alter the respiratory rate.
• A chemosensitive area is located adjacent to the rhythm centre.
• This area is highly sensitive to CO₂ and hydrogen ions.
• Increase in CO₂ and hydrogen ions activates the chemosensitive area.
• Activation of the chemosensitive area signals the rhythm centre to adjust the respiratory process by which CO₂ and H⁺ ions are eliminated.
• Receptors present in the aortic arch recognise changes in CO₂ and H⁺ ion concentration and send signals to the rhythm centre for remedial actions.
• The role of oxygen in the regulation of respiratory rhythm is insignificant.

• pCO₂ plays an important role in oxygen transport.
• In the alveoli, because of low pCO₂, oxygen binds with haemoglobin resulting in the formation of oxyhaemoglobin.
• In tissues, high pCO₂ facilitates the dissociation of oxyhaemoglobin.
• O₂ gets bound to haemoglobin at the lung surface where pCO₂ is low and dissociates at the tissues where pCO₂ is high.

• When a person goes up the hill, the altitude increases.
• At the higher altitude, the concentration of atmospheric oxygen becomes lesser, i.e. the partial pressure of oxygen decreases.
• This increases the demand of oxygen.
• To increase the oxygen supply to the blood, the person starts breathing rapidly. His heart rate also increases to meet the oxygen demand.

• Insects have trachea as their respiratory organs.

• Air enters the trachea by openings called spiracles present on either side of the insect’s abdomen.
• Every segment of the abdomen has a pair of spiracles.
• The trachea branch into smaller and smaller tubes until they reach the tissue level.
• Oxygen which enters the trachea is exchanged with the tissues by diffusion. At the same time, CO₂is passed to the trachea from tissues and is expelled from the body. 

• The oxygen dissociation curve is a graph obtained when the percentage saturation of haemoglobin with oxygen is plotted against the partial pressure of oxygen. 

• When the first molecule of oxygen binds to haemoglobin, its affinity for the second molecule of oxygen increases.
• Hence, the formation of oxyhaemoglobin is faster which is indicated by the steep slope of the S curve.
• When the formation of oxyhaemoglobin comes to rest or when there are no haemoglobin molecules available for binding, the curve achieves a plateau phase.  

Hypoxia is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply.

Causes of hypoxia:

• At high altitudes, low concentration of oxygen may lead to hypoxia.
• Any respiratory disorder such as emphysema, bronchitis and asthma may lead to hypoxia.
• Heart problems such as tachycardia cause hypoxia.
• Conditions such as anaemia which are related to a less number of red blood cells cause hypoxia.

Symptoms of hypoxia:

• Rapid breathing, rapid heart rate and shortness of breath are symptoms of hypoxia.
• Lethargy, anxiousness, inability to communicate and confusion are some symptoms associated with hypoxia.

On the basis of causes, there are different types of hypoxia:

1. Anaemic hypoxia: It is caused by the deficiency of haemoglobin.
2. Cytotoxic hypoxia: It is caused by cyanide poisoning.
3. Stagnant hypoxia: It is caused by heart failure or reduced pumping activity of the heart.
4. Hypoxic hypoxia: It is due to insufficient oxygen in the air (e.g. at high altitude).
5. CO poisoning: Carbon monoxide binds to haemoglobin irreversibly and the oxygen transport is reduced.

• The volume of air inspired or expired during normal respiration is called the tidal volume.
• A healthy man breathes 12−16 times per minute.
• The tidal volume for a healthy person is 500 ml per breath which is 6000–8000 ml per minute.
• Hence, the tidal volume in an hour will be 3,60,000−4,80,000 ml.